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# Questions about the op amp used in unity-gain configuration

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#### derknand

##### Newbie level 3
If we do need a op amp with low output resistance when we used the op amp in a unity-gain configuration?
I think the negative feedback in the unity-gain configuration has made the circuit get low output resistance.So it will not care the value of the output resistance of the op amp.Here we assume that the Av is very large and the load is small.

Is it right?
And if we need a op amp with low output res in a unity-gain configuration?

Thanks a lot!

Re: Questions about the op amp used in unity-gain configurat

too high output resistance in unity gain op-amp....will make the output voltage higher than input...unity is not achive.

Re: Questions about the op amp used in unity-gain configurat

There are several reasons for using an op-amp in unity gain configuration. If you need to drive a small resistive load, then certainly you need a low output impedence, which is the reason you stated. Related to that, when you drive small resistive loads, you usually also need high current. The input signal source to the op-amp may not be able to provide the current needed to drive the load, so the unity-gain op-amp acts as a buffer.

Well some property's of negative feedback is to make bigger the input resistance and lower the output resistance. And it not depend on the Av.
However for Av =1 you have te biggest BW

Re: Questions about the op amp used in unity-gain configurat

Miguel Gaspar said:
Well some property's of negative feedback is to make bigger the input resistance and lower the output resistance. And it not depend on the Av.
However for Av =1 you have te biggest BW

I dont agree with Miguel Gaspar
In this condition, output impedance is depend on Av (open loop voltage gain of op-amp)
Output impedance = openloop output impedance / AV

Regards,
Davood.

Re: Questions about the op amp used in unity-gain configurat

Negative feedback decreases the output resistance and inreases the input resistance for series-shunt feedback configuration like unity gain amplifier.
A: Opamp open loop gain
B: Feedback gain
1+AB= Loop gain

A is very big and B=1 for unity gain amplifier (voltage follower)

Av=A/(1+AB) ≈1/B --> Av≈1
Zout(with feedback)= Zout / (1+AB) ≈Zout/A
Zin(with feedback)=Zin*(1+AB) ≈Zin*A

Re: Questions about the op amp used in unity-gain configurat

I think the negative feedback in the unity-gain configuration has made the circuit get low output resistance.So it will not care the value of the output resistance of the op amp.Here we assume that the Av is very large and the load is small.

Is it right?

1. you assume the open-loop gain Av is very large.
2. You want to have a large output resistance.

However, if you have a large output resistance, when you put a small resistive loading at the output, the open-loop gain will become small. In that case, because you dont have a large open-loop gain anymore, you don;t have close-to-unity closed-loop gain.

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