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Design >> Analog Design >> A simple question.
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Message started by James on Mar 25^th, 2005, 7:28am

Title: A simple question.
Post by James on Mar 25^th, 2005, 7:28am

For a common Source stage Amplifer with Resistive Load Rd.

It is said in Razavi's book that:

For large values of Rd, the effet of channel length modulation in the transistor becomes significant.

Why does the effet of channel length modulation become large for Large Rd?

many thanks

Title: Re: A simple question.
Post by Ken Kundert on Mar 25^th, 2005, 9:27am

The model of the output of a common source amplifier is a controlled current source in parallel with a resistor, r_o. The channel length modulation determines the size of the resistor. The more channel length modulation, the smaller the resistor.

If the load resistor R_d is small, then it overwhelms r_o. If the load resistor is large, then R_d does not overwhelm r_o, meaning that r_o affects the overall gain, distortion, etc.

-Ken

Title: Re: A simple question.
Post by James on Mar 25^th, 2005, 4:51pm

Thanks, your explanation is very clear. I fully understand it now