Circuit Idea/How to Present Circuits

contents - understand - invent - stage

Once understood how an unknown electronic circuit works with the help of heuristics, we naturally have the desire to explain it and present its operation to others - a skill necessary not only for teachers but also for everyone who deals with circuits. Therefore, in this chapter we will discuss the technology of circuit presentation.

How do we Present Circuits Using Heuristics? edit

In order to understand how an unknown circuit works, in the previous story, we went the reverse way of its creation. By breaking it down into simpler devices and trying to recognize the most general principles in its operation, we have actually retraced the evolution of the electronic device. We will now use this valuable information in the correct sequence for presentation purposes (from the beginning to the end and from the imperfect passive circuit to the perfect circuit).

We can best represent the circuit operation if we "invent" it again ("reinvent" or "pseudo-invent"). By recreating the act of invention, we form a mental toolkit of circuit tricks with which we can later really invent electronic devices.

If we do not have enough time for a complete reinvention, we may limit the presentation only to a simple building of the electronic circuit. The stages of such a representation carried out with the help of heuristic means are two - presenting the structure and presenting the operation. We will illustrate them in the next part of the exposition again through the example circuit of an op-amp inverting voltage summer, and for variety here it will be with three inputs. For the purposes of good presentation, we will reveal the circuit evolution in the direction from the passive to the active solution.

Technology of Presenting edit

Stage I: Introducing the circuit structure edit

1. Formulating the task solved by the electronic device. At the beginning, we present in a clear form the task solved by the device. For the particular example circuit of an inverting summer, it is: Sum the voltages of grounded input voltage sources; the resulting output voltage is also reffered to ground. It arises from the common ground problem of the most elementary circuit of a series voltage summer, according to which it is not possible to connect all input voltage sources to ground (see the story about parallel voltage summer).

2. Presenting the basic idea on which the device is built. Next, we formulate the fundamental idea on which the circuit operation is based (it is derived in the understanding stage). Specifically, there are two fundamental ideas in the example voltage summer that need to be revealed as follows:

Fig. 1. Block diagram of a parallel voltage summer. We can solve the problem with the common ground by summing the currents in a parallel manner instead of summing voltages in a series manner (by converting voltages to current and current to voltage).

Idea 1: Instead to sum in a series manner the input voltages, we decide to use an indirect parallel summing by first converting them into currents, then summing the currents and finally, converting the total current into voltage (Fig. 1). Figuratively speaking, we sum the voltages of "hard" voltage sources, forcing them to act in parallel through "softening" resistances at a common output point. This solves the common ground problem because in a current summer all input sources and the load are grounded.

Fig. 2. Fluid analogy of a passive summer (voltage <–> pressure, current <–> flow, resistance <–> constriction)

3. Illustrating the basic idea through analogies. In order to better reveal the basic ideas in the presented electronic device, we can make associations with many analogous life situations:

Fluid analogy. Several pressure sources act through constrictions (throttles) at a common point (for example, several people inflate a common air mattress through the air resistances of individual openings - Fig. 2).
Heat analogy. Several heat sources heat a common massive body through separate thermal "resistances" (for example, several stoves heat a common room through separate air thermal "resistances").

4. Presenting the block diagram of the device. We draw the block diagram of the device that implements the idea in the most general form, independent of the specific implementation.

5. Building the device. At this stage, we build the device sequentially, step by step, using the elementary building blocks from the collection. This is shown in the following figures, arranged sequentially as frames of a movie with explanatory text (in fact, these are actually frames of an animated Flash movie).

Fig. 3a. Building a voltage summer: Adding a current summer.

STEP 1: Add a current summer. We take a current summer, with the help of which we can sum currents from grounded input current sources.

Fig. 3b. Building a voltage summer: Adding input voltage-to-current converters

STEP 2: Add input voltage-to-current converters. But we want to sum voltages, not currents. That is why we include voltage-to-current converters at the summer inputs.

Fig. 3c. Building a voltage summer: Adding an output current-to-voltage converter

STEP 3: Connect a current-to-voltage converter at the summer output. But we need a voltage output, not a current output. That is why we connect the opposite current-to-voltage converter to the summer output.

Fig. 3d. The voltage drop across the output resistor is both useful and harmful.

STEP 4: Reveal the technical contradiction. The "harmful" voltage VR (the resistance R) must both exist and not exist because it is useful for the load but harmful for the input sources.

Fig. 3e. The undesired voltage drop VR is compensated by an equivalent voltage VR

STEP 5: Compensate the voltage by an "anti-voltage". We include in series with the resistor R an additional voltage source with an equivalent voltage VS = VR, which compensates for the "damage" caused by the resistor R.

Fig. 3f. We take the voltage of the compensating voltage source as an output

STEP 6: Choose where to take the output signal from. We use the compensating voltage of the additional voltage source as an output voltage (we feed the load from the "copy", not from the "original"). Thus, the load draws current from the auxiliary and not from the input source.

Fig. 3g. By adding an op-amp to the passive voltage summer, we obtain an active op-amp summer.

STEP 7: Finally implement the circuit with real electronic elements. An op-amp can function as a compensating voltage source. We insert it and get the final circuit of an op-amp inverting voltage summer.

Stage II: Demonstrating the circuit operation edit

The most important part of the presentation is the demonstration of the circuit operation. Here, all the possibilities of the human imagination must be used to reveal in the most attractive way the deep essence of the phenomena.

1. Demonstration through imaginary thought experiments. This is the most popular, accessible and at the same time efficient way by which the operation of electronic devices can be illustrated. This requires only two things:

on the part of the demonstrator - figurative speech that stimulates the "projection" of images in the minds of listeners
on the part of the viewers – imagination to allow them to visualize things

2. Demonstration through animation - the circuit operation is presented "cinematically" by breaking it down into separate frames. The circuit operation can be presented in two successive steps:

First, we replace the active elements (transistors and operational amplifiers) in the circuit with man-controlled elements that perform their "algorithm" of action (as, for example, in Fig. ). During the demonstration, we project individual frames step by step and accompany them by verbal explanations. This allows the valuable information hidden in the transition to be displayed. For example, in the circuit of the op-amp inverting summer, we replace the op-amp with a "man controlled voltage source", a zero indicator and an actor. The role of the latter is performed by us first, and then we can assign it to one of the participants of the demonstration.
Next, we replace the man-controlled active elements in the circuit (transistors and op-amps) with program-controlled elements programmed with their "algorithm" of operation. We project the frames in a slow motion and in a continuous sequence (ie, as an animation).

3. Demonstration by real experiments. Electrical phenomena are invisible to us humans because we have no senses with which to feel them. That is why we need converters of invisible electrical quantities into tangible quantities accessible to our senses. Traditional measuring devices are not smart - they do not "know" what kind of objects are in front of them and, accordingly, in what way to visualize the information received from them. Thus, they are forced to represent electrical quantities in some standard, universal, generally accepted form - a linear movement of an arrow, a two-dimensional graphic on a screen or a digital code on a display.

For the purposes of representation, however, we need another kind of technical means in which to pre-embed an original way of representation refracted through the lens of our imagination. So programmed, this kind of "didactic x-ray" will visualize the invisible electrical quantities in the form of analogies, potential bars and diagrams, current loops, volt-ampere characteristics, etc. Its role can best be performed by a personal computer connected with suitable analog-to-digital peripherals to the experimental setup (Fig. 4).

Fig. 4. A computerized laboratory setup for demonstration of the circuit operation

Under the control of appropriate software, all possible interpretations of the phenomena taking place in the studied object can "come to life" on the screen. To be more convincing, we can supplement this experimental setup with traditional measuring devices that indicate electrical quantities in a standard form.

We can demonstrate the operation of the device in this evolutionary way, in three successive steps:

1. Replace the active elements in the circuit (transistors and operational amplifiers) with manually controlled elements that perform their "algorithm" of operation. Now these are very real elements (in our example – we turn the regulator of a laboratory power supply in the role of a "hand-controlled" operational amplifier), and the computer recreates the corresponding interpretation on the screen.

2. Assign the role of active elements to the personal computer (emulate them). To do this, we disconnect the wire from the output of the "man-controlled" operational amplifier and hook it to the computer-controlled digital-to-analog converter.

3. Replace the computer-controlled elements with real active elements (in our example - with a real operational amplifier).

Heuristics to aid in the presentation of electronic circuits edit

1. Building courses on analog circuits. In these courses, electronic circuits are not given ready-made in their complete, perfect and final form but are built sequentially - each subsequent circuit building on the basis of the previous simpler circuit building blocks.
Circuit Building Tutorial (well illustrated, in Bulgarian)
Negative Feedback Circuit Builder

2. A library of circuit building blocks in analog circuitry. Contains a total of 90 circuit building blocks from the field of analog circuitry:
Collection of Circuit Building Blocks