# Increasing gain and bandwidth of 2 stage amp with CMFB circuit

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#### surno

##### Junior Member level 3 Hi~ all~~

I desinged fully differential op amp. The structure of op amp is two stage amp with CMFB circuit.

The specification is like that : Gain is more than 60dB, unit gain bandwidth is more than 100MHz and phase margin is more than 85degree.

The simulation result is like that : loop gain is 57dB(@40MHz) and phase margin is 64degree(@40MHz). 40MHz is unit gain bandwidth.

Now I have question. What is relation loop gain and open loop gain of op amp. And how to increas gain and bandwidth if total current is fixed 100uA.

#### yxo

##### Full Member level 4 question of CMFB

>>What is relation loop gain and open loop gain of op amp.
gain of the feedback amplifier can be obtain as
Af = A / (1 + A*betta). where A*betta - loop gain. betta - feedback factor( example for inverting amp R2/R1) .A -open loop gain

>>And how to increas gain and bandwidth if total current is fixed 100uA.
nowise! Or change schematic topology

#### paley

##### Member level 4 question of CMFB

I think the spec is difficult a little when total current is 100uA

#### ljy4468

##### Full Member level 4 question of CMFB

Hi surno~

You said "phase margin is 64degree(@40MHz). 40MHz is unit gain bandwidth. "
But I think , when you using a closed-loop configuration, It is probably loop gain=0dB place.
that is, amp' open loop UGB is not same as closed loop gain =0dB place.
Loop gain is open loop gain Ao - Closed loop gain 1/B, so, loop gain is AoB in dB scale.
and so, we should see phase margin at AoB=0dB place

### surno

Points: 2

#### surno

##### Junior Member level 3 question of CMFB

dear ljy4468

I measured phase margin at A0B=dB place.

I have another question. If loop gain is much large, closed loop gain is changed?

#### betty1007

##### Member level 1 Re: question of CMFB

surno said:
Hi~ all~~

I desinged fully differential op amp. The structure of op amp is two stage amp with CMFB circuit.

The specification is like that : Gain is more than 60dB, unit gain bandwidth is more than 100MHz and phase margin is more than 85degree.

The simulation result is like that : loop gain is 57dB(@40MHz) and phase margin is 64degree(@40MHz). 40MHz is unit gain bandwidth.

Now I have question. What is relation loop gain and open loop gain of op amp. And how to increas gain and bandwidth if total current is fixed 100uA.

it is not easy to achieve this ,using two stage structure

#### surianova Re: question of CMFB

betty1007 said:
surno said:
Hi~ all~~

I desinged fully differential op amp. The structure of op amp is two stage amp with CMFB circuit.

The specification is like that : Gain is more than 60dB, unit gain bandwidth is more than 100MHz and phase margin is more than 85degree.

The simulation result is like that : loop gain is 57dB(@40MHz) and phase margin is 64degree(@40MHz). 40MHz is unit gain bandwidth.

Now I have question. What is relation loop gain and open loop gain of op amp. And how to increas gain and bandwidth if total current is fixed 100uA.

it is not easy to achieve this ,using two stage structure

I try to answer for the open loop and loop gain as below:

1. Phase margin is never measured in closed loop and it must measure in opened loop. You must break the loop to measure phase margin.

GH is the loop gain where G= open loop gain and H= feedback gain.

Loop gain is use to measure phase margin at 0 dB and find out the phase shift.

Why we chose GH or loop gain to measure phase margin?

bcoz in control system for closed loop:

closed loop gain= G/ (1+GH)

if GH = -1 , the close loop gain will go infinity bcoz G/0 which is untable and will oscillate and this is according to Barkhausen's criteria.

that 's why we use GH or loop gain at GH=1 or 0 db to measure the phase shift to determine the phase margin.

2. Closed loop gain is the gain you measure in the feedback loop. The system is in closed loop.

The closed loop gain is much lower than open loop gain (G) and loop gain (GH)

bcoz of formula as below:

closed loop gain= G/ (1+GH)

if G very high, basically the closed loop gain = 1/H

I think for incresing the GBW, maybe you can try to use active feeback architecture which increase the GBW or use active inductor to increase the bandwitdh.

#### betty1007

##### Member level 1 Re: question of CMFB

surianova said:
betty1007 said:
surno said:
Hi~ all~~

I desinged fully differential op amp. The structure of op amp is two stage amp with CMFB circuit.

The specification is like that : Gain is more than 60dB, unit gain bandwidth is more than 100MHz and phase margin is more than 85degree.

The simulation result is like that : loop gain is 57dB(@40MHz) and phase margin is 64degree(@40MHz). 40MHz is unit gain bandwidth.

Now I have question. What is relation loop gain and open loop gain of op amp. And how to increas gain and bandwidth if total current is fixed 100uA.

it is not easy to achieve this ,using two stage structure

I try to answer for the open loop and loop gain as below:

1. Phase margin is never measured in closed loop and it must measure in opened loop. You must break the loop to measure phase margin.

GH is the loop gain where G= open loop gain and H= feedback gain.

Loop gain is use to measure phase margin at 0 dB and find out the phase shift.

Why we chose GH or loop gain to measure phase margin?

bcoz in control system for closed loop:

closed loop gain= G/ (1+GH)

if GH = -1 , the close loop gain will go infinity bcoz G/0 which is untable and will oscillate and this is according to Barkhausen's criteria.

that 's why we use GH or loop gain at GH=1 or 0 db to measure the phase shift to determine the phase margin.

2. Closed loop gain is the gain you measure in the feedback loop. The system is in closed loop.

The closed loop gain is much lower than open loop gain (G) and loop gain (GH)

bcoz of formula as below:

closed loop gain= G/ (1+GH)

if G very high, basically the closed loop gain = 1/H

I think for incresing the GBW, maybe you can try to use active feeback architecture which increase the GBW or use active inductor to increase the bandwitdh.
GBW is always equal to gm/CL. THE only way to increase GBW is increase id.and what is active feeback architecture ?

#### surianova Re: question of CMFB

betty1007 said:
surianova said:
betty1007 said:
surno said:
Hi~ all~~

GBW is always equal to gm/CL. THE only way to increase GBW is increase id.and what is active feeback architecture ?

Razavi got one architecture for limiting amplifier using active feedback. Maybe you can check that journal.

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