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Output of N MOS common source amplifier does not seem to be correct

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shomikc

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Hi. I am just trying to check my design of N MOS CS Amplifier. The voltage gain is 0.884 as per the book but I am not getting it. Can you please help.
 

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Did you think about C1 high-pass cut-off frequency? Change C1 to 1 uF for a first try.
 
My problem is there seems to be a DC offset of 1.15 volt. Why is that happening. And peek to peak it is 0.5 mV when it should be theoretical 0.884 mV because the voltage gain is 0.884 and the input I am using is 1mV.
The answer in Neamen Ex. 4.7 is 0.884 and I calculated it to be 0.882. But I don't think it should be 0.5 mV
 

1) Your Mosfet has no part number so we cannot see its spec's.
2) The current is only 0.35mA, it is almost cutoff. Maybe the Mosfet works better at a much higher current.
 

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    shomikc

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It's a Common Drain Amplifier, not Common Source..
! Bulk is connected to Source !
 

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    shomikc

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shomikc said:
My problem is there seems to be a DC offset of 1.15 volt. Why is that happening.
Click to expand...
The "offset" is there because a class A amplifier circuit needs a bias current. Respectively the circuit has an output DC bias of Id*Rs.
shomikc said:
And peak to peak it is 0.5 mV when it should be theoretical 0.884 mV because the voltage gain is 0.884 and the input I am using is 1mV.
The answer in Neamen Ex. 4.7 is 0.884 and I calculated it to be 0.882. But I don't think it should be 0.5 mV
Click to expand...
Did you notice that the example refers to specific transistor parameters Vtn, Kn and \( \lambda \). MBreakN uses quite different default MOSFET parameters. If you set the NMOS model parameters respectively, you can expect similar behavior. Read PSpice manual about MOSFET model parameters.

1593332986215.png
 

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    shomikc

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