Commong Gate Ampilifer

[MahmoudHassan]

What are the disadvantage of common gate amplifier with reference to gain and Noise Figure ?

 

[goldsmith]

Hi Muhammad
Advantages , instead of other configurations ? for example in compare with CS or CD ? CG , amplifier has lower input impedance , and better frequency response in compare with CS , and it has voltage gain and it's current gain is around unity , but in practice it will be lower than unity ( e.g 0.9 or less ) . can these parameters help you ? if you need more information you should describe your problem , ( a bit more ! )
Best Wishes
Goldsmith

 

[MahmoudHassan]

Thanks a lot for your reply

This is this complete problem i need to solve number (a) only
The other one i solved it


i need it's disadvantages with reference to gain and noise figure

 

[goldsmith]

Hi again , if the meaning by noise , is temperature noise , if you consider low input impedances for CG amplifier ( it's input impedance is low ) , thus : Vn =sqrt( R*BW*K*T ) it demonstrates that noise is as low as possible , because of low input impedance , thus the out put noise is low , too . K is Boltzman constant , it which is around 1.38*10^-23 and T is temperature in kelvin and BW is band width .
Best Wishes
Goldsmith

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BTW :are you sure that your circuit is correct ? i don't think so ! because in CG , input is source and out put is from drain !

 

[Baas Rietrot]

With reference to gain: The CG-amplifier have a very small input impedance (Rin=1/gm), say Rin = 50Ω and your source impedance Rs = 50Ω, this will effectively give you half the gain you designed for! It is important to consider the input loading factor with a CG-amplifier. If Rs increase your gain will decrease.

Note: The MOSFET is the wrong way around...

 

[godfreyl]

The JFET is drawn the wrong way around but it doesn't matter - the circuit will still work. JFETs are normally symetrical so it doesn't matter which pin you use as source and which as drain.

As for question A, I don't see any noise disadvantage, assuming you want input impedance matching.

You could probably get higher gain with a common source amplifier by running the FET at a higher current, but then you'd need a 50 Ohm input resistor for impedance matching so the noise figure would be the same (I think).

 

[Baas Rietrot]

JFET arrow usually on the gate terminal, that is why MOSFET was assumed.