# Quick question about open loop / closed loop op amps and mosfets

#### hrstar24

Joined Apr 7, 2009
32
So with the following circuit (Q1 and Q2 are matched, as well as Q3 and Q4): How do I go about calculating the overall small signal gain of the circuit?

So far I am just assuming that since IN- is connected to Vout, this circuit is just merely a voltage follower op amp, so the gain from the differential amplifier (Q1 - Q4) is 1. This means I only need to worry about the gain from Q5 right?

#### Ron H

Joined Apr 14, 2005
7,014
The closed loop gain is exactly 1 only if the open loop gain is infinite. You have to calculate the open loop gain (loop broken between Q5S and Q2G) before you can calculate the closed loop gain.

#### hrstar24

Joined Apr 7, 2009
32
Ok I have calculated the open loop gain of the circuit, so how do I go about getting the closed loop gain once I have the open loop gain?

I have this equation:

Vout = Vin ( K / (1 + K/A))

where K is the closed loop gain, and A is the open loop gain. Since this is just a simple voltage follower, we make K = 1, plug in the open loop gain, and solve for Vout/Vin. Is this the correct approach?

Last edited:

#### Ron H

Joined Apr 14, 2005
7,014
Ok I have calculated the open loop gain of the circuit, so how do I go about getting the closed loop gain once I have the open loop gain?

I have this equation:

Vout = Vin ( K / (1 + K/A))

where K is the closed loop gain, and A is the open loop gain. Since this is just a simple voltage follower, we make K = 1, plug in the open loop gain, and solve for Vout/Vin. Is this the correct approach?
The equation I am familiar with is

Vout/Vin = A/(1+Aβ)

where A is the open loop gain and β is the feedback ratio.

I suppose your equation solves for actual closed loop gain, given ideal closed loop gain, so that's another way to do it. I hadn't seen that method.

• hrstar24