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How can a CMOS be used as an amplifier?

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Newbie level 6
Jul 12, 2009
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I am new to the Analog field. I was reading the Book by Behzad razavi "Design of Analog CMOS ICs'. I understood how an nMOS and a pMOS can act like an amplifier individually . But then I came to know that the CMOS technology is used for designing RF chips.
Can anybody tell me how a CMOS ( an nMOS connected to pMOS ) can be used for amplification.
I am not sure if my understanding of CMOS is currect. For me a CMOS is a basic inverter circuit which can be used to make higher level digital circuits.

A reply would be of great help.

cmos as amplifier

You are right. The CMOS inverter mostly is used in digital circuits.
However, it has a rather steep transition range between the on and the off state.
Always, such a region can be used for amplification (like in each "normal" amplification stage), when you can fix a bias point in the middle of this range.
And, luckily, this is possible with a CMOS stage. This is because an input voltage of Vdd/2 (at both gate nodes) will produce an output voltage at Vdd/2 (middle of the transition region). Therefore, connect a rather large resistor between output and input - and you have a fixed bias point due to negative feedback. If you then inject a signal via a coupling C you have an inverting amplifier. As an alternative, you can inject the input via another resistor (lower than the feedback resistor) - and you have an amplifier with signal feedback and lower gain. Try it.

bias cmos inverter amplifier

A Cmos inverter like a CD4069 or a 74C04 can be used as an amplifier. They are symmetrical.
They cannot drive a low resistance and they have pretty bad distortion when the output level is high.

A negative feedback resistor from its output to its input self-biases it at close to half the supply voltage. Its gain and frequency response is affected by its supply voltage.

Here is an old graph from RCA:
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