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# Why is the PMOS used for a constant current source, not NMOS?

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#### kissmoh

##### Junior Member level 1
Why is the PMOS used for a output resistance of the current source, not NMOS?

Hi, I'm studying microelectronic circuits theory using sedra smith book.

The question is, this book is saying that in IC design, for load resistance, tr is used as a constant current source for some reasons and PMOS is usually used for it (im studying cascode amplifier circuit now and, if this will be help to understand what i meaning, this is written in ch. 6.2.1 and 6.2.3).
What is the reason to use PMOS(just as 'single' tr connected to drain node of the other NMOS which is for amplification) for constant current source as a load resistance in IC design, not NMOS?, is there any particular reason? (sorry for my poor english skill )

There were some comments saying that PMOS is usually used for constant current here and there, but i couldnt find the reason.

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Look for the impedance seen from drain and source of transistor. From source we see low impedance equal to 1/gm, while from drain it is "high" rds. So if your amplification stage is made on nmos, to obtain high gain this nmos must see high drain impedance of complementary fet.

cira529

### cira529

Points: 2
The transistors in such a circuit have to be complementary for proper operation.
So the input transistors are typically NMOS and the load is typically PMOS.
But it could be done with the input transistors PMOS and the load NMOS.

Basically the drains of the two transistors have to be facing each other for the circuit to work since you need both to be a high impedance to get high gain. If you look at the current direction and the gate voltages required you will see why this requires complementary transistors.

The transistors in such a circuit have to be complementary for proper operation.
So the input transistors are typically NMOS and the load is typically PMOS.
But it could be done with the input transistors PMOS and the load NMOS.

Basically the drains of the two transistors have to be facing each other for the circuit to work since you need both to be a high impedance to get high gain. If you look at the current direction and the gate voltages required you will see why this requires complementary transistors.

ty,
But I cannot prove the 'If you look at the current direction and the gate voltages required ' for my self.
Can i learn about it more detail? or use some material to learn about it?

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Perhaps this will help.

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