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Old 25th May 2014, 04:06 PM   #1
BigE is offline BigE  Canada
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Default op amp class a bias

I have a phono Section that uses two op27's cascaded. First stage is I to V. Second is a differential stage with most of the riaa.

The op 27 has dc offset nulling. Can I run the first stage with negative bias, and correct the second stage to zero bias thus biasing the second stage into class a?

Or since the second stage is differential, is it not possible? The stage is Peter Daniels.
Thanks!

Last edited by BigE; 25th May 2014 at 04:09 PM.
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Old 27th May 2014, 07:55 AM   #2
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A phono stage with a current input? Did I miss something?

Anyway, class A biasing is something that's applied to the output stage, usually by sourcing or sinking current into one of the supplies (depending on what's more beneficial). It is most useful when the opamp has copious output current capability but is run at low idle current. Fast but power-efficient types like LM6171, or the trusty MC33178/179. An OP27 is not particularly powerful by today's standards, I wouldn't want to load it with more than 5 mA or so.

Last edited by sgrossklass; 27th May 2014 at 08:06 AM.
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Old 28th May 2014, 05:05 PM   #3
BigE is offline BigE  Canada
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Thanks for your reply!

Nope, you did not miss anything. It's a phono stage with current input dedicated to MC cartridges.


Once the output is biased into class A, a coupling cap becomes necessary correct?

Will using the DC offset nulling features of the chip once again take it out of class A?
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Old 28th May 2014, 09:34 PM   #4
juma is offline juma  Germany
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No. Loading the output stage with a CCS doesn't change DC conditions related to input/output feedback loop.

No. Setting the DC conditions has nothing to do with turning the the output stage from PP to SE mode (that's exactly what CCS loading does).
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Old 29th May 2014, 03:21 AM   #5
BigE is offline BigE  Canada
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Thank you!

That is great news. I hear that some chips are much improved by running single ended. Will have to test operation in LT spice. It could be tricky to implement given the existing board is so very tight.

Last edited by BigE; 29th May 2014 at 03:30 AM.
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Old 29th May 2014, 03:00 PM   #6
juma is offline juma  Germany
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Quote:
Originally Posted by BigE View Post
... It could be tricky to implement given the existing board is so very tight.
Not at all. Make a simple, one JFET, 3mA CCS and solder it between opamp's output pin (pin 6 on single opamp chip, pins 1 and 7 in dual opamp chip) and the opamp's negative rail (pin 4 on both versions of opamp case).
Of course, for dual opamp chip you'll need two CCSs per chip.
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Old 29th May 2014, 05:07 PM   #7
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If you can use 2SK246 for CCS.
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Old 30th May 2014, 09:26 PM   #8
BigE is offline BigE  Canada
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I have 2 pieces 2SK117GR.

Does it matter if it is on the output or on the negative input? No series resistor is necessary?

Must check local shop for 2SK246Y.
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Old 30th May 2014, 11:28 PM   #9
BigE is offline BigE  Canada
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So I bought some 246GR from the local shop.... 60 cents each, all 8 pieces are fake. IDSS is zero.

Same test jig shows that IDSS on the two pieces of 2sk177GR is 3.3 and 4.8..... my K170 show 6.5, 7.3....

Dang!
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Old 31st May 2014, 09:36 AM   #10
AndrewT is offline AndrewT  Scotland
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have you read "how to" bias the output stage of a ClassAB opamp, into single ended ClassA?
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