practical opa2134 gain limits

[cuibono]

A simple question, but I haven't been able to find a simple answer. I'm building a very simple electret mic preamp with an OPA2134, and I am wondering how high the practical gain limit is. Also, I feel I should be limiting the amount of NFB at the same time, which should dictate both the gain resistor ratios and their actual values.


Anyone got some numbers?

[BWRX]

Well the typical open loop gain of the opa2134 is 120dB, so that's your limit...

To have a 20kHz bandwidth the max gain you can use is approximately 50dB or about 316V/V.

You can find all the info needed to determine these values in the data sheet for the part.

[cuibono]

thanks - I've looked at the data sheets quite a bit, but I just don't know how open loop gain, bandwidth and closed loop gain interrelate. Does anyone have a resource for learning some intermediary opamp design?

I'm still looking for what people think the max NFB should be (before messing with the low level detail)...

Thanks again

[Chucko]

I'm afraid I don't agree with the "too much negative feedback is bad" theory. When working with IC op amps, IMHO, more NFB is better. NFB is the only thing that keeps the op amp linear.

Which brings us back to the original question. 8 MHz GBW is pretty modest by today's standards. It implies you only have 52 dB of gain (400x) to play with at 20 KHz. That's not a lot if you're asking the 2134 to work as a mic preamp and stay linear at the same time.

So, how much gain do you need? If you need more than, say, 10x (20 dB), I'd suggest a compound gain stage with the OPA2134 as the front end and a video op amp, say the LT1358 or LM6127, as the output stage. See Jerald Graeme's excellent book Amplifier Applications of Op Amps for details on how to do this.

As for actual resistor values, in general you should keep the impedance seen at the inverting input as low as practical for noise reasons. Low feedback resistor values also require smaller compensation capacitor values. How low you can go depends mostly on the output drive capability of the op amp.

I hope someone finds this useful.

[cuibono]

Thanks for the suggestions. 50dB gain should suffice. I used to work with a very competent microphone designer, and he was always complaining that too much NFB ruined low level detail. I know just about nothing when it comes to design...

That book looks good