Second pole of two stage opamp

[bsrivastava]

In conventional two stage opamp the miller cap is added between first dtage and second stage. This is frequency compensation and helps in making one pole as dominant pole and pushing the other pole towards infinity.
For the second pole ater compensation the RC product is approximately going to be same [ R is same and C is slightly increased] so, how can i say that reciprocal of RC will go towards infinity.
Though text book says that R will reduce and so the RC product, which in turn push the pole towards infinity but I am missing the explaination "R will reduce" , why??

thanks in advance,[/img]

 

[heartwide]

Hi bsrivastava:
"R will reduce",it is true.with calculating the equation of small signal modeling,you can find that the nondominant pole would become Gm/C, where Gm is the transconductance of the second stage of amp, not R you refer to above. obviously, Gm/C has been pushed towards infinity than RC.

 

[nandu_r]

hi
the output impedance of the second amplifier(R) will reduce because of the negetive feed back (through the miller capacitor)
thats why the output pole moves away from the origine

hope i answered your question

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[rajkumaru]

Hi,
For better explaination, you can refer to Allen Holberg book(Six-Chapter).