Circuit Idea/How to Invent Circuits

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Once we can understand and present existing electronic circuits, we would like to create, synthesize, and even invent new circuits using heuristics. This skill is needed not only by designers and inventors but also by teachers in the field of circuit engineering. Therefore, this story discusses the technology of the invention of electronic circuits.

Types of Inventions
Pseudo-invention. We have already used this imaginative invention to effectively present the operation of electronic circuits. As a result, we have accumulated a set of circuit engineering tricks and techniques with which we can truly invent new electronic devices.

Subjective new invention. As a result of the understanding and presentation of electronic circuits, things that have already been invented are most often "invented", which usually causes the ridicule of others ("invented the wheel"). Indeed, society does not directly benefit from these subjective inventions. But they are extremely useful to get a deep insight into the essence of phenomena, to realize the meaning of technical solutions and to form inventive thinking.

True invention. If we have a chance, we can reach a real invention - an objectively new technical solution unknown to this moment not only to the inventor but also to everyone. In terms of the creative process, there is no difference between the individual forms of invention. Only the material benefits for the inventor and society are different.

Technology of Inventing
Here we will sequentially disclose the steps of the invention using for illustration the exemplary circuit of our op-amp inverting summer. The story of this invention will not be entirely authentic but will be largely imaginary. Therefore, it will not fully correspond to the actual path taken by the inventor of this circuit. Since the device has long been invented and exists, this is again a pseudo-invention in a sense but now it is much closer to a true invention. To do this, we will temporarily forget that the device has already been invented and replay the stages of its invention. This approach is justified here because the point of this chapter is more to show the way the inventor thinks, the way they move when come up with the new idea, and not so much the idea itself.

The difference between inventing and building circuits is that in inventing there is no ready-made solution to the problem and the end result is not known, while in building the scenario that the presenter follows is clear. In other words, there is an element of the unknown in invention.

We can accomplish the invention of the exemplary device using heuristics in the following steps.

Stage I: Inventing the circuit structure
1. Formulate the task that the electronic device must perform. Two ways of setting the task are possible:


 * The first is to passively wait for the task to be "dropped from above". Here the first danger lies, because the task may not be formulated correctly.


 * The second way is to set the task ourselves - for example, we are dissatisfied with some circuit and want to improve it.

For the purposes of this work, we assume that the task is assigned to us in the first way and it is: Create a voltage summing device (voltage summer).

2. Start by solving the task. We begin with the obvious solution – the first one that would occur to us to deal with the task at hand and that we can find in the literature.

In the specific case, we choose the simplest way to sum the voltages – by connecting the input voltage sources in series according to KVL, and thus assemble the most elementary possible series voltage summer. Its device is extremely simple - only a loop (piece of wire).

3. Usually a problem occurs. In most cases of practice, the first solution is not suitable (otherwise we would be satisfied with it and would not make an invention).

In the specific case, we come across the basic problem of the common ground for analog circuit engineering, which consists of the following. The individual stages of the devices in the circuit engineering (input sources, load, etc.) are usually connected with one of their outputs to the common ground of the system (the point with zero reference voltage). In the case of a two-input summer, assuming that the input sources are connected to the common ground, the output (load) remains "floating". The rule is that in order not to cause a conflict (short circuit) between the input voltage sources when we try to connect them in series, only two of the stages should have a connection to the common ground.

4. Look for a solution to the problem. But we cannot find it among the existing solutions in the literature. We try to find a solution to the problem in the literature but the existing solutions are usually compromises and do not satisfy us. In the case of the two-input summer, for example, we would be suggested to include in its output a converter with a differential input and an unbalanced output, which is complicated and expensive.

5. Reveal the technical contradiction. To get to the heart of the matter, we need to uncover the technical contradiction. It usually sounds paradoxical (for example, something must both exist and not exist). In the case of the summer, it turns out that the input voltage sources must be connected in series and parallel at the same time, which is practically impossible.

6. Look for the new idea. Thus we come to the actual part of the invention, which can be carried out in the following sequence:


 * Apply one of the principles we know so far to build electronic devices (from the principles collection).
 * Stimulate the emergence of new ideas in every possible way. For this purpose, we use all possible methods to stimulate creative thinking and avoid meeting with "idea killers".
 * Let new ideas "crystallize" calmly, without pouncing on them while they are still unformed. This, the most important stage of the invention of the voltage summer, can be carried out in the following steps:

STEP 1. As soon as the series way of summing voltages, according to Kirchhoff's II law, does not work for us, then we try the parallel way of summing currents, according to KCL. Just as the loop is the most elementary voltage summer, the node is the most elementary current summer. It has the advantage that the input current sources and the load are connected to the common ground.

STEP 2. However, we want to sum voltages. So we need to convert them to currents by including voltage-to-current converters between the voltage input sources and the summer inputs.

STEP 3. However, the summer made in this way has a current output, and we want it to be voltage. Then we include the opposite current-voltage converter in the output. We can reinvent it from the Ohm's elementary electrical circuit but powered by a current source (ie, we now assume that the current determines the voltage). To do this, we vary the current as an input and measure the voltage as the output value.

7. Next problem occurs. However, it turns out that the voltage drop VR across the resistor R affects the operation of the circuit. The currents through the input resistors depend not on the input voltages themselves but on the difference between them and the output voltage.

8. Reveal the new technical controversy. In the voltage summing circuit, it is: The voltage VR (the resistance R) must both exist and not exist because it is useful to the load but harmful to the input sources.

9. Look for the next new idea. We begin again to look for a solution to the problem in the surrounding reality. For the specific case, we notice many situations in our daily life, in which a harmful disturbance prevents us from realizing a certain goal: we get sick - we start taking medicine until we recover our health; we are tired - we rest; we get hungry - we eat; we spend money - we work hard to recover it; a weight appears in the left pan of the scale - we destroy it with an "antiweight" in the right pan; we commit a violation and immediately pay the fine, etc.

10. Derive the following inventive principle. We summarize the examples and thus derive a new inventive principle, which we can figuratively call "compensation of disturbance by anti-disturbance". In our electrical example it reads: Support the input sources with an additional voltage VS = VR, which actively copies the voltage drop across the resistor R and compensates it.

11. Apply the principle in the specific circuit diagram. Now it remains only to apply the principle in our particular device. To do this, we break the circuit and include a compensating voltage source in series with the resistor R. It produces a voltage of the same value, which compensates for the damage done by the resistance R. To get a better feel for the idea, at this first stage we take on the functions of an active follower - we change the voltage so that it is always equal to the drop VR across the resistor R (empathy).

Now it remains to solve the last problem - where to get the output signal, i.e. where should we plug in the load? At the output of the current summer (position 1) the voltage is already destroyed and therefore it is pointless to include the load there. The resistor R has the output voltage we need but if we connect the load to it (position 2) two problems arise. First, the load will divert some of the current through itself (especially if it is low-impedance). Second, the load is "floating" (ungrounded). So where is the output of this circuit?

If we go back to the above analogies and remember how we acted in all these cases, we will find that we often use an indirect rather than a direct assessment: the amount of medicine we take gives us an idea of ​​our disease; how tired we are is understood by how long we rest; the amount of food we swallow shows how hungry we were; the sum of the weights in one pan of the scale gives an estimate of the unknown weight in the other pan, and so on. These examples allow us to refine the principle of "removing disturbance by anti-disturbance" into the principle of "measuring disturbance by anti-disturbance": An idea of ​​the value of a harmful disturbance gives us indirectly the value of the useful "anti-disturbance" by which we have destroyed it.

Now we know where to take the output signal of the inverting summer - the compensating voltage at the output of the op amp is an exact inverted copy of the original voltage and this will be the output of the circuit. In this way, we supply the load from the "copy" and not from the "original". Thus, the load draws current from the auxiliary and not from the input source.

Stage II: Exploring the circuit operation
First we prepare the circuit for the study (as before). We then examine it sequentially in the following steps, looking for unusual inputs:

1. Consistently vary the parameters of the circuit individual elements at a constant value of the input quantities. What this actually means is that we temporarily abandon the traditional circuit inputs and look for new entry points. We can use rather unusual "inputs" - the supply voltage, the output of the circuit, "harmful" quantities (for example, the temperature), etc.


 * In our inverting summer circuit we can start varying the resistance of the resistor R ie. the value of the harmful disturbance. The op amp responds to this by changing its output voltage accordingly. So we invent the resistance-to-voltage converter and convert the disturbance into an input signal. Of course, a more common application of this phenomenon is the scaling of the output signal.
 * Then we can change the resistance of the input resistors (R1, R2...). We get resistance-to-voltage converters again, but with opposite characteristics. Thus, we can individually adjust the weighting coefficients of the individual inputs.
 * If we change the resistance of the load, we will be convinced again that the circuit behaves as a constant voltage source.

2. Deepen the study by simultaneously changing several signals or circuit parameters. In practice, we are most often limited to two quantities, which we change unidirectionally (common mode) or oppositely (differentially).


 * If, for example, in the circuit of the inverting summer we change the input voltages simultaneously and in the opposite direction, the output voltage will not change - an idea for a differential circuit.


 * In the same way, we can change the resistances of the input resistors simultaneously and in the opposite direction - we get a differential resistive sensor, etc.

Stage III: Summarizing the results
We can use the ideas gained in inventing one particular electronic circuit and in inventing other analogous circuits (as we did before). This is how:

1. Try to find commonalities between the principles underlying the invented device and other electronic devices already invented. All sorts of other parallel OOB circuits can be devised by applying the same idea (assisting the input source and using the offset voltage as the output signal). Examples: "ideal" ammeter, current-voltage and voltage-current converters, inverting amplifier, integrator and differentiator, logarithmic and antilogarithmic converter, etc.

2. Add the principle to the collection of principles for inventing electronic devices. In the specific case, we first formulated the universal (non-electrical) principle of removing disturbance by "anti-disturbance", which we then further developed into the principle of measuring disturbance by "anti-disturbance". Then we concretized it in the circuit principle of compensating voltage by "anti-voltage" and measuring voltage by "anti-voltage".

3. Draw a block diagram of the device implementing the principle and add it to the collection of block diagrams.

Web resources
Here are some circuit stories showing the process of inventing specific circuits:

Battery backup voltage regulator (an invention story) 3-LED voltage indicator (an inventor's story)