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

Thread Starter

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.
 

Thread Starter

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.
 
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