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Problem in Hspice calculating Gain

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pnsakanjankumar

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I need to calculate gain of the Common source amplifier and that value should be written in output file(.out) but i don't how to calculate it
input is ac+dc and output is also ac+dc i want to calculate small signal gain how to separate dc to calculate it
please help me
 

hi
one solution can be adding a large resistor load and a capacitor in series to block dc component and measuring the output at resistor
 

kk one thing i didn't understand while writing the code is
Vin in gnd 1 ac 1 sin(1.5 100mv 100khz)
|| ||
-------------- i didn't understand what is "ac 1" and what is the use of writing 1.5 ?
Can u give me typical values of gain we get for common source amplifier ?
 

for gain measurement you should do transient analysis with sin voltage sources not ac
such as : Vin in_node gnd 0 100m 100k
the 1.5 in yours defines the voltage offset of the voltage source(its a dc value)
gain value depends on your MOS and load type; typically it is in the order of ten
 

Thanks for your reply

I have another doubt
i tried designing a cascode amplifier using Three Mosfts
see this pic Cascode.JPG
i used W/L for Nmos 10u/.2u and PMOS 20u/.2u when i simulated it then the Vds of PMOS is -11.76134m therefore i am not able to get amplification i tried using different values of W/L but still Vds is approx around 60m.
And one more point the region of operation of pmos is always Linear no matter what i change its still the same
Please help Waiting for Your reply
Thanks
 

so what about vb1 and vb2 ?
you should choose them correctly to have your mosfets work in saturation region , I think this is the problem
 

so how did you choose them?
check the conditions for MOSFET saturation, choose a typical Vov(overdrive) for each MOSFET, to have your MOSFET work in saturation and have a good output swing ...
refer to a CMOS design textbook such as Prof. Behzad Razavi's or Martin's
good luck
 

I think firstly you should check the Vt of Nmos, then determine the Vin to ensure that Q1 does not operate in the cutoff region.
 

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