Minimizing noise with single-ended opamps

Question

Opamps often get used with one input at a DC voltage, usually the main ground per the standard inverting amplifier example:

For low noise, a BJT input stage like OPA1611's is often chosen for the opamp:

But, this then causes flicker noise (I am concerned mainly about low frequencies near 1 Hz) on top of the "ground reference" due to the IN+ NPN transistor, so it seems that total noise could be reduced to half (or 0.707) if the IC were designed knowing that IN+ was always at a DC voltage. Basically, I would rather see an opamp pinout with 1 input pin and 3 supplies than see an opamp pinout with 2 input pins and 2 supplies.

So, why can't I find low noise "single-ended amplifiers" instead of these "differential amplifiers"? Does there exist a single-ended BJT amplifier for low noise at low frequencies?

Am I missing something or is the noise just sacrificed to allow more usage applications?

Answer 1

In answer to your question, yes, they've sacrificed ~40% noise for wider applications and DC performance. It's impossible to get good DC performance without some kind of differential front end.

If you need to avoid the weirdness of zero-drift amplifiers with their nasty current spikes coming out of the inputs and EMI sensitivity, and are concerned about low-frequency 1/f noise you can make your own single ended preamplifier (perhaps with a bunch of JFETs in parallel) and follow that up with something like an LT1028.

You can also get the \$\sqrt{2}\$ increase in voltage noise back by purchasing two amplifiers and effectively paralleling them so the outputs add, however the current noise goes the other way.

Answer 2

Low noise single-ended amplifiers often go by the name pre-amplifier in the low-frequency world. The www holds many rabbit holes worth of literature.

They are usually just used as another amplification stage in front of op-amps. Their voltage gain makes the noise of the op-amp almost meaningless.

Typical voltage noise values can be way below 1 nV/rtHz, but if you are after particularly low 1/f noise, some cherry picking might be required.

For low noise at frequencies well below 1 Hz forget about anything without modulation. The easiest way to have 1/f rejecting modulation is via zero-drift aka chopper-stabilized op-amps.

Answer 3

So, why can't I find low noise "single-ended amplifiers" instead of these "differential amplifiers"? Does there exist a single-ended BJT amplifier for low noise at low frequencies?

Look for chopper amplifiers. they specifically rectify the flicker noise to a higher frequency and then filter it out, then construct the DC signal again.