Hello,
I am new to the amplifier designer. In my demodulator circuit, I have designed CS amplifier to shift the DC level of the modulated signal [attached]. Although I got my desired result, but when I decrease the gain of the amplifier by changing the load resistor then I got an irregular output voltage waveform. I would like to know why this is so?

Specifications: Vdd=5V , Vth= 805mV, Vgs=1.552, Id=548uA , gm=1.06mS, w/L=20
Rd=5K ohm (Av= 5.3 & I get my desired Vout)[attached] .
Rd=2K ohm (Av= 2.156 & i get irregular Vout )[attached]

 

how are you generating the input waveform and coupling it to the gate of the MOSFET?

Do you have a specific MOSFET in mind?

 

how are you generating the input waveform and coupling it to the gate of the MOSFET?

Do you have a specific MOSFET in mind?

Thanks for your response. I am modulating a message signal using a multivibrator and then capacitively coupled with demodulator. The modulated signal is fed to the CS amplifier on demodulator for lifting it DC level up. I am using CADENCE software with nmosm MOSFET's. I have attached block diagram for your reference.

 

Hard to say if this is a simulation issue, or just the way your 'generic' NMOS transistor behaves. My gut feel is this isn't working as an amplifier but a switch, and the issue at Rd = 2k is that you don't have enough gate voltage to get it out of the linear region at the higher drain current so the MOSFET isn't turning on fully.