Odd OpAmp topology - Gain Calculation

[knutolai]

Hi all!

Was looking through CGS for inspiration and came by this unusual inverting amplifier configuration; the "High Gain Stage" that has a voltage divider placed in the negative feedback loop (OpAmp pins 6 & 7). I've had no luck searching google on the. In the circuit notes (linked below) Ken states its a method to add gain without using a bigger feedback resistor. Does anyone have any insight on how I would calculate the gain for such a circuit? Alternatively a searchable name for the topology would be very helpful. Thanks!



https://www.cgs.synth.net/modules/cgs49_twf.html

[GFR]

Imagine the output is at the 10k resistor in the feedback loop. Calculate the gain. The level at that point is 10/110 times the real output, so the gain to the real output is 110/10 times the gain you calculated. So you have 11x more gain without needing a 11 times greater resistor.

[knutolai]

Not able to draw up a diagram atm. So looking at the picture below. The gain would be (Rf / Rin) * (110k / 10k) ?

Where Rf is the leftmost 100k feedback resistor & the (110 / 10) derived from the rightmost 100k and the 10k pointing to ground?

[R.G.]

What he said.

The T attenuator in an opamp's feedback loop is one way of getting a high gain and a high input impedance without huge resistors.

Remember that what happens at an opamp's inverting input is that the current coming in from the input resistor has to be pulled out by the current going to the output through the feedback network. So the output has to use a very much larger voltage to suck enough current through that T network.

[knutolai]

Ok so searching for T Network inverting OpAmp I found the equation for calculating the gain.





http://www.ti.com/lit/an/slaa068b/slaa068b.pdf

Thank you!

[PRR]

> the "High Gain Stage"

Funny thing: it is not high gain.

Put Dist Perc pot for just-clean. You have 100K driving 10K (pot), gain of 0.1. Then 100K:100K inverter resistors for gain of 1. Then the 100K:10K divider, making "1" into 10. 0.1*1*10= unity.

There's gain before that, and on the other side of the Dist Perc pot, but this part is always right about unity.