Increasing gain and bandwidth of 2 stage amp with CMFB circuit

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.

please help me. Thanks~!![/quo
it is not easy to achieve this ,using two stage structure

Click to expand...

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.

please help me. Thanks~!![/quo
it is not easy to achieve this ,using two stage structure

Click to expand...

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.

Click to expand...

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.

please help me. Thanks~!![/quo
it is not easy to achieve this ,using two stage structure

Click to expand...

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.

Click to expand...

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

Click to expand...

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 ?

Click to expand...

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

Click to expand...

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