Sometimes the old stuff is still the best. At room temperature, a
mechanical chopper combined with a high-ratio step up transformer and
high impedance follower amplifier is unbeatable for low noise, low
impedance signal amplification. The solid state devices that replaced
mechanical choppers in most applications have much higher resistance,
so more intrinsic noise.
You should save the chopper for possible future use - and if there is
an input transformer that goes with it, especially save that. Some
circuits used a transformer (typically 1:100 ratio) to reach high
gains for tiny signals into the nV DC region - without it, the noise
level swamps the signal. The transformer is the key to the noise
performance, but it wouldn't do much good without the low
on-resistance of the chopper. The main problem with mechanical
choppers is that they wear out - I think 10,000 hours is a typical
life specification, and there's no way to know the history of a
salvaged part. Transformers should last virtually forever, unless abused.
As an example, I built a transformer-based preamp for my lock-in
analyzers, that provides 1000X gain with 300 pV/root Hz noise, when
the source Z is low enough - around a few ohms. I could make this
unit measure DC by putting a chopper in front of it, but it must have
very low resistance to preserve the noise performance.
There is no conventional IC or discrete amplifier that I know of that
can directly amplify with this noise performance, without using a
transformer. The analyzer front-ends typically use "regular" low
noise, broadband IC amplifiers that have around 7 nV/root Hz - about
twenty times as much. The lock-in analyzer makers used to offer
special transformers for this purpose.
There are tradeoffs, of course - the transformer circuits only work
well over a limited frequency range like 1 Hz to maybe a few kHz,
unlike the ~0 to 100 kHz that an analyzer can run. Also, they only
make sense in low-Z circuits - higher Z adds lots of noise, and
dramatically affects the transformer frequency response. As I recall,
300 pV/root Hz is about the intrinsic noise of 50 ohms at room
temperature, so in the more normal high impedance realm, conventional
amplifiers are good enough.
Ed
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