unity gain buffer design problem

[greenjuice]

I want to make unity gain buffer.

So I have some question.

I think to make in(+) and in(-) equal(virtual short)

amplifier has negative feedback and large open loop gain.

Is it right? Or only negative feedback make amplifier virtual short?

and I made unity gain buffer using one stage differential amp and I did it

But after that, I made unity gain buffer using two stage differential amp.

however it didn't work. only one side transistor is saturated. and another side transistor is in

triode region. I don't know why this happened.

please advise consideration to design unity gain buffer amplifier to me.

 

[KlausST]

Hi,

it seems you are talking about ASIC design. If so, then please post in the ASIC design section.

***

Generally a unity gain buffer is an amplifier with gain of +1.

With an OPAMP you can IN+ use as input
and connect IN- to the output. Maybe with a resistor.

Then you have high impedance input and low impedance output.

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There are unity gain buffer ICs available.
***
A schemaitc could help to better understand your problem.
Additional informations like technology, bandwith, supply voltage and so on will lead to more appropriate answers.
***

Klaus

 

[greenjuice]

at 1.3v supply like this, i design 40u current source in differential amp. so current of each input TR should flow 20u.

and at second stage i design 3 times ratio. so current of second stage should flow 60u.

But current source has only 33u current and in(-) is almost off.