Frontend OP amp test

[6666]

Simple differential pair can have both sufficient gain and high cutoff. And comes with superior noise too.

Davor have you done any modelling for this idea ?

[mickstv]

With the 5532 tests above, is this being used as a two stage preamp? This is because a wider bandwidth and faster settling can be had by sharing the gain over two stages. I never use a single 5534 now as the gain does not recover as fast as the recovery curve as shown on the scope traces would lead you to believe. Sum in a 10kHz signal and you will see this.

Eric.

All the CRO pictures are single stage amp only. The frontend is the typical 1k resistor and diode protection. It was setup this way as a quick method to test differences in op amps. Obviously the benifits of low noise amps are lost then you use a standard resistor type frontend, due to Johnson noise.

[Davor]

Davor have you done any modelling for this idea ?

Yes I have. My main goal with differential pair was making a low noise pre for VLF, so I did not pay much attention to PI needs, but there is no problem to tweak these. I'll come up with my solution soon so that it can be tested against the op amps.

[Davor]

All the CRO pictures are single stage amp only. The frontend is the typical 1k resistor and diode protection. It was setup this way as a quick method to test differences in op amps. Obviously the benifits of low noise amps are lost then you use a standard resistor type frontend, due to Johnson noise.

Yes, 1k is equivalent to 4nV/sqrt(Hz), but it is still in a playground of a NE5534 and some sexier configuration would not yield any improvement regarding noise. However ... using the very same 1k in an inverting configuration may speed things up a lot. You could try the inverting configuration in comparison with the "standard" non-inverting.

I know, Aziz will come and give me a lecture on noise, but please try it anyway

The whole idea of non inverting configuration being less noisy makes sense in case you have a naked op amp input exposed to the signal source. As long as you use a serial resistor of any value, it will have exactly the same noise performance - inverting or non-inverting.

[mickstv]

Yes, 1k is equivalent to 4nV/sqrt(Hz), but it is still in a playground of a NE5534 and some sexier configuration would not yield any improvement regarding noise.

Hi Davor, surely a mosfet blocking circuit instead of the standard frontend configuration would reduce the overall input noise.

[mikebg]

Why do the big players in the industry use low noise electronics IC's etc, if internally generated noise isn't an issue ?

Because they are not ham radio designers.
A ham radio designer thinks how to decrease TX power and increase S/N ratio and modulation index.
IMNHO the big players think for circuit diagram only, not for block diagram. There are several professional designers who think how to suppress AIR&GND signal in RXs input.
Can you see thermal noise in output of your preamp? Please show a picture!
(HINT: Use a noisy opamp if you think that this will help :-)