How does Cgd effect Cin and Cout??

[zhangjavier]

How to estimate the node capacitance at gate and drain of a common-source amplifier? I am not sure the Cgd effect Cd...For Cin I know Miller approximation can give a good estimation. Cin=Cgs+Cgd(1+Av), but how does Cgd effect Cout at the drain??

 

[Sadegh.j]

To answer this question, two situations can be considered.

Common source with and witohut source degeneration.

As you properly mentioned, the result obtained using the Miller effect is not applicable for the output. Many electronic text books have used KVL and KCL equations to find Cout. I'll check the exact equation tomorrow and give you the exact formula for Cout.

I would like to mention a book called "Microelectoric circuits" written by Sedra or another text book written by B. Razavi as two very good sources in which you can seek your answer.

Cheers

 

[DenisMark]

See Razavi's book pp.167-...
Cout=..+Cgd(1+1/Av), the last term is negligible.

 

[zhangjavier]

To answer this question, two situations can be considered.

Common source with and witohut source degeneration.

As you properly mentioned, the result obtained using the Miller effect is not applicable for the output. Many electronic text books have used KVL and KCL equations to find Cout. I'll check the exact equation tomorrow and give you the exact formula for Cout.

I would like to mention a book called "Microelectoric circuits" written by Sedra or another text book written by B. Razavi as two very good sources in which you can seek your answer.

Cheers

Thank you...I am also wondering that whether Cgs effect the drain capacitance since cgs is connected with cgd...

Added after 4 minutes:

See Razavi's book pp.167-...
Cout=..+Cgd(1+1/Av), the last term is negligible.

I know this equation, but I don't see it in other textbooks. I am a little
doubt about this since the miller effect is usually used to calculate the forward
transmission and I am not sure about its validity to calculate the output capacitance.

 

[DenisMark]

See Razavi's book again.
If we use Miller theorem for simplify calculation of pole position, the results'll not provide resonable estimate. Output pole correspond Cout~Cdb+Cgd(1+1/Av)+Cload. Av varies with frequency and Cout varies too. In many cases Cload>>Cgd,Cdb.
Therefore the existence of zeros isn't considered.
Summary: For first estimation, such calculation is valid, then use laplace transformation and spice simulation.

 

[zhangjavier]

DenisMark:

Thank you for you explaination. In fact what I doubt is that in Gray's book it is said that Miller effect is not useful for calculating high-frequency reverse transmission or output impedance. So I have been a lillter confused about how to calculate Cgd's effect on Cd.

 

[Guest]

If you know Miller theory, the Cgd can be separated in two effective impedance at both nodes;
if you have a DC voltage gain from in to out (-A), for input Miller cap, we have Cgs(1+A) but for output miller cap we have Cgd(1+A^-1), so if your DC gain is high enough you can estimate the later cap to Cgd.
the most important issue using miller is that this gain is only DC gain, and at higher frequencies is not true.
Hope to be useful!

Regards,
SAZ