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Old 19th June 2002, 04:32 AM   #31
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I've tried perhaps 10 or 20 different versions of this circuit now with different resistor values and in all cases it suffers from excessive LF gain.

In some cases it has HF gain problems as well, but lowering R4 and R5 to 10 Ohms or so seems to flatten out the HF response.

The only way I've been able to reduce the LF gain is by reducing the gain to near unity, which takes the fun out of "amplification."

A couple of details about the prototype:

1. All fets are mounted to the same heatsink.
2. The measurements posted are unbalanced (- input connected to ground
3. It is built on a breadboard.


A couple of additional questions for the group:

1. How do you derive balanced outputs from your signal generators? Would a low-distortion op amp unity gain converter be the way to go? My generator doesn't have balanced outputs.

2. Anyone know how to make a balanced distortion measurement with an HP8903B?

Thanks again for your help!

Mike
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Old 19th June 2002, 04:57 AM   #32
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Question R9 & R10

What are these guys doing in there? If they are for DC feedback for the output stage, it might work better to use low-value (.2-.5 ohms or so) source resistors instead.
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Old 22nd June 2002, 05:30 PM   #33
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Hi Joe:

My thought was that R9 and R10 would allow the input differential to swing a higher voltage without disturbing the voltage referenced by R11 and R12. But my scheme must be reaking havoc on the frequency response. I've added .2 Ohm resistors to the output sources, which helps the bias, but if I eliminate R9 and R10 The output clips at 10 Vp instead of 20Vp. I wonder if I should flip the circlotron around and drive the output across the drains of Q1 and Q2. Hmmm. I hope to post some more info this weekend.

Thanks again for your help.

Mike
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Old 22nd June 2002, 06:24 PM   #34
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Mike, if you don't mind another suggestion, you could also try leaving R9 and R10 where they are and referencing the output stage sources to a common bias voltage source. The attached zip file shows a (very crude -- sorry!) picture of this. For the actual bias source, you could just use an adjustable voltage divider across the -20V supply, i.e. a pot and a resistor. If you get a lot of bias drift with temperature, you could then go with something more elaborate.

ADDENDUM -- You could also derive a bias voltage from the drains of the input differential pair, and that might work better once you have solved the frequency response anomaly and know how to avoid it.
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Old 25th June 2002, 12:53 AM   #35
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Joe: Thanks for the suggestion. It helped to determine that the frequency response problem is not related to the output section. I built a copy of the front-end only and I'm getting the same results. I'm beginning to suspect something more sinister. I think the change you suggested may ultimately be the way to go, but for now I've got to troubleshoot this front-end. I'm perplexed, there's nothing to it... I'll let you know what I find out. If I can figure it out I'm certain this thing will work quite well.

Thanks

Mike
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Old 25th June 2002, 02:27 AM   #36
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Just curious, Mike: if you are actually driving the inputs balanced, are you using a floated signal generator to do it? I could imagine this being a potential source of problems if the signal source isn't designed for balanced or floated operation. In any case, you could check for problems on the source end by measuring the signal appearing at the inputs of the diff pair.
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Old 25th June 2002, 08:24 PM   #37
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Joe: I'm driving the circuit unbalanced - negative input connected to ground. My audio generator is single ended - negative input connected to earth through the chassis. I was wondering if the oscilloscope might be a problem, it is a battery operated PC scope, so I assumed (perhaps falsely) that the input was floating. As a crude double check, I used my DVM to test and swept through a few frequecies. The result was the same - a long downward taper from 100Hz to 20KHz. This is my first attempt at a balanced circuit, so I'm not familiar with the proper measurement techniques. I've found very little info on the Internet or elsewhere with respect to floating scope measurements. And still, I'm not even completely sure the measurement is the problem...

Thanks for your help. When it's perfected, I'll owe you a couple of PCB's!

NP: No word as of yet. You're just going to let me wrestle with this one for awhile aren't you...! No pain, no gain. (no pun intended)
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Old 25th June 2002, 09:57 PM   #38
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Mike, is the taper on the inputs as well as the outputs?
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Old 25th June 2002, 10:38 PM   #39
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No, just the output. The input looks fine.
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Old 25th June 2002, 11:10 PM   #40
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Sorry for the "20 questions," but -- have you double-checked that R3 and R8 are 220 ohms and not 22k ohms?
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