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19th June 2005, 08:41 PM  #1 
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Join Date: Aug 2004
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Simple(ish) discrete opamp
I've been pondering making a design for a discrete opamp. Something that is not too complex and yet offers performance as good as IC opamps.
Attached is one design I am considering (only simulated at the moment  it's too damned hot to do any real work). It has a singleended input instead of differential for simplicity (lower component count plus no need for Miller compensation). This makes it 'current feedback' (as in lowimpedance feedback node  not to be confused with 'current feedback' meaning feedback proportional to output current). Biasing for the Vas is accomplished with current mirrors. This helps with symmetry, in particular compensating for the potentially unequal Vgs of the two Vas JFETs. Both the input stage and Vas are cascoded, which in conjunction with the current mirrors provides both constant current and constant voltage operation for those transistors, minimizing thermal memory distortion. Output stage uses JFETs, for simplicity. Simulated performance compares well with IC opamps, with 134dB of openloop gain, 30MHz GBP, very high input impedance, low output impedance, low distortion and around 180V/us of slewrate. It's not quite as simple as I had set out to create, but it's not too bad. Comments and criticism welcome!
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19th June 2005, 09:33 PM  #2 
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Join Date: Apr 2005
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At first sight like all it is good, but something guards. Did not look more deeply.
For the information look. 
19th June 2005, 09:54 PM  #3 
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Join Date: Jun 2004
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Hi
what are J5 & J6 for? How about compensation and stability issues? What's the minimal (closed loop) gain? I like its simplicity, but I don't understand the issues above so it's hard to comment now. best regards 
19th June 2005, 10:55 PM  #4  
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Re: Simple(ish) discrete opamp
Quote:
sound. 

20th June 2005, 01:14 AM  #5  
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Re: Re: Simple(ish) discrete opamp
Quote:
Quote:
Compensation is not needed. It should be unitygain stable, with the caveat that comes with currentfeedback designs that the resistor from output to feedback node must be present and of sufficiently high value (I haven't worked out the minimum yet). Of course that might be different in reality. Quote:
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20th June 2005, 01:34 AM  #6 
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well
I thought J7/J8 were cascodes for bjt VAS and adding another common gate stage (J5/6) doesn't change things much. Do I miss something here? regards 
20th June 2005, 02:11 AM  #7 
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Well you're right that the BJTs do provide gain, and if the JFETs weren't there then they would work entirely as the Vas, but the JFETs provide gain too, and so are also the Vas.
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20th June 2005, 04:59 PM  #8 
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Join Date: Feb 2005
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In practise, your current mirrors will need emitter resistors. And, when you add them, you can easily multiply current without needing multiple transistors  just divide the corresponding leg emitter resistor with the current multiplier.
Otherwise, it looks very interesting! Let us know how it works for real when you ave the chance to try it out... 
20th June 2005, 05:19 PM  #9 
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Join Date: Dec 2004

EVIL'S Intersting CIRCUIT...very imaginative....waiting to get the real life response.....go and build up the discrete opamp fast, nice and easy.......

20th June 2005, 06:32 PM  #10  
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Join Date: Apr 2005
Location: Russia

Re: Re: Re: Simple(ish) discrete opamp
Excuse, it was very fast the thrown sight, but all the same, there will be a feeling that it to work well.
Quote:
ilimzn (In practise, your current mirrors will need emitter resistors.) completely I support. 

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