PMOS differential amplifier

Discussion in 'General Electronics Chat' started by anhnha, Apr 2, 2014.

1. anhnha Thread Starter Active Member

Apr 19, 2012
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This is a PMOS differential amplifier with current mirror load. I'd like to ask some questions that is confusing.

M3 and M4 have two functions in this circuit.

1. They are loads for circuit.
2. They are used to biased the circuit. M3 and M4 makes sure that DC bias current in two branches are equal (I1/2)
Is that right?

2. crutschow Expert

Mar 14, 2008
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1. Yes
2. No. The current through the two branches is determined by the relative voltage between Vref and M2's gate voltage. M3 and M4 have little effect on that.

The function of M3 and M4 is to convert the differential signal from M1 and M2 to a single ended output going to M5.

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3. anhnha Thread Starter Active Member

Apr 19, 2012
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Please, what type of signal are you talking, voltage or current?

4. Jony130 AAC Fanatic!

Feb 17, 2009
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Of curse we are talking about current. But voltage gain also increase thanks to this current mirror.
The voltage gain without current mirrors is equal to (gm*Rd)/2 but if you use current mirror as active load
the voltage gain becomes gm*RL.
Because without current mirror the IL current can change form 0A to I1.
But with the current mirror IL can now swing from -IL to IL.

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5. anhnha Thread Starter Active Member

Apr 19, 2012
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Thanks.

I read that the current mirror converts differential double ended current to a single ended output current.
But as in the picture the output current is always zero because the gate is open.
Then what is the purpose for the conversion?

It seems that you are right. The current mirror will increase voltage gain.
I am still confused. The output current of the differential amplifier is fed to the gate of M5. But because Ig of M5 is always zero, how can I change from -I1 to I1?

6. crutschow Expert

Mar 14, 2008
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I just realized that M3 and M4 can't act as current mirrors in your circuit since they are the wrong polarity. M3 and M4 must be NMOS transistors for them to operate as current mirrors. (A differential circuit requires opposite polarity transistors for the differential input and the current mirror transistors for proper operation).