unity fb in differential opamp

[deepa]

what is the configuration for a unity buffer in a diff amp..is it just connecting the +ve output to the -ve input and -ve output to the +ve input..but then there seems to be a direct conenction from input to the output????

 

[suria3]

Connect the output terminal to the negative input terminal to design a voltage buffer. The feedback should be negative.

 

[satyasiva]

yes ur giving total output as feedback.

 

[dodoro]

negative output connected to positive input,
and positive output connected to negative input.

 

[deepa]

if it is connected in this manner then there is no need of the amplifer at all,there is a direct path from the input to the output.is there some other way to use the diff amp in unity gain fashion ,

 

[melc]

Why don't you read datasheet of any fully differential OA ?
THS4121 or AD8132...

 

[deepa]

wat is the info i should look in a data sheet,,i know how on open loop diff works or how it works in switched capacitor circuits,i don know how its can be put in feedback to be used as a buffer of unity gain

 

[satyasiva]

if it is connected in this manner then there is no need of the amplifer at all,there is a direct path from the input to the output.is there some other way to use the diff amp in unity gain fashion ,

it is not like that. just see..u r not shorting +ve input to positive o/p. ur shorting -ve i/p to +ve o/p. such that -ve feedback occurs.

 

[zizu]

it is not like that. just see..u r not shorting +ve input to positive o/p. ur shorting -ve i/p to +ve o/p. such that -ve feedback occurs.

It is indeed a unigain amplifier with negative feedback, but it can't be a buffer.

 

[panditabupesh]

No you can not use it as a unity buffer this way- an option would be to be use same value for the feedback and input resistances or capacitors. DDAs are an option for similar application.

Bupesh

 

[deepa]

what s an DDA,and can you show the circuit ?

 

[panditabupesh]

DDA - differential difference amplifier. Search for DDA will find plenty of references.

I still recommend using a fully-differential amplifier with identical feedback and sampling resistances/capacitors - does not stretch the common-mode of your input pair.

Bupesh