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Design >> Analog Design >> Designing an output stage!
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Message started by samiran on May 20^th, 2010, 6:06am

Title: Designing an output stage!
Post by samiran on May 20^th, 2010, 6:06am

Dear All,

I need to design an output stage (third stage) for a two-stage compensated opamp. I have gone through some related papers and text. After reading them, I have built-up some interpretations:

1. The main design specifications whilst designing the o/p stage are a) output swing, b) power dissipation (or efficiency), c) linearity (or THD). Please let me know are there anything else I need to consider.

2. My 2stage diff-amp works with 1.8 V - 0 V supply rail. So, the output stage also need to be designed among those. Which topology should I go for?

3. After inserting the third stage is it required to again compensate the whole circuit? And any other design metric dependent upon both the driver stage and output stage needs to be considered?

4. Any simple design guideline is very much appreciated.

Please help me.

Thanks & regards
Samiran

Title: Re: Designing an output stage!
Post by Mayank on May 20^th, 2010, 7:20pm

Quote:

The main design specifications whilst designing the o/p stage are a) output swing, b) power dissipation (or efficiency), c) linearity (or THD). Please let me know are there anything else I need to consider.

Yes, The kind of load it needs to drive & how much of it ?

Quote:

2. My 2stage diff-amp works with 1.8 V - 0 V supply rail. So, the output stage also need to be designed among those. Which topology should I go for?

Depends on the above mentioned factors + the answer.

Quote:

3. After inserting the third stage is it required to again compensate the whole circuit? And any other design metric dependent upon both the driver stage and output stage needs to be considered?

Yes, Esp. if the output driver's a gain stage.

Quote:

4. Any simple design guideline is very much appreciated.

Again there are numerous topologies ranging from a simple common drain config to complex drivers like class AB & others. Choose the one which fits your specs. There's plenty of material on output stages.

--
Mayank

Title: Re: Designing an output stage!
Post by love_analog on May 31^st, 2010, 7:48am

You need to tells us what kind of load and gain you desire.
Typically you put a 3rd stage if you have to drive a low resistance and prevent that from loading the 2 stage opamp.

Title: Re: Designing an output stage!
Post by ampbuilder on Jun 22^nd, 2010, 2:29pm

Can try Common Emitter/Source; it alone can swing to +/-sat voltage off the rail which is about 0.2V for a BJT but watch out, gain depends on load.