103 Original scientific paper Journal of Microelectronics, Electronic Components and Materials Vol. 53, No. 2(2023), 103 – 117 https://doi.org/10.33180/InfMIDEM2023.205 How to cite: Ž. Rojec, “Towards Smaller Single-point Failure-resilient Analog Circuits by Use of a Genetic Algorithm", Inf. Midem-J. Microelectron. Elec- tron. Compon. Mater., Vol. 53, No. 2(2023), pp. 103–117 T owards Smaller Single-point Failure-resilient Analog Circuits by Use of a Genetic Algorithm Žiga Rojec Department EDA, Faculty of Electrical Engineering, University of Ljubljana, Slovenia Abstract: Failure-resilient analog circuits are difficult to design, but artificial intelligence can help crawl the topology solution space. Using evolutionary computation-based topology synthesis we evolve analog arcus tangent computational circuits, resilient to any rectifying diode or resistor high-impedance single failure or removal. We encode analog circuit topologies as individuals with an upper-triangular incident matrix. Circuits are evolved using a combined technique utilizing parts of NSGA-II and PSADE, based on a special three-dimensional robustness function. We show that topology size for a failure-resilient circuit can be classes smaller than hand-made component-redundancy-based solutions. Our best failure-resilient topology comprises six diodes, three resistors, and a voltage offset source. Keywords: analog circuits, analog circuit synthesis, circuit optimization, failure-resilience, circuit robustness Manjšanje analognih vezij odpornih na odpoved poljubne komponente z uporabo genetskega algoritma Izvleček: Analogna vezja, ki so odporna na napake, je težko načrtovati. Pri prečesavanju prostora možnih topologij lahko pomaga umetna inteligenca. Z sintezo topologij, temelječi na evolucijskem algoritmu, smo razvili analogno računsko vezje za inverzni tangens, ki je odporno na visokoimpedančno okvaro posamezne komponente (diode ali upora) ali njene odstranitve. Topologija analognega vezja je v algoritmu zapisana v obliki zgornje-trikotne vpadne matrike. Vezja razvijemo z uporabo kombinirane metode z uporabo večkriterijskega optimizacijskega algoritma NSGA-II in PSADE, kjer je za usmerjanje sinteze razvita posebna tri-kriterijska funkcija robustnosti. V članku prikazujemo kako zmanjšati velikost topologije, odporne na odpoved komponente, na razrede manjšo velikost od ročno izdelanih robustnih topologij, ki temeljijo na redundanci posameznih komponent. Naš najboljši rezultat je analogno računsko vezje za inverzni tangens, ki je sestavljeno iz šestih diod, treh uporov in odmičnega napetostnega vira. Ključne besede: analogna vezja, sinteza analognih vezij, optimizacija vezij, odpornost na napake, robustnost vezij * Corresponding Author’s e-mail: ziga.rojec@fe.uni-lj.si 1 Introduction Design of an analog circuit is a challenging task, espe- cially when the product has to meet high standards and fulfill tough requirements. Designers often use various simulation tools to predict temperature, humidity, and electromagnetic behavior during circuit operation. Furthermore, to predict the blueprint manufacturability and maximize the produc- tion yield, they also use statistical methods, such as Monte Carlo analysis [1]. However, customer requirements might get even hard- er. When a device is targeted for use in harsh conditions (i. e., space exploration, aeronautical missions, auto- motive, robotics), we expect the product to be robust against extreme temperature swings, high ionizing and electromagnetic radiation levels, high working cur- rents, and more. That kind of stress can lead to compo- nent faults and premature device failure. Furthermore, failed components in remote and unmanned missions could not be replaced easily. 104 Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 Researchers have already focused on hardening elec- tronic devices against failures per se [2]. The classical ways of doing that include component redundancy, overdesign, shielding and insulation, thermal manage- ment, and so on. Most of the time such solutions signif- icantly increase the size, weight, and finally, the cost of the device. The upper methods usually aim to protect every circuit component as if it was the main breaking point of the system. Researchers have already proposed systems resilient to failures that occur in vivo. Meaning, the circuit has the ability to persist functional when one or more com- ponents fails during the circuit operation [3]–[6]. Such systems usually utilize duplicated circuit modules to form redundant sub-systems which are controlled by various voting mechanisms [3], [7]. However, the de- multiplexer then becomes the weak part of the system. This paper shows an alternative method of evolv- ing failure-resilient analog circuits. Using an intensive evolutionary search, we can find novel analog circuit topologies that exhibit robustness to any electronic component (semiconductor diode or resistor) high- impedance failure or removal, without a dedicated ac- tive demultiplexing system. We show in this work, that by using an evolutionary to- pology synthesis tool, we can greatly reduce the size and the number of needed components to achieve failure-resilience of an analog circuit, compared to ca- nonical hand-made design. To the best of our knowledge, this is one of the few published works on the automated synthesis of a pri- ori robust, failure-resilient nonlinear computational analog circuits [3], [4], [8]–[15], and also one of the first attempts of redundancy reduction by using evolution- ary search. The paper is organized as follows. We summarize previ- ous work on robust topology synthesis in Section 1.1 and describe our motivation in 1.2. We describe the applied topology synthesis technique in Section 2. Re- sults are given in Section 3, summarized in 3.8 and con- cluded in Section 4. 1.1 Previous work The discovery of novel circuit topologies has been done by hand for over a century. This is changing with the availability of novel tools, relying on artificial intel- ligence [16]. Since the beginning of this research area [17]–[19], computer-aided circuit synthesis has be- come human-competitive and trustworthy for fabrica- tion [16], [20]. We believe, rather than replacing a hu- man expert in the industry, AI might help in the rapid exploring of undiscovered topology space, thereby helping and speeding up the design process. Reviews of existing analog circuit synthesis techniques can be found in existing literature [21], [22]. However, we give a brief overview of existing topology synthesis efforts for extremely robust and failure-resilient analog circuits below. 1.1.1 Synthesis method Analog topology synthesis is an extremely non-linear and complex task, which is why most existing ap- proaches in this field search topology with a method, based on the Darwinian selection of the fittest, i.e. evo- lutionary or genetic algorithm. Somehow special are the works of Zebulum and Key- meulen, et. Al., who presented an evolutionary algo- rithm that is being run on the controlling unit of the circuit under failure, in vivo [4], [12]. Evolutionary methods demonstrate a capacity to tack- le unconventional challenges. One compelling reason that supports the continued relevance of evolutionary computation, even when compared to neural networks like GNNs, is that they do not always require prior train- ing to align with the defined cost function. However, emerging tools rooted in GNNs, like CktGNN, showcase impressive capabilities in generating robust circuit topologies [23]. 1.1.2 Synthesis goals and degrees of robustness Passive filters are usually the entry point for showing the performance of analog circuit synthesis tools. Most of the works on failure-resilience also experimented with the synthesis of robust passive analog filter cir- cuits, dealing with various degrees of component faults. Resistor/capacitor/inductor removal was con- sidered in [9], [15], while in addition [3], [7] also stud- ied the complexity of partial and full short-circuit and high-impedance faults. Studies [24]–[27] only consid- ered R/L/C parameter perturbation without full com- ponent failure. Other authors reported syntheses of - compensator circuit [8] and - inverter, amplifier, and oscillator [13] resilient to bipolar transistor removal, - PID controller with R/L/C removal resilience [10], - transistor-fault resilient amplifier [11], - half-wave rectifier, NOR gate, and voltage-con- trolled oscillator for extreme temperature swings (in situ evolution) [12] 105 - XNOR gate, analog multiplier, and inverter resil- ient to arbitrary faults in the controlling unit FPTA (Field Programmable Transistor Array) [4] - the natural logarithm and square-root analog computational circuits resilient to semiconductor diode short-circuit or high-impedance malfunc- tion [28] 1.2 Motivation 1.2.1 Failure-resilience For this work, let us define failure-resilience as an analog circuit topology property, where any of the basic com- ponents (diode or resistor) can be removed or replaced with high-impedance failure, with the circuit showing minimal-to-zero deformation of nominal signal process- ing abilities. The voltage source and the 10 k W input- pullup resistor are excluded from the definition. The methodology incorporates various failure scenar- ios using specialized “failure-defining” Spice models, as demonstrated in our prior work [28], where we suc- cessfully synthesized analog circuits resilient to both high-impedance and short-impedance failures in semi- conductor diodes. In this paper, we primarily concen- trate on minimizing topologies that are fully resilient to high-impedance failures. However, due to high compu- tational costs, we do not address short-circuit failures for all component types in this paper; this topic is left for future research. 1.2.2 Size of failure-resilient circuits Failure-robustness comes with a cost. It is generally paid by (often significantly) higher total number of needed components for the same nominal task as a non-robust circuit would perform. For a system to sur- vive such rigorous change, as one or any component removal/failure, redundant elements and connections must be available in the system. Let us consider an example of a non-linear, computa- tional analog circuit from Figure 1. The circuit outputs an inverse tangent of input voltage signal between 0 and 10 V. It is a hand-designed linear voltage divider, with diodes used to switch between five linear seg- ments, which closely interpolate the mathematical function [29]. Due to its simplicity, the topology is often used instead of the amplifier-chain summing circuit. If any of the components in the dotted square (except for the voltage source) fails (or is removed), the circuit’s transfer function severely changes as seen in Figure 2 with absolute error range plot and Figure 3 with rela- tive error plots. The most common and straightforward approach to achieving failure-resilience property is to introduce redundancy on a single-component level. In the case of an arctan circuit, every diode has to be paired in parallel and every resistor has to be (at least) tripled in parallel. Two diodes in parallel give a sub-circuit where, theoretically, any of the two diodes might enter high- impedance failure without transfer function transfor- mation. Single resistor with resistance R n has to be re- placed with three parallel resistances 3 R n to maintain 33% relative error of sub-circuit in case of one resistor entering high-impedance failure. Figure 4 shows a hand-designed topology that fulfills the failure-resilience criteria. Fair nominal response and narrow error range in failure cases are presented in Figure 5 and Figure 6. Evidently, the circuit topology hence the number of needed components goes off- scale. While the nominal non-robust topology includes 10 resistors and 5 diodes (excluding the input resistor, see 1.2.1), the hand-made robust version comprises 30 resistors and 10 diodes. In CMOS technology, for exam- ple, resistors occupy large chip areas [30]. In addition, those resistances are multipliers of the nominal values, which further multiplies the needed area for fabrica- tion. The circuit total cost would be above comparison to the nominal non-robust version. However, novel studies of analog topology synthesis imply, that number of needed components for failure- resilience might somehow be lower than expected in hand-made designs [3], [7]. The possible reason for that phenomenon is that open-ended topology synthesis allows component-level redundancy to be replaced with system-level redundancy. 1.2.3 Topology size as a synthesis constraint In this study, we explored the lower limits of topology size for a failure-resilient computational analog circuit. We show, that for the arcus-tangent circuit, the topol- ogy could be reduced from 40 critical components in hand-made design down to 8 components by evolu- tionary-based synthesis. This also has fewer compo- nents than used hand-made non-robust design (15). Figure 1: Canonical hand-designed piece-wise linear arctan computational circuit topology. Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 106 Our study provides step-by-step size-reducing results for further investigation and a better understanding of underlying mechanisms. Primary contribution of this paper lies in the demonstration of a novel application of evolutionary methods, resulting in the attainment of system robustness that has not been ob- served in any existing systems or circuits within the literature. Figure 2: Hand-designed non-robust arctan circuit: nomi- nal response (black) completely covers the arctan func- tion. The range of various failure responses is given in blue. Figure 3: Relative error curves of nominal (solid) and component failures (dotted and dashed). Figure 4: Hand-designed piece-wise linear arctan computational circuit, robust to any single component high-impedance failure or removal. Figure 5: Hand-designed failure-resilient arctan circuit: nominal response (black) covers the arctan function. The range of various failure responses is given in blue. Figure 6: Hand-designed failure-resilient arctan circuit: relative error curves of nominal (solid) and component failures (dotted and dashed). 2 Methods In this section, we provide details of the methods used in this circuit synthesis. The applied approach is mostly based on [28]. 2.1 Analog Circuit Representation Upper-triangular incident matrix is a well-proven method of encoding an analog circuit topology [22], [28], [31]. It is based on a fixed set of available compo- nent terminals. Each building block can comprise one or more input/output terminals (see Figure 7). Usually, the building-block terminals are located on the left side of the fixed set, and outer connections are located on the right-side of the set. The set is then mirrored in two dimensions, forming a connection matrix, where the logical one represents an existing zero-impedance Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 107 connection between the terminals on both axes. The matrix is filled with logical ones on a diagonal so that by definition, every terminal is connected to itself. Only the upper matrix triangle is used to exclude half of the redundant mirror connections from the bottom trian- gle, to reduce the effective matrix size, without sacrific- ing any topology search space [31], [32]. Additionally, in the inner-connections sector of the matrix, we allow every possible connection, while in the outer-connec- tion section only one positive logical value is allowed per line, filtering-out any connections between outer terminals. Figure 7: An example of an upper-triangular matrix, representing a simple T-shaped analog circuit topol- ogy [31]. Components with adjustable parameters (i.e., resist- ances, capacitances, transistor widths and lengths, etc.) have their values organized in a separate array, called value vector. While the topology matrix is purely bina- ry, the value vector is a numeric entity. 2.2 Genetic Reproduction and Sizing For evolutionary computation and mimicking natu- ral genetic reproduction, we use the topology-matrix crossover technique, described in [31]. Every terminal is connected to other terminals via the logical values that reside on a column and a row, intersecting the diago- nal element, that represents the connection to itself. By exchanging the two lines of the matrix with another topology matrix, the information of the terminal con- necting with the rest of the circuit is transferred. Figure 8 shows two examples of newly-created offspring with one terminal (N=1) and three terminal (N=3) informa- tion being exchanged. Note that in the applied algo- rithm, the number of exchanged terminal connections N is a randomly-chosen number from the set {1,2,3}. Figure 8: Topology crossover examples. For better illus- tration, parent no. 2 is a full upper-triangular matrix [31]. The value vector is being optimized using two different methods. The first one is a reproduction mechanism, inspired by a well-known intermediate crossover [33]. The choice between topology-matrix or value-vector crossover is initiated by the evolutionary algorithm. In one case offspring will inherit a modified topology and in another a modified parameter. Another parameter tuning technique in this work is an established PSADE (Parallel Simulated Annealing and Differential Evolution) [34]. Due to its computational expensiveness (yet effectiveness), it is triggered only every 10 th generation on one to three best individuals. 2.3 Fitness function The fitness function should encompass the desired properties of the circuit. Additionally, it should filter out individuals with unwanted properties and help to guide the searching algorithm through the valley of local minima. We will briefly review the applied fitness function below, but the full justification of chosen cri- teria is given in [28]. In the case of open-ended topology synthesis, the fit- ness function definition is rather complex and com- Vin Vout R1 R2 C1 ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Inner-connections Outer-connections R1 R2 C1 R1 R2 C1 Forbidden sector without connections N=1 N=3 offspring 1 offspring 2 parents Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 108 prises several stages. The first is an evaluation of the circuit’s transfer function, i.e. signal processing quality, using a DC analysis in Spice simulator. In the case of arctan circuit design (let us denote the mathematical function as g) we calculate the root mean square error (RMSE) between V out (V in ) and g(V in ). We call the result fitness and denote it as f. Calculation of failure-resilient circuit fitness needs to be carried out for every predicted failure scenario. In our work, failure-resilience is defined as the high im- pedance failure of any resistor or semiconductor diode (see 1.2.1). In the case of 30 resistors and 10 diodes, the total number of RMSE calculations must be 41 – that is one for nominal (no failure) scenario f nom , and 40 for every critical device failed, multiplied by the number of failure types considered (only one failure type in this case). Vector f comprises all RMSE results: 1,11 ,, , , ..., , ..., nom FN F ff ff    f (1) where N is the total number of critical components and F is the total number of failure types [28]. Failure-resilient circuit evaluation is carried out in mul- tiple dimensions, and forms a three-dimension robust- ness vector r: max nom f f        f r (2) where f nom is RMSE result of no-failure, nominal circuit topology, f max is the maximum of vector f and α f is the standard deviation of the same vector [28]. Vector r gives insight into a single failure-resilient candidate - nominal performance - performance in case of worse single-point failure and - statistical failure scattering. This separation gives a chance to the NSGA-II algorithm to non-dominantly sort the individuals into Pareto- fronts and by that maintain the genetic diversity, thus avoiding premature convergence. In the specific case of a failure resilient circuit synthesis, a practitioner might encounter a false-robustness phe- nomenon, which we explain below. Let us consider an example of a simple diode half-wave rectifier (Figure 9, left). If D0 fails or is removed, the rec- tifier is no longer working, and statistically, one critical component (diode) makes a 100% chance of circuit failure. Imagine a topology modification, that would harden the circuit against the D0 removal or high-im- pedance failure. Let us have four additional diodes to fulfill that requirement (one would be enough, but we assume the search algorithm does not know that). The search algorithm can encounter a topology with four diodes with no effect on the nominal transfer function (example in Figure 9 (right)). Still, if D0 fails, the circuit does, too. However, if any of D1-4 fails, the circuit still delivers the transfer function. It appears as only 20% of critical components (diodes) cause a fatal scenario for the circuit. The latter circuit might get promoted because of its better “robustness” value. Obviously, this is not the case, because D1-4 are not electrically con- nected and do not play any role in signal processing. That kind of circuit has to be ranked out since it does not contribute to real circuit robustness. Figure 9: False-robustness problem [28]. Inclusiveness [28] successfully unfolds the false-robust- ness problem. Using modified diode models and SPICE simulator commands we determine which of the com- ponents are electrically connected (included) and have an effect on signal processing. Inclusiveness (denoted by I) is calculated as a ratio between the number of all critical and included components. Having an updated robustness definition: max nom f fI        f r (3) circuits with greater inclusiveness are promoted over the circuits with floating or flawed connected com- ponents. However, this can lead the synthesis to build larger circuits with excessive redundancy, so compo- nent number limits must be set elsewhere in the algo- rithm. In our case, the top number of available devices is set in the pre-defined component set, which also de- fines the topology-matrix size. Note that only the inclu- siveness of diodes was considered in our work. 2.4 Synthesis algorithm The search and sorting algorithm utilize major ideas from NSGA-II [35]. Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 109 The evolutionary algorithm is initiated by a randomly generated population. Then every individual is evalu- ated according to the fitness/robustness from Section 0. Sorting is performed in three steps, following NSGA-II. In the first step, individuals that do not dominate each other (are not beaten in any combination of objectives) are assigned to a front (i.e. Pareto front). The remaining individuals are put in a second, third, etc., front, with the same non-dominance criteria. A new generation assem- bly is the second step. We aggregate the new generation starting with individuals from the 1 st front, and continue with available individuals from further fronts. Because a union of parents and offspring is usually larger than available space in the new generation, there is a front of individuals, that does not fit as a whole to the new gen- eration. A selection between non-dominated individu- als needs to be undertaken. This is done in the third step, the crowding distance calculation. The crowding dis- tance is the distance between two neighboring points (i.e. individuals) along each of the objective axes. Rank- ing individuals with higher crowding distance helps to a more even distribution in a front of individuals. After the assembly of the new generation, a parent se- lection process takes place. With the tournament, some randomly selected individuals are chosen from the generation. The selected individuals compete based on their front number (lower is better) and crowding distance (higher is better). Two tournaments take place to choose two future parents. Having selected two parents, their genetic material gets reproduced. This can be done by mating their genetic material as in 0 or by mutating it. Control over mating/mutation is a statistical probability, set at the beginning of the algorithm. Similarly, a probability pa- rameter controls whether the topological or paramet- ric part of the gene will be mated/mutated. We repeat the synthesis algorithm until at least one of the stopping criteria (i.e., design requirements, max. number of generations, timeout.) is met. When ten gen- erations have passed, we run a PSADE [34] parameter optimization on three of the best circuits from the popu- lation and thus fine-tune the ambitious individuals. Figure 10 summarizes the main synthesis algorithm steps. 2.5 Finding minimal topology Our objective was to evolve circuits with consistent performance even if devices are removed. Initially, we aimed to incorporate as many “redundant” com- ponents as possible. However, circuit size doesn’t al- ways reflect actual functional contributions, leading to “dummy” or electrically connected but non-functional components. To address this, we introduced “Inclusiveness” to pre- vent circuits dominated by dangling sub-circuits, enhancing evolutionary outcomes. Individuals with greater inclusiveness measure propagate more effec- tively. Our experimentation revealed a paradox when maximizing redundancy while minimizing circuit size simultaneously. Hence, we perform separate stages for minimizing and maximizing circuit schematics. We are listing two more reasons, why the size of circuit sche- matics is not another objective of NSGA-II search. Our topology representation method using an upper- triangular incident matrix limits arbitrary extensions during evolution runs. Varying matrix sizes in the evo- lutionary pool cause inconsistent crossovers and mat- ing patterns. The third concern relates to the computational com- plexity of NSGA-II and evaluating circuits under differ- ent failure scenarios. A variable maximum component number during evolution would increase computa- tional effort, impacting NSGA-II’s performance and cir- cuit robustness evaluation. As a result, we chose not to experiment with variable component numbers to mini- mize computational burden. 3 Results Our experiment comprised eight independent topol- ogy searches. For each synthesis we predefined the set of available components, that is N d diodes and N r resistors that are subject to possible high-impedance failure. V off and a R in input resistor (the latter was non- optional) were also available with each synthesis but were excluded from failure consideration. Initial population Evaluation Sorting Tournament (parent selection) (according to rank and crowding-distance) (calculate fitness/robustness) Reproduction (offspring creation) Offspring evaluation Criteria met? END True False 10th generation? True False PSADE parameter opt. on 3 of best individuals Figure 10: The applied evolutionary algorithm flow- chart [31]. Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 110 The main part of the experiment was discovering the possibilities of finding topologies with fewer compo- nents than in hand-designed examples (e.g., from Fig- ure 4), that perform arcus tangent analog calculation and exhibit the failure-resilience property (1.2.1). The genetic algorithm parameters were fixed through the experiment and are summarized in Table 1. Table 1: Genetic algorithm properties. Parameter Value Population 1000 Tournament 3 Mating prob. 0.6 Topology reproduction prob. 0.8 Resistance values were limited to the range between 10 and 100 k W, and voltage source with DC range of 0 to 6 V. Every synthesis was conducted on an i9 HP desk- top, utilizing 16 computational threads on 8 processor cores. 3.1 Synthesis with a max of 12 diodes, 12 resistors With the ambition to cut the number of needed com- ponents for the circuit, we gave the first upper limit of Nd max = 12 and Nr max =12. This is already a significant cut of the total number of components (Nd + Nr) in com- parison to hand designed example from Figure 4 which comprises 40 components. The algorithm can, howev- er, synthesize a topology with fewer elements. Starting with a random population, without any prior knowledge available in the population itself, we let the combined NSGA-II algorithm run for 306 generations (roughly 15 hours). The outcome is presented in Figure 11. The final topology comprises all 12 available diodes. Some resistors were excluded from the final topology since they do not have any signal-processing effect (such as short-connected resistors, or resistors con- nected to simulator-helper nodes). The voltage source was also not included in the final design. We excluded some of the components already from topology sche- matics in Figure 11. We summarize the circuit performance in three param- eters: nominal topology RMSE is 0.312, the worst failure RMSE is 0.370 and the standard distribution of all cases (nominal and failures) is 0.026. One can visualize those results in Figure 12 and Figure 13. Figure 12: Synthesized arctan computational circuit (Nd max = 12, Nr max = 12): nominal response (black), arctan function (red, dashed-dotted). The range of vari- ous failure responses is given in blue. Figure 13: Synthesized arctan computational circuit (Nd max = 12, Nr max = 12): relative error curves of nominal (solid) and component failures (dotted and dashed). Together with a voltage source, six available resistors were not used in the final circuit. That is why we con- Figure 11: Synthesized arctan computational circuit (Nd max = 12, Nr max = 12), robust to any single component high-impedance failure or removal. Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 111 ducted our experiment with tighter device component limits. 3.2 Synthesis with a max of 10 diodes, 10 resistors The next synthesis was limited to Nd max = 10 and Nr max =10. We stopped the algorithm after 822 generations (that was after 33h). The outcome is presented in Figure 14. The final topol- ogy comprises all 10 available diodes. Two resistors were not included in the final topology. Figure 14: Synthesized arctan computational circuit (Nd max = 10, Nr max = 10), robust to any single component high-impedance failure or removal. Circuit performance: nominal topology RMSE is 0.158, the worst failure RMSE is 0.270 and the standard distri- bution of all cases (nominal and failures) is 0.032. One can visualize failure ranges in Figure 15 and Figure 16. This circuit performs better than the one from the pre- vious synthesis, according to the three observables. It also comprises 2 diodes less and four resistors more. Figure 15: Synthesized arctan computational circuit (Nd max = 10, Nr max = 10): nominal response (black), arctan function (red, dashed-dotted). The range of vari- ous failure responses is given in blue. 3.3 Synthesis with a max of 8 diodes, 8 resistors We proceed with Nd max = 8 and Nr max = 8. We stopped the algorithm after 432 generations (11h). The outcome is presented in Figure 17. The final topol- ogy comprises 6 diodes and 6 resistors that can fail dur- ing the circuit operation. Two resistors and two diodes were not included in the final topology. Figure 17: Synthesized arctan computational circuit (Nd max = 8, Nr max = 8), robust to any single component high-impedance failure or removal. Circuit performance: nominal topology RMSE is 0.149, the worst failure RMSE is 0.152 and the standard distri- bution of all cases (nominal and failures) is 0.017. One can visualize failure ranges in Figure 18 and Figure 19. Figure 16: Synthesized arctan computational circuit (Nd max = 10, Nr max = 10): relative error curves of nominal (solid) and component failures (dotted and dashed). Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 112 Because the algorithm kept solving the problem using less than the maximum of available components, we proceed and further tighten the Nd max and Nr max criteria. 3.4 Synthesis with a max of 6 diodes, 6 resistors We stopped the Nd max = 6 and Nr max = 6 synthesis after 2340 generations (48 h). Figure 20 shows the outcome. The final topology uses all available diodes and three out of six available resistors. Circuit performance: nominal topology RMSE is 0.106, the worst failure RMSE is 0.110 and the standard distri- bution of all cases (nominal and failures) is 0.008. One can visualize failure ranges in Figure 21 and Figure 22. 3.5 Synthesis with a max of 5 diodes, 5 resistors The Nd max = 5 and Nr max = 5 synthesis was stopped after 2582 generations (36 h). As shown in Figure 23, the final topology comprises all available components. Although the synthesis comprises only ten critical components (plus voltage source and input resistor), the performance was not yet diminished. The nominal Figure 18: Synthesized arctan computational circuit (Nd max = 8, Nr max = 8): nominal response (black), arctan function (red, dashed-dotted). The range of various fail- ure responses is given in blue. Figure 19: Synthesized arctan computational circuit (Nd max = 8, Nr max = 8): relative error curves of nominal (solid) and component failures (dotted and dashed). Figure 20: Synthesized arctan computational circuit (Nd max = 6, Nr max = 6), robust to any single component high-impedance failure or removal. Figure 21: Synthesized arctan computational circuit (Nd max = 6, Nr max = 6): nominal response (black), arctan function (red, dashed-dotted). The range of various fail- ure responses is given in blue. Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 113 topology RMSE is 0.108, the worst failure RMSE is 0.165 and the standard distribution of all cases is 0.022. See failure ranges in Figure 24 and Figure 25. 3.6 Synthesis with a max of 4 diodes, 4 resistors Searching for the bottom limit, we conducted the Nd max = 4 and Nr max = 4 synthesis. We finished it after 1077 generations and 12h. The final topology comprised 4 resistors and 4 diodes (Figure 26). Figure 26: Synthesized arctan computational circuit (Nd max = 4, Nr max = 4), robust to any single component high-impedance failure or removal. The nominal topology RMSE is 0.173, the worst failure RMSE is 0.217 and the standard distribution of all cases is 0.028. See failure ranges in Figure 27 and Figure 28. We have discovered, that this synthesis is a probable bottom limit in our experiment. To illustrate, how a smaller design poorly fits the requirement, we show one more synthesis. Figure 22: Synthesized arctan computational circuit (Nd max = 6, Nr max = 6): relative error curves of nominal (solid) and component failures (dotted and dashed). Figure 23: Synthesized arctan computational circuit (Nd max = 5, Nr max = 5), robust to any single component high-impedance failure or removal. Figure 24: Synthesized arctan computational circuit (Nd max = 5, Nr max = 5): nominal response (black), arctan function (red, dashed-dotted). The range of various fail- ure responses is given in blue. Figure 25: Synthesized arctan computational circuit (Nd max = 5, Nr max = 5): relative error curves of nominal (solid) and component failures (dotted and dashed). Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 114 3.7 Synthesis with a max of 3 diodes, 3 resistors Using limits Nd max = 3 and Nr max = 3 synthesis, we fin- ished the search after 3188 generations (11h). See Figure 29 for the topology. The nominal topology RMSE is 0.497, the worst failure RMSE is 0.507 and the standard distribution is 0.010. Failure ranges are shown in Figure 30 and Figure 31. We can observe a two-piece approximation of the arctan function, which yields high RMSE. Figure 30: Synthesized arctan computational circuit (Nd max = 3, Nr max = 3): nominal response (black), arctan function (red, dashed-dotted). The range of various fail- ure responses is given in blue. Figure 31: Synthesized arctan computational circuit (Nd max = 3, Nr max = 3): relative error curves of nominal (solid) and component failures (dotted and dashed). 3.8 Result Summary Table 1 summarizes the experiment results. Surpris- ingly, tightening the number of available diodes and resistors has led to improved circuit performance in Figure 27: Synthesized arctan computational circuit (Nd max = 4, Nr max = 4): nominal response (black), arctan function (red, dashed-dotted). The range of various fail- ure responses is given in blue. Figure 28: Synthesized arctan computational circuit (Nd max = 4, Nr max = 4): relative error curves of nominal (solid) and component failures (dotted and dashed). Figure 29: Synthesized arctan computational circuit (Nd max = 3, Nr max = 3), robust to any single component high-impedance failure or removal. Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 115 both nominal functionality and robustness, with its best at Nd=6, Nr=3. Although initial syntheses involved searches over Nd max > 6, Nr max > 3 topology space, the Nd = 6, Nr = 3 best solution was not discovered in these. Table 2: Results of a conducted experiment. Every row is an independent topology synthesis with different num. of component limits. The first row is the hand- made robust design. Nd max Nr max Nd Nr f nom f max s f N/A N/A 10 30 0.116 0.262 0.047 12 12 12 6 0.312 0.370 0.026 10 10 10 8 0.158 0.270 0.032 8 8 6 6 0.149 0.152 0.017 6 6 6 3 0.106 0.110 0.008 5 5 5 5 0.108 0.165 0.022 4 4 4 4 0.173 0.217 0.028 3 3 2 3 0.497 0.507 0.010 There might be several reasons for that phenomenon. The first, most obvious one, is an enormous search space for topology search. Within one synthesis run, we cannot sample every possible circuit, but rather crawl the space using the evolutionary search. This is why two evolutionary syntheses with the same goal but different initial settings might not produce the same outcome. The second reason is more related specifically to the robustness definition in our experiment. As noted, our problem definition does not reward circuits with fewer components, but rather the opposite. Inclusiveness (see 2.3) rewards circuits that electrically include all available components to push means of redundancy into the circuit and avoid false robustness. During the synthesis, while the objectives might already be met with requirements, the inclusiveness criteria might draw the search toward more included components, which makes the search too wide and lasting long. We conclude, that with such-defined search problem, the hard limits on the topology size and the number of available components are key to an efficient small-size failure-resilient topology search. 4 Conclusions Using the topology synthesis tools, we can find to- pologies, that exhibit novel properties, such as failure tolerance. We showed that failure-resilience in analog circuits can be achieved with smaller-than-expected topologies, by introducing system-level redundancy instead of much more expensive component-level re- dundancy. Using an evolutionary-based topology syn- thesis tool, we introduced novel topologies of analog arcus tangent circuit. The most compact one comprises six diodes, three resistors, a voltage source, and an in- put resistor. Each of the diodes and the three resistors can fail or be removed, with almost no computational error. Based on this research, we can conclude that the inte- gration of system redundancy for single-point failures was achieved by imposing a strict limitation on the maximum size of available components. We showed, that to achieve such resilience, surprisingly low num- ber of electrical components is needed. In the realm of CMOS design, reducing the number of components doesn’t necessarily translate to cost savings on its own. However, we conducted a brief analysis of the total resistance for both robust circuits, encompassing both hand-crafted and synthesized de- signs. Total resistance can provide a rough estimate of circuit area in certain CMOS processes. For instance, the total resistance of a hand-designed circuit (as shown in Fig. 4) amounts to approximately 219 k Ω, whereas the resistance of the best synthesized circuit totals around 20 k Ω (a difference of a decade). Furthermore, reducing the number of components can have a direct impact on cost savings in the realm of discrete electronics, such as PCBs. In the domain of discrete resistors, the resistance value itself does not significantly affect the cost of the device, assuming fac- tors like manufacturer, package, power rating, and tol- erance remain the same. With this in mind, the minimi- zation of robust topologies emerges as a pivotal factor in achieving cost-effective and highly reliable circuits. In comparison to previous experiments, this study con- siders not only diodes, but also resistors to be a possi- ble point of failure. We experimented with evolutionary search for circuits that are robust to both, short-circuit and open-circuit failures in all possible failure points (components), including some experiments including transistors. However, we acknowledge that further in- vestigation and modified approaches are required to address this specific problem effectively. We believe our work will inspire further practitioners in the field of analog circuit topology synthesis. 5 Supplementary material The source code of the synthesis tool is available online at https://github.com/zigarojec/MatrixCircEvolutions. Ž. Rojec; Informacije Midem, Vol. 53, No. 2(2023), 103 – 117 116 6 Acknowledgments I would like to thank my colleagues from the EDA de- partment of the Faculty of Electrical Engineering, the University of Ljubljana for all the support in my work. 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