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Design >> Analog Design >> Folded Cascode Opamp Design
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Message started by analog_fever on May 1^st, 2009, 11:08am

Title: Folded Cascode Opamp Design
Post by analog_fever on May 1^st, 2009, 11:08am

I successfully completed the classic 2 stage OpAmp design with Miller compensation, and am looking at advanced Op Amp designs.

I started with Folded Cascode Opamp design and am in a confusion not knowing where to start. I went through Johns Martin, and Jacob Baker's books. Both seem totally different in the way Folded Cascode Op Amps are handled - biasing and compensation. I am not sure which one to follow.

Also I am in search of a specification for the opamp parameters to use for the design.

Can somebody please guide me to get started, and provide pointers on how to get started, literature to read for a beginner, and a specification if possible.

Thanks in advance.

Title: Re: Folded Cascode Opamp Design
Post by raja.cedt on May 2^nd, 2009, 1:08pm

hi,
i think cascode and folded cascode design is same,But for this design don't follow text book use gm/id method. If you want some starting step use allen holberg.

Thanks,
Rajasekhar.

Title: Re: Folded Cascode Opamp Design
Post by thechopper on May 7^th, 2009, 7:14pm

Hi,

I would start thinking of basic specs for the opamp such as:
- Supply voltage
- Input CM range
- GBW
- Accuracy (open loop gain)
- Offset

These basic parameters will start forcing you to use some particular topology. For example, you in case you need a relative large GBW, then a folded cascode migth be an option since you are avoiding low frequency secondary poles and at the same time achieving large open loop gain due to the high impedance from the cascoded devices.
A telescopic amplifier might be another option.
When you start considering supply voltage, that might force you to choose between the two topologies I mentioned above.
If you are looking for good offset performance then you migth want to go to for a simpler topology.

The selection of a particular topology is highly tied to your actual specification. So I suggest you first fix your set of specs and then analyze each topology to decide which better fulfils your requirements.

Hope this helps
Tosei