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In the above circuit I was asked to find the input capacitance at the gate of Q1, Cg1, using the Miller approximation, to determine the frequency of the pole formed at the amplifier input in terms of Rsig, use the Miller approximation to find the input capacitance of Q2, and hence determine the total capacitance Cg2 at the drain of Q1, finally to use Cg2 to obtain the frequency of the pole formed at the interface between the two stages. Below's my attempt. Unfortunately I got stuck trying to find the frequency of the pole formed at the interface between the two stages. I'd appreciate some help with that.

\$M_{in}=[1/(j\omega C_{gd})]/(1-K))\$, where \$K=-g_m r_o\$ for CS.

Hence, the frequency of the pole formed at the amplifier input in terms of Rsig:

\$f_p=1/(2\pi R_{sig}(M_{in}+C_{gs}))\$

I think the input capacitance of Q2 would be:

\$C_{g2}=C_{gs} + C_{db} + K/[(j \omega C_{gd})(K-1)]\$

The big question -- in order to obtain the frequency of the pole formed at the interface between the two stages, do I now simply multiply \$C_{g2}\$ by \$r_o\$ to determine the time constant?

peripatein
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  • Cin1 = Cgs1 + Cgd1(1 + |K1|) and Cin2 = Cgs2+ Cgd2(1 + |K2|) + Cdb1 – G36 Jun 09 '17 at 14:36
  • @G36 But would the pole formed at the interface between the two stages be at f=1/(2picin2*ro)? – peripatein Jun 09 '17 at 15:25
  • Yep, the pole will be f=1/(2picin2*ro) – G36 Jun 09 '17 at 15:39
  • @G36 two more minor queries, if I may -- is C_out=Cgd2*(1+|K|)/|K|+Cdb2? And, how may I know whether the amplifier has a dominant pole? Must I find all the poles and then see whether one is significantly higher than the rest? – peripatein Jun 09 '17 at 17:02
  • Your Cout looks good. And yes again for your second question. Fh will be the lowest pole. – G36 Jun 09 '17 at 17:14
  • @G36 do I use open circuit time constants for finding all the poles or rather Miller? – peripatein Jun 09 '17 at 17:16
  • It will depend on what the question ask you to do. – G36 Jun 09 '17 at 18:06
  • @G36 determine whether there is a dominant pole and if there is one find at what frequency – peripatein Jun 09 '17 at 18:08
  • @G36 if I found using Miller's theorem the following three poles: fp1=30.32MHz at the amplifier input; f = 3MHz at the interface between the two stages; fout=158.36MHz, can I infer that the amplifier indeed has a dominant pole at f=3MHz or is further evaluation necessary? – peripatein Jun 09 '17 at 23:51
  • If your calculations are correct then the dominant pole is 3 MHz – G36 Jun 10 '17 at 14:37

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