Ask a question about Operation Amplifier

[wjxcom]

Hi all: I am a fresh men to design analog circuit. I want to ask a question about OP: what's the relation between the setup time and the unit gain bandwidth?

我想知道，建立时间和单位增益带宽的关系是什么？

 

[richloo]

The setup time u r refering to transient reponse time?
It has dependent on the slew rate. unity gain bandwidth can be related by using this 2 relationship.

SR=Ibias/Cc
Cc=gm/UBW

SR=slewrate
Ibias=bias current of the OP differential pair input
Cc=compensation capacitor
gm=transconductance of input transistor
UBW=unity gain bandwidth

 

[alok_ky]

Lower the set up time, higher the bandwidth.

 

[wjxcom]

Hi, richloo: I think I know why SR=Ibias/Cc.

But can you tell me detail why Cc=gm/UBW?

 

[Alles Gute]

Hi, richloo: I think I know why SR=Ibias/Cc.

But can you tell me detail why Cc=gm/UBW?

Suppose a two stage opamp with miller Cc,
The dominant pole will be p=Rout1*(gm2*Rout2*Cc), BW=1/p
And the gain is Gain=gm1*Rout1*gm2*Rout2
Hence the UGB=BW*Gain=gm1/Cc, i.e., Cc=gm1/UGB