Help me design a 80dB opamp with 10uF capacitance load

[lqy]

I need to design a high gain (80dB) OPAMP with a large capacitance load (10uF), It is very difficult to compensate it with a classical two stage OPAMP, could someone please give me some suggestions or practical structures? thanks very much!

 

[Robert Qi]

resistor in series with opamp

folded cascode amp may do

 

[willyboy19]

multi stage gain of op amp

Due to the existence of equivalent series resistance (ESR), there will be a zero created with this 10uF cap. As this ESR resistance can vary by more than 40%, this may cause stability problem if you use a single-stage (such as folded cascode) opamp.

Solution: carefully designed multi-stage opamp.

 

[h.martin]

Re: A OPAMP design

maybe you should gave more parameters.

 

[Chethan]

Re: A OPAMP design

If the load is just capacitive then a folded cascode opamp would solve ur problem.

 

[Shaber_Mezbah]

A OPAMP design

try folded cascode or telescopic OTA .. if you are only terget for capacitive load.

 

[vbhupendra]

Re: A OPAMP design

You can use Buffered opamp.

 

[pk3316]

A OPAMP design

It seems ok, you need two gain stages to maximizee you bandwidth and keep your gain large than 80 dB, and then you need a good class AB output stage to draw large current for the large load cap. for phase margin, use miller compensation.

 

[suthan]

Re: A OPAMP design

folded cascode opamp will solve ur problem..try it out !

 

[yeewong_su]

A OPAMP design

yeah. I think the AB output with the cascode will be ok

 

[nandi_ranjan]

Re: A OPAMP design

Hi

Following Books may help..

The Art and Science of Analog Circuit Design by Jim Williams
Analogue Electronic Circuit Design by Jan Davidse
Electronic Circuit and System Simulation Methods by Lawrence T. Pillage, Ronald A. Rohrer,Chandramouli Viswesvariah
Fundamentals of Analog Circuits by Thomas L. Floyd, Dave Buchla
Linear Integrated Circuits by Jack Winzer
Operational Amplifiers and Linear Integrated Circuits by Robert F. Coughlin
Analysis and Design of Analog Integrated Circuits by Paul R. Gray, Robert G. Meyer
Analog Integrated Circuit Design by David Johns, Ken Martin, Kenneth W. Martin
Analog Integrated Circuits by Edwin W. Greeneich
111 Digital and Linear Ic Projects by Don. Tuite
Analog Integrated Circuits by Miklos Herpy
Analog Integrated Circuits by Grebene
Analog Integrated Circuits by Sidney Soclof
Analog integrated circuit design by Alan B. Grebene

Thanks
Ranjan Nandi

 

[jiaming_lee]

Re: A OPAMP design

Due to the existence of equivalent series resistance (ESR), there will be a zero created with this 10uF cap. As this ESR resistance can vary by more than 40%, this may cause stability problem if you use a single-stage (such as folded cascode) opamp.

Solution: carefully designed multi-stage opamp.

I agree with you very much. ESR should be considered carefully. Two stage opamp with high current buffer stage can help you. 10p is not a very big load. I ever did one with 10p load in 0.8um process, gain exceeds 100dB

 

[avinash]

Re: A OPAMP design

with such a high value o/p capacitance you should go for the folded cascode topology.in folded cascode opamp design ,higher the value of load cap will provide much stable opamp.

 

[yaxazaa]

Re: A OPAMP design

Don't just think use Miller cap. Instead think by generating a zero through feed-forward.

 

[jinxingsun]

A OPAMP design

Self biased folded cascode amp can do!

 

[beckchm]

Re: A OPAMP design

folded configuration self-compensation

 

[Karthikeya]

Re: A OPAMP design

you shud try using folded cascode with gain enhancement amplifiers for 80db gain....but i dont know if it can support such high cap load.
also, which technology are you using?in latest technologies it wud be a tough task to achieve that spec...

regards

 

[fasmatikos]

Re: A OPAMP design

I agree with Shaber_Mezbah...It work for me , i had a similar project 4 monts ago

 

[krishmaniac]

Re: A OPAMP design

anyone can share the design steps for the folded cascode opamp.read through allen's book but got confused.

Added after 37 minutes:

esp when the systemtic offset is specified and input swing is not given.

 

[tjr]

Re: A OPAMP design

u refer this opamp design reference manual from Texas