Why don't BJT IC opamps use resistive degeneration in the input stage? - diyAudio
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Old 21st May 2012, 07:22 AM   #1
godfrey is online now godfrey  South Africa
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Default Why don't BJT IC opamps use resistive degeneration in the input stage?

They often use resistors at the output for current limiting and there's normally at least one somewhere to set the idling currents, so why no input stage degeneration?

In fact chip designers seem to go out of their way to avoid using resistors as far as possible. Any thoughts?
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Old 21st May 2012, 10:43 AM   #2
DF96 is offline DF96  England
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Resistors use a lot of silicon? Better to use three or four matched BJTs than a resistor.
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Old 21st May 2012, 12:02 PM   #3
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Yeah, considering all of the benefits of at least some degeneration in a differential input stage, I don't get it either. Even if the transistors are matched, there is further benefit from degeneration, at least based on my experiments in simulation.
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Old 21st May 2012, 12:26 PM   #4
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How do you know they don't? The schematic on the data sheet is just conceptual.
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Old 21st May 2012, 12:33 PM   #5
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Quote:
Originally Posted by richie00boy View Post
How do you know they don't? The schematic on the data sheet is just conceptual.
Well, here's the NE5532 schematic. No degeneration in the input pair, but plenty of other resistors shown. I don't claim to understand this schematic very well.
Attached Images
File Type: jpg NE5532 schematic.jpg (45.2 KB, 80 views)

Last edited by dirkwright; 21st May 2012 at 12:36 PM.
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Old 21st May 2012, 12:55 PM   #6
AndrewT is offline AndrewT  Scotland
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Is it noise specification that over-rules all other design choices?
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Old 21st May 2012, 01:48 PM   #7
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Quote:
Originally Posted by AndrewT View Post
Is it noise specification that over-rules all other design choices?
Do degeneration resistors actually add that much noise, if any at all? I don't know really. It doesn't seem to me that they'd add much if any noise.
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Old 21st May 2012, 02:48 PM   #8
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I can only think that they don't use them because they want the maximum gain in the first stage. Wouldn't max gain (in addition to the CCS in the tail) translate to higher CMRR for the differential input pair?
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Old 21st May 2012, 03:00 PM   #9
Bonsai is offline Bonsai  Taiwan
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1. Noise - especially an issue where the opamp is used in a high gain configuration
2. Maximize loop gain at LF (remember the DC performance of opamps is a large part of their utility value)
3. They most often use Cdom MC, so provided output loads are not unduly capacitive, they tend to be stable, so no need to use input stage degen to reduce loop gain enough to put the output stage pole below the UGF (pole splitting effect of Cdom MC helps here as well)

Others already mentioned above as well.
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Old 21st May 2012, 03:05 PM   #10
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There also may be a game of "specsmanship" going on here.
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