Small-signal AC amplification and DC offset

odm4286

Joined Sep 20, 2009
265
Hello everyone,

I am playing around with small-signal AC amplification and have a question about DC offset. Let's say you have a signal that swings from 100mV to -100mV or 200mV in magnitude. If a DC offset is introduced and the signal is then amplified by a gain of 10 you will end up with a signal that is about 2V in magnitude but will swing around a point determined by the amount of DC offset introduced.

This is where I am stuck, what is the best way to eliminate this DC offset after amplification? I'd like to have a signal with a larger magnitude but still swings positive and negative through zero. For now, all of my circuits use transistors, capacitors, and resistors only. I'd like to avoid using ICs, op-amps for example, to develop a better understanding of what is actually going on.

ronsimpson

Joined Oct 7, 2019
2,886
Please post a schematic of you amplifier and say how much offset you are seeing.

MrChips

Joined Oct 2, 2009
30,456
Here are two ways to remove DC offset:

1) Use a summing amplifier and subtract the offset.
2) Use a DC blocking capacitor after the amplifier, i.e. a high pass filter.

odm4286

Joined Sep 20, 2009
265
Please post a schematic of you amplifier and say how much offset you are seeing.
I am not in front of the machine that has LTSpice on it at the moment but I will post a schematic later tonight. It was more of a general question. I want to know how to handle a situation where you have a very small AC signal, let's say a sine wave, that you want to convert to a square wave. Well, if I used a Schmitt trigger to do this it would rely on a zero-crossing signal.

I felt like this was a tricky problem, a signal too small to serve as the input to the Schmitt trigger directly but by amplifying it you remove the zero-crossing characteristic needed for the Schmitt trigger to generate the desired waveform. Sorry if this is a little bit of a ramble. Hopefully, I got my point across.

MrChips

Joined Oct 2, 2009
30,456
Use an analog comparator circuit to convert to a square wave. Then you can set the voltage at the crossing.