How do I "understand a circuit?"

I'm interested in learning to design circuits for my own use. I'm also a Physics student (my course has an electronics module) so have had some experience with circuits.

I was just wondering; when I see a circuit (and have the appropriate values for the components) how do I understand the circle?

For example, a simple circuit like the one attached. How would I calculate what the circuit was doing?

"Thinks", If I increase the voltage on the base, the same voltage appears on the emitter but with a lower impedance, so it can drive more current back into C1 and C2. If the crystal was replaced by an inductor, this inductor would resonate with C1 and C2 in series, and they would form a tap on the circuit, so any current that gets fed into the tap results in more voltage at the base.
So the voltage builds up until the transistor limits (runs out of Vcc) or saturates (runs out of Ic). The oscillations then change phase and on and on it goes.
The crystal acts like an inductor over a very small frequency range (.01% of its labelled frequency), so the frequency is determined primarily by the crystal and a tiny amount by the other circuit components.
And that's it, look for what a voltage change would do and follow that change through the circuit.
Frank

Resistanceisfutile said:

I'm interested in learning to design circuits for my own use. I'm also a Physics student (my course has an electronics module) so have had some experience with circuits.
I was just wondering; when I see a circuit (and have the appropriate values for the components) how do I understand the circle?
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Click to expand...

My recommendation is not to start with a relatively complex circuit.
Why not start with a simple transistor amplifier?
However, this requires at least to know and to understand what a transistor is doing.

This circuit operates entirely differently to what is described above. A crystal does not act like an inductor and the transistor does not saturate and it does not run out of Vc or Ic.
Here is the operation of the circuit. This has NEVER been described before in any book.
When a transistor is placed in a circuit and power is turned ON, a spike occurs through the circuit and this is what starts some circuits operating. That why some circuits don't start when the voltage gradually rises.
Other circuits start to oscillate due to the fact that a small amount of noise is generated in the base-emitter junction of the transistor.
That's what happens in this case.
Secondly, when the circuit turns ON, a spike is passed to the crystal via the capacitor in series with the crystal.
This spike causes the crystal to oscillate and it produces a very small sine-wave waveform that has a frequency corresponding to the fundamental frequency of the crystal.
Now we have two "noisy waveforms" produced by the circuit.
The transistor detects this frequency via the base and the same amplitude is produced on the emitter.
The emitter passes this waveform back to the base via C1 and the circuit oscillates with a very small amplitude of about 50mV at the exact frequency of the crystal.