Gain, amplifier related question

Hello Esme,

It is not exactly clear what sort of response is expected from this circuit. It appears that the first stage U1 is a low-pass filter, followed by a high-pass filter in the second stage U2; possibly resulting in a band-pass response? The way that R2, the 8.2 K resistor drives R3, and 8.2 Ohm resistor looks odd. There is no output taken from U1. Are you sure this connection correct? Also, the non-inverting input of U2 does not have input bias current path so the output will pull up against one of the output rails. A bias current path to ground is needed.

It may be that this topology isn't capable of devloping gain because both U1 and U2 are connected as unity-gain buffers. If you have some information about this topology and what you are attempting to accomplish and can share that, then we may be able to proivde a solution.

Regards, Thomas

PA - Linear Applications Engineering

Hi Thomas,

I double checked the resistances of R2 and R3, yes, they are supposed to be 8.2k and 8.2k. This is just a part of a circuit board, and the board works.

We want to increase the gain up to 10db and 20 db, and frequency pass band to stay the same.

Is it possible to do that?

(U1 and U2 don't have to be unity-gain buffers)

Thanks,

Esme

Hi Esme,

Even if I change R2 and R3 in your original schematic from 8.2 Ohms, to 8.2 K, and add a 1 Meg input bias current resistor to U2, pin 3, the circuit has a gain far below 1 V/V. It looks to be on the order of about -20 dB with the potentiometer set to 50 %. You can see the response below. I contend, unless the circuit was designed to have high loss, that there is something incorrect about the schematic. I can remove U1 completely from the circuit and there is no change in the gain and frequency responses. That should be an strong indicator that something is not correct with the circuit.

If you are sure the circuit response is what you need, with the exception of low gain, move and use U1 as a gain stage to follow the U2 output.

Regards, Thomas

PA - Linear Applications Engineering

122ckt.TSC

Something like this? (please see attachment)

But the gain is even smaller..

The maroon curve is the original circuit with U1 and U2 as unity-gain buffers

the green curve is the new circuit

Thanks,

Esme

Also this is the original schematics and I attached the file (This particular circuit is on the top right)

On the input side, it is connected to LED

20161203025357.pdf

(since I cant find RC4580 in TINA-TI, I just used U1 and U2, I don't think that matters, right?)

Thanks,

Esme

That is very helpful! Thank you so much!

One last question, is it possible to shift the gain to the left?

(The maroon line is the current one you make, is it possible to shift to the blue line?)

Thanks again,

Esme