differential amplifier

[sourav]

hi all,
If drain resistance in a differential amplifier is replaced by a current mirror, whether its voltage gain will increase or decrease or remain same? why?

sourav

 

[siddharthshukla]

i think u mean to say the current source ISS is replaced by an ideal one. in this case,the gain increase slightly and the o/p CM level becomes completely independent of the i/p CM level

 

[hmsheng]

Gain of differential amplifier with resistor load: Gm*Rload
Gain of differential amplifier with current mirror load: Gm*(rop||ron), where rop is the impedance looking into PMOS from the output port and ron is the impedance looking into NMOS from the output port.
Generally, rop||ron >> Rload, so the voltage gain of the current mirror load differential amplifer is much large than that of the resistor load one.

 

[abhisheksbox]

by replacing drain resistance with current mirror it converts balanced configuration to unbalanced output mostly and the gain of single ended output amplifier is hingher then conventional balanced output opretional amplifier.

 

[siddharthshukla]

sry i misunderstood the question,as the other ppl have pointed out ,the gain increases when we use a current source load but a problem tat arises is that the o/p CM level is not well defined .you need to use a CMFB circuit to define the O/P CM level

 

[jecyhale]

hi all,
If drain resistance in a differential amplifier is replaced by a current mirror, whether its voltage gain will increase or decrease or remain same? why?

sourav

It is single end output as you do.

I thinks the voltage gain depands on your design.

 

[sourav]

hi siddarthshukla,
I donot know what is cmfb ckt? cm is common mode voltage or anything else.

sourav

 

[amitjagtap]

CMFB is common mode feedback ckt. it is when u need differentail output.

 

[sohpie2007]

There is a free differential amplifier design example for you to get:

**broken link removed**

 

[Sadegh.j]

well it depends on the resistance of the current mirror. It normally increases since a resistor whose resistance is ro consumes too much power. Thus not used.