Mosfet current mirror as Active load instead of resistor

mosfet current mirror

when I use a Common source Amp, I usually use Current Mirror as active load instead of passive resistor.



it is desired that output impedance of Amp is low,

But Current Mirror equation tell me that
"if there is high output impedance, Currnet mirror has good output current

two story tell a truth. but if current mirror have good output current, that is, currnet mirror have high output impedance, what happen to amplifier? is it worse?

or I don't need to worry about that? tell me which is right.[/img]

current mirror active load

if you want a low output impedance, you can't use common source config. you have to go for source follower instead.

current mirror using mosfet

The gain of the amp. is gm*Rout. We need to maximise gain so we increase Rout as increase in gm would need increase in current. IF you have an operational apmlifier in feedback you desire that the forward path gain is large so that the net gain is a function of feedback passive element only, so one desires large gain.

You desire low impedance if you have to drive large capacutive loads and you need larger amplifier bandwidth or say faster responce. So that would be application dependent.

If the active current source load has higher output impedance, that means goodnews. Higher gain, better VDD suppluy rejection so output would be effected less with supply variation

current mirror mosfet

books for active load design
thomas lee
razavi
gray 4th

current mirror as active load

It is not desirable to have low output impedance. because the gain of AMp is reduced. the main reason that current mirrors are used is that we had a single ended amplifier which has high Gm.

mosfet active load

From what I understood from your question:::

An amplifier needs to have a low output impedance. Not any amplifier.,a general Opamp(voltage in and voltage out) should have a low output impedance. This is because an opamp supplies voltage. A voltage source should have a low output impedance.

A current mirror, on the other hand supplies current, so you need high output impedance, so all the current can be supplied to the load.

Now how do we make an opamp? You design a high gain amplifier and feed the output back to the input. So if you design a high gain OTA type amplifier with MOS current mirror as load.,you need high output impedance at the output of the current mirror. The reason you need this is so you can get a very high open loop gain. For a differential CMOS OTA, the impedance seen at the output is VERY high because it is in OPEN LOOP. Once you take that OTA output and connect it back to input, the output impedance is low.,(because of feedback, the impedance is divided by the gain.,gm*gds)

Hope that was clear

active load mosfet

U must not worry about high active load current. Why do you use active load ? Because u can not implement a big resistor in value with a small passive resistor (poly silicon) in dimention in dimension. This is a trade of. U earn in resistor value with small dimension. But u loss in power consumption.

Thanks