Do two or more subcircuits connected together change behavior?

Question

This circuit for example:

This is an op-amp circuit. It contains several subcircuits.

I want to know how I can analyze this circuit. I found that I need to break the circuit into subcircuits, but I still don't understand. Do subcircuits change behavior when they are connected together?

Does each subcircuit still work like when they are unconnected to another subcircuit? And if it still works like when they are unconnected, how can I show that this is true in my analysis?

And if I want to learn more about this, what articles do I need to read?

Answer 1

All the subcircuits depend on each other. They are designed and built that way. Their inputs and outputs are compatible and biased to correct operating point by the red biasing circuits.

For example the cyan output circuit does not work alone, it needs the green Vbe multiplier to work. But you can replace it with ideal voltage source if you want to simulate cyan circuit only.

If you want to separately simulate a subsection, it is possibly by biasing the circuit and signals to similar operating points with ideal current and voltage sources. But for that you need to already know a lot about how the circuit operates.

Since you pasted a picture of op-amp internals from Wikipedia here without attribution, you can also read the Wikipedia article for details how that circuit works.

Answer 2

This isn't supposed to be an answer but became too long for a comment.

I want to know how I can analyze this circuit.

Please don't get offended but this seems a bit too much for a beginner level. Today, even in the electrical and electronics engineering departments of universities the internals of an op-amp is kept out-of-scope.

I found that I need to break the circuit into subcircuits, but I still don't understand.

Breaking into sub-circuits and analysing each section is a good way. But

• Sectioning requires familiarity, of course. You can't make sectioning randomly. I assume you are the one who marked the sections with colourful dash-dot lines. So you should be familiar with each section.
• Sections may be meaningful with adjacent sections i.e. the sub-circuits interact with each other. So, for example, the long-tailed pair may not be fully analysed/understood without analysing the current source. Likewise, the Vbe multiplier can't work without a current source.

Now I can see a long-tailed pair, current sources/limiters, current mirrors, Vbe multiplier, and a class AB amplifier. If you want to analyse the entire circuit, you should know how to analyse these transistor circuits/amplifiers first. Then, looking at the circuit, you can analyse the interaction between each. This will hopefully make the total analyse easier.