Electronic – Amplify a signal having a DC component without using an opamp


I am building a simple modular synthesizer. I would like to keep everything simple and prefer to use discrete components over integrated circuits. My goal is to improve my knowledge about electronics.

How can I amplify a signal which has a DC component (e.g. 1V DC + a sine wave which is 1V peak to peak) without using an opamp?

It is easy to amplify a pure AC signal with a transistor, some resistors, and 2 capacitors (to decouple DC at input and output). However, now I want to also amplify the DC component, so the decoupling with the capacitors is not possible.

I am looking for a solution that doesn't have DC drift with temperature change, but it doesn't need to be perfectly linear.

Best Answer

To keep the drift relatively low (it will be inferior to a monolithic op-amp because of the poor matching and thermal coupling) you can use a differential pair and use feedback like an op-amp.

Here is an example of a unity-gain stable amplifier made with discrete parts, from this website.

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C1 provides frequency compensation- and can be changed for stability. Q1 and Q2 should be thermally coupled for minimum drift (eg. put thermal compound between the two, face to face, and shrink wrap around them).

Output is class A so no crossover distortion, but drive capability is limited. Bias is power supply voltage dependent, so PSRR is poor. You can play with this in LTspice or on the bench.

Performance won't come close, in most respects, to a modern monolithic amplifier designed for audio applications.