Induction Balance Stuff - Single/Multi Frequency Response, GB, Disc, Measurements, Ideas, Fun, etc.

[Aziz]

Hi all,

I am testing different solutions via LTspice simulations.

Mode of transmitter
- Direct injection
- Transformer coupled injection

Active nulling via:
- Simple mixer ( a*(Vsig+Vcarrier) + b*Vcarrier = 0)
- Difference amplifier topoplogy

Amplifier topology regarding noise generation and source impedance issue:
- bipolar amplifier (NE5532)
- j-fet input amplifier (OPA2134)

There are a lot of trade-offs, design considerations, pros/cons, etc.. depending of the mode.

So far:
The NE5532 can be operated at approx. 7 nV/sqrt(Hz) input referred noise at operating frequency.
The OPA2134 at approx. 9-10 nV/sqrt(Hz).
Do we really need input referred noise of <1 nV/sqrt(Hz)? How much would cost us using some more parts to the NE5532?

It takes time to travel and analyse all the possible ways.
Aziz

[Aziz]

Hi all,

so far, the j-fet input op-amp OPA2134 does not get any significant improvement. It is more noisy.
I can use even the NE5532 in direct mode as the non-inverting (+) input does have enough high input impedance. En (input referred noise voltage density) is approx. 9.3 nV/sqrt(Hz) in direct mode.This is even slightly better than OPA2134. On the other hand, the OPA2134 is a rail-to-rail op-amp providing more dynamic range.

I will look at the ultra-low-noise front-end with the BJT ZTX951 (PNP) + NE5532 as a feedback loop driver. This will reduce the noise density 10 times.
And yes, it makes really sense to use the ultra-low-noise front-end in this case.

I will be in Berlin for the next 5 days. The analysis will be continued, when I am back.
Cheers,
Aziz