[volt-nuts] Cool looking old stuff

[ed breya]

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