question regarding correct biasing of opamp input pins

[alexs]

op amp biasing

Hi all and good day!

I have question regarding correct biasing of opamp input pins. I am studying desing of fully differential opamp and have misunderstanding of correct biasing procedure. As I understand from books opamp is self powered and biased element so it not need biasing at normal conditions.
How can biasing is applied at the opamp input pins?
I found in literature that I can use source follower at the and of the opamp to perform self biasing and next apply bias to the input using feedback.

I am designing opamp with single power supply so I need input bias voltage at the half of power supply voltage.

Thanks in advance for all reply!!!

 

[betwixt]

Re: op amp biasing

It depends very much on the type of op-amp, some have self biassing built in, some don't.

In general though, if you have DC negative feedback and sufficient current in the feedback path to supply the input stages, the output will adopt the same voltage as the non-inverting input. I am ignoring the effects of fixed drops or offsets in the feedback loop which you may have to take into account.

So with negative feedback, if you use a potential divider to set the voltage of the non-inverting (+) pin to half supply voltage, the output will also be at that voltage. Ignoring the tiny effects of input bias currents, that means you use two resistors with the same value in the potential divider and connect it between supply and ground.

Brian.

 

[LvW]

Re: op amp biasing

Only a small, however, important remark from my side:

In general though, if you have DC negative feedback and sufficient current in the feedback path to supply the input stages, the output will adopt the same voltage as the non-inverting input.

This is only true for 100% DC feedback, which means: DC noninverting gain equal to unity !

 

[betwixt]

Re: op amp biasing

I'm going to wriggle out of that one by adding "and no DC current is drawn from or added to the feedback path" !

Of course it is perfectly possible to have gain above unity as long as the static DC condition is met.

Brian.

 

[alexs]

Re: op amp biasing

So if I understand correctly I can also use opamp without selfbiasing circuit.
For example I have cascade of two opamps (without selfbiasing), can I use one source of biasing (current mirror or voltage divider) at the input differential pair of first opamp to bias all opamp cascade?

 

[LvW]

Re: op amp biasing

.............
Of course it is perfectly possible to have gain above unity as long as the static DC condition is met.
Brian.

Does a dc gain above unity make sense? I am afraid we are going to confuse ALEXS.

 

[alexs]

Re: op amp biasing

LvW,

For me DC gain is not important.

 

[betwixt]

Re: op amp biasing

I'll shut up - You're not nearly as confused as you think I am.

Depending on the schematic, yes you can use the same bias network for several amplifier stages - as long as it is only bias and not signal you share.

Brian.

 

[alexs]

Re: op amp biasing

So I can use exteranl biasing at input and DC coupled next stages to share biasing, is this right?

 

[FvM]

op amp biasing

Unfortunately, you never clarified the intended circuit. You can design a DC coupled cascaded OP circuit.
Usually it implies a DC feedback respectively a high-pass rolloff for the AC gain.

But: A picture is worth a thousand words

 

[alexs]

Re: op amp biasing

I am studying design of fully differential OTA for active filter application. OTA is simple two stage (see picture) and CMFB circuit.
What is possible solutions for biasing such type of OTA?

 

[Tahmid]

op amp biasing

Generally, the non-inverting input should be held at a voltage equal to the centre of Vdd and Vss. The same reference can be used for all opamps in one circuit.