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

Discussion in 'Homework Help' started by hrstar24, Dec 13, 2010.

1. ### hrstar24 Thread Starter Active Member

Apr 7, 2009
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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?

2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
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.

3. ### hrstar24 Thread Starter Active Member

Apr 7, 2009
32
0
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: Dec 13, 2010
4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
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 likes this.