Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

single opamp compensation

Status
Not open for further replies.

020170

Full Member level 4
Joined
Jan 31, 2005
Messages
231
Helped
3
Reputation
6
Reaction score
1
Trophy points
1,298
Activity points
2,221
hi

last night I designed fully differential folded-cascode amp with Unit gain frequency 500Mhz

( 1p capacitor load )

but it have poor phase margin. so I think this circuit is needed to compensate.

but it's phase plot is so weird. and I don't understand how to compensate opamp.

In Edaboard, I found the pdf - book is labeled that "compensation tutorial"

but it is not practical. Is there anyone who have compensation tip or good E-book?

thanks.

**broken link removed**
 

Could you please post your schematic here as well?
 

    020170

    Points: 2
    Helpful Answer Positive Rating
There are dobule poles in you system. so you only have "NO" Phase Margin.

Do you need so large B.W. ?
The output impedance is so high ,here induce a pole.
 

    020170

    Points: 2
    Helpful Answer Positive Rating
Importantly, is the Y-axis right? I mean you have a Y axis which calls of degrees. I do not think so. I do not think that there is an opamp which will cause a 5 degree phase shift.

Secondly, have you put the options as .OPTIONS UNWRAP for the phase plot? If you have done so, then the cause is due to two RHP zeroes and not multiple poles. The problem in the design could be due to the sizing of the cascode transistors which are at the current folding end. So, kindly verify them.

Also, have you used the positive input for testing the amp because there is no phase shift at DC. Are you doing a open loop test? If that is the case, you are actually looking for a lot of phase margin(which I think is next to impossible at those frequencies).

Kindly clarify me on these things.
 

    020170

    Points: 2
    Helpful Answer Positive Rating
Vamsi Mocherla said:
Importantly, is the Y-axis right? I mean you have a Y axis which calls of degrees. I do not think so. I do not think that there is an opamp which will cause a 5 degree phase shift.

Secondly, have you put the options as .OPTIONS UNWRAP for the phase plot? If you have done so, then the cause is due to two RHP zeroes and not multiple poles. The problem in the design could be due to the sizing of the cascode transistors which are at the current folding end. So, kindly verify them.

Also, have you used the positive input for testing the amp because there is no phase shift at DC. Are you doing a open loop test? If that is the case, you are actually looking for a lot of phase margin(which I think is next to impossible at those frequencies).

Kindly clarify me on these things.
RHP zero/pole and LHP zero/pole have different effects on the circuit.
what is the difference?
 

    020170

    Points: 2
    Helpful Answer Positive Rating
tsanlee said:
There are dobule poles in you system. so you only have "NO" Phase Margin.

Do you need so large B.W. ?
The output impedance is so high ,here induce a pole.

output impedance is low ,especially about 1.8K
 

The output impedance of a folded-cascode cannot be that low. Did you get your DC biasing correct?
 

    020170

    Points: 2
    Helpful Answer Positive Rating
020170 said:
tsanlee said:
There are dobule poles in you system. so you only have "NO" Phase Margin.

Do you need so large B.W. ?
The output impedance is so high ,here induce a pole.

output impedance is low ,especially about 1.8K

when you do the simulation,you must confirm that all the transistors work in the saturation regjion.if they all work properly,the output resistance can't be so small from my experience.
 

    020170

    Points: 2
    Helpful Answer Positive Rating
all mosfet is under saturation.

in last I have a solution. everybody thanks.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top