# Knowing the dominant poles of a small circuit

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#### reilly

##### Newbie level 4
Hi, do anyone know how to read the dominant poles by eye sight? I heard some experience designers know how to view the poles just at a glance of the circuit. Any place or book teach this method or at least some guide line on how to look at the dominant poles. This can give a designer a quick overview of the circuit before running a simulation. Thanks in advance.

#### easytarget

##### Member level 1
I know for amps, you have to look for Miller capacitance (C) accross which there is the largest gain, then very often the dominant pole is 1/(A*C*R), A and R are the gain and resistance accross C, assuming A is large. For a common source, C is Cgd, for a two-stage op-amp, C is between the outputs of the two stages. I hope this helps.

### reilly

Points: 2

#### nxing

for each capacitance, find the resistance associate with it. The largest value of C*R is the dominant pole.

#### vale

in the equivalent circuit, let Vin=0, find the frequency points at which Vout !=0. the dominant pole is the minimum one.

### reilly

Points: 2

#### surianova

reilly said:
Hi, do anyone know how to read the dominant poles by eye sight? I heard some experience designers know how to view the poles just at a glance of the circuit. Any place or book teach this method or at least some guide line on how to look at the dominant poles. This can give a designer a quick overview of the circuit before running a simulation. Thanks in advance.

The best way to find dominat pole in a circuit is to find the most high impedance node in the circuit, because pole frequency is f=1/RC. As R is higher, the pole
frequency is lower.

#### sunking

find the most high impedance, and find the most the big cap about the net, f=1/RC, is the dominant poles

#### avinash

##### Full Member level 3
the node having the largest product of resistance and capacitance seen from the node to ground will be the dominant pole.

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