How to understand circuits

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  • #1
Hello, I'm in my 2nd year of EE. I have done circuit analysis and some basic into to diodes, transistors and op-amps.

We have a class in which we are given circuits for a audio amplifier and a power supply. We are required to understand the circuits completely. How they work and why each component is necessary.

I am not sure how to approach this. I have learned how to calculate voltages, currents etc. But not design. Is this common in universities? My understanding of semiconductors is weak so i am going over op-amps and diodes again.

How can I learn to understand circuits?

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  • #2
The first circuits course is usually analysis ... how to understand what is going on ... and you focus on solving for the currents and voltages, starting with static cases using Ohm's law and KVL/KCL (batteries, power supplies, resistors, some diodes and transistors, op amps), then transient cases using Laplace transforms (switches), then alternating currents (fixed frequency, phasors, power transfer).

During this process you will start to recognize certain standard elements: voltage dividers, bridges, rectifiers, simple filters, etc. Design is the process of putting together these elements to achieve a particular goal. This is very similar to how you learn computer programming: you learn a language, its syntax and objects, then you write programs which do what you want.

Typically your second class will be a systems and signals course for linear systems; then you will learn how you can design analog systems starting from the abstract goals - this is a fairly mathematical course, but you will learn a lot. If it is a good course there will be labs where you build circuits which illustrate the theory; usually these will be radio circuits.

The real design work comes as you take further courses where you will learn how to think about transistor logic, power systems, and controls. In the meantime you can move ahead on your own by learning more about basic circuit elements, and building stuff yourself. Lots of practice with a good SPICE program is also useful ... then you can simulate your designs prior to doing a breadboard.
  • #3
When you review what you know about active devices (transistors, op amps, etc) focus on how they are used to make "building blocks" for circuits. For example an op-amp used as a linear amplifier will always have some components (often resistors) that form a feedback loop and set the loop gain. Usually there will be other components that set the input impedance. A transistor common-emitter amplifier stage has some components to set the DC operating conditions (there is more than one way to do that) and other components to set the AC gain of the stage.

If you are given the circuit of a complete audio amp, you should be able to identify the individual amplifier stages, what configuration each stage uses, whether there are feedback loops that go across several stages, etc, just by "looking at" the diagram, without doing any calculations yet. Then you can start to analyze each stage independent of the others.

For multi-stage amplifiers, it's often easiest to work "backwards" from the output to the input, rather than starting at the input.

What you described in your question isn't really "design". it's "analyzing a design made by somebody else". But that's a first step towards "analyzing a design you made yourself". Most of the work you do when designing something is actually "analysis", not "creative thinking".

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