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How does an amplifier work?

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karthik_reddy00

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basic questions

1.how does an amplifier amplify a signal ?

2. why do we need to apply bias voltages ?
 

basic questions

2. bias voltage is the energy needed for the circuit to work

1.you control a larger signal with a small signal, that what is usually called amplification
 

Re: basic questions

Bias voltages are used to bias a transistor into it's correct point of operation. This point of operation determines your small signal parameters, for example gm of a mosfet.
Small signal parameters are used to linearize the operation of your device around it's point of operation.

Now, how does an amplifier amplifies a signal?
Assume you are working in a CMOS technology. I think you can try to understand it by considering a common source amplifier. Load this amplifier with a purely resistive load, Rout. Assume the mosfet as an ideal current source, delivering the current gm*VGS, where VGS is the input voltage. This current will flow through the output resistor, so the output voltage Vout to be: gm*VGS*Rout. So the amplification will be: Vout/VGS = gm*Rout. You can state that the amplification is based on pulling a current through a resistor in such a way that the output voltage swing becomes bigger than the input voltage swing. In this case you need a gm*Rout > 1 to become amplification.

This also explains why good biasing of the transistor will be needed: in most cases you'll need a big gm to become a high amplification.

You can refer to various analog design books for this basic amplifier and small signal parameters. I hope this explains it a bit.
 

Re: basic questions

karthik_reddy00 said:
1.how does an amplifier amplify a signal ?

2. why do we need to apply bias voltages ?
Amplifier amplifies signal at the cost of biasing voltage..therefore we need biasing voltage..we also need biasing voltage because without this the SEMI-CONDUCTOR cannot operate..

Regards
 

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