query regarding ICMR

[rishabh_31ec]

Hi I have a confusion regarding ICMR in opamp design (circuit is attached) :

(1) Is it required to provide the DC voltage bias at the gates of PMOS drivers of differential amplifier ( encircled by red color) along with PMOS tail current source.

(2) Is the range of this dc voltage bias at the gate of PMOS driver of differential amplifier for which all transistors remain in saturation is called ICMR.

PLEASE CLEAR MY DOUBT.........

 

[KlausST]

Hi,

did you mean ICMR = Input Common Mode Range?
If so: In an closed loop opamp circuit both In- an In+ have (about) the same voltage. The allowed voltage range at these inputs is ICMR.
Design your application that both inputs are in the allowed voltage range.

If you run the opamp with one or both inputs outside tihis range the circuit may not work like specified. Saturation may occur, phase reversal may occur, latch up with its latch_up_time may occur.
Increased supply current, and and and.

Make sure to avoid this situation.

 

[rishabh_31ec]

Hi, Klausst,
thanx for your valuable response. I mean to ask:

(1) I have designed opamp with driver PMOS of first stage differential amplifier are biased by a gate voltage of 0.5V. Now when this bias dc gate volatge is varying between -0.1 to 0.9V all the transistors of 4-stage opamp remain in saturation.
so is it true to say the INPUT COMMON MODE RANGE = 1V (0.9+0.1). The gate biasing of both PMOS is needed ???

If so then is it true to say that in order to operate the opamp properly we have to provide the AC input ~1uV along with a dc voltage between -0.1 to 0.9V at the gate of driver PMOS, or

We do not need to bias the driver PMOS of differential amplifier with any dc voltage. In order to find the ICMR I have to tied both the input togethet and apply a dc voltage and vary this voltage till all MOS remain saturated.

 

[kenambo]

Hi, you are absolutely right..

Common mode range id the range of input DC voltage for that all the mosfets are in saturation (except the switches).

And your test setup is correct.. and no need to give a DC voltage for the operation..

for example, if you are going to use it as a comparator means... you give a reference voltage in one input and apply a signal to be compared..

if it is an differential amplifier .. you give two input signals in the input pins..

thanks

 

[rishabh_31ec]

Than Kenambo.
So I have to remove dc bias voltage at the gates of PMOS drivers (input V+, V-) of first stage differential amplifier. we have to apply only signals at gate ( INPUT ).

 

[kenambo]

yeah.. give two signals which are 180 degree out of phase and measure the Ac gain..

hope you get correct result..