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Input referred noise of a MOSFET (Read 5388 times)
Hrishikesh
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Input referred noise of a MOSFET
Oct 16th, 2013, 1:55am
 
Hi..

Through an ac simulation, I see that the input referred noise of a MOSFET reduces with increasing frequency, stays constant for some range, and again rolls off at about 1-10 GHz. I understand that low frequency noise is dominated by flicker noise and the frequency independent part is thermal noise. Can anyone tell me why it should further decrease at high frequencies?
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raja.cedt
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Re: Input referred noise of a MOSFET
Reply #1 - Oct 16th, 2013, 2:51am
 
How did you simulated, please show me the spice setup. Please try to simulate y21(short circuit current gain).

Thanks,
Raj.
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harpoon
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Re: Input referred noise of a MOSFET
Reply #2 - Oct 17th, 2013, 2:33am
 
sounds like there is some low pass filter in the simulation ...

as raj mentioned, post the schematic/spice netlist and we may be able to help ...
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Lex
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Re: Input referred noise of a MOSFET
Reply #3 - Oct 17th, 2013, 4:08am
 
Could be in model parasitic gate resistance/capacitance.
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Hrishikesh
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Re: Input referred noise of a MOSFET
Reply #4 - Oct 17th, 2013, 10:03pm
 
Thanks for the responses. The netlist is very simple. I have an 50x1 NMOS with the drain connected to 5V supply through a 10K noiseless resistor. The source is grounded and the gate is biased at 0.8V. I am doing an AC simulation by sweeping the frequency. To get the input referred noise in V/sqrt(Hz), I am dividing the total noise at the output by the amplifier gain.

It appears that the effect is due to the Cgd of the NMOS. Because, the roll off frequency reduces as I connect a capacitor between the gate and drain. I can also see a zero present at that frequency. But I always thought that the input referred thermal noise is 4kT*2/(3gm) V2/Hz. It would be great if any of you can throw some light on the effect of Cgd.
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