An interesting 2 stage op-amp question!

[spbhu]

A very interesting problem, for compensation of a 2-stage opamp, we normlly use miller capacitor to do pole splitting, and a nulling resistor is ususlly used to cancel the RHP zero, and sometimes even to cancel the non-dominant pole.
How large the resistor should be ? The book by Johns Martin mentioned that, normally it can be selected such that the LHP zero [1/(Rnull -1/gm)Cm] locates at about 1.2 times the unity gain frequency of the op-amp before nulling resistor is used? What's the reason behind this, why donot we move it to even lower frequency? I believe it is not just because of the area of the resistor.
Anybody can explain this? Thanks very much....

 

[safwatonline]

pole zero duplet , slower response

 

[mists]

if this zero move inside unity gain frequency, you will have pole-zero doublet which will degrade you opamp's setting time.

 

[spbhu]

I know the doublet phenomenon, but why it is 1.2times the orginal unity-gain frequency? Is it because of detailed derivation or optimum settling time consideration? Because if you put the zero at about the exact orginal unity-gain frequency, the location will be even further away from the second pole, the doublet effect will be minimized, isn't it?

 

[wonbef]

the RHP zero can degrade you opamp's phase margin.

 

[leelamadhav]

U can refer to an example in allen hollberg.. the RHP zero is 1.2 times ther UBW because we need a phase margin of 60deg...