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Old 9th June 2004, 03:01 AM   #1
sam9 is offline sam9  United States
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Default Wrong tail current cause oscillation?

Can an incorrect tail current cause oscillation in the differential pair? Actually I think the tail current is correct but I'm grasping for a reason that the differential pairs (mirror image topology) get very hot. With no input, 250hz-1kHz appears at the bases of the input transistors and at the NFB feed back depending on the rail voltage. Mean while the output section remains completely cold.

This is a complaint I've never heard of.
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Old 9th June 2004, 10:22 AM   #2
djk is online now djk
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Do you have base resistors?

If there isn't enough current in the diff pairs to charge the Miller capacitors in the VAS pair you may have a problem.
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Old 9th June 2004, 02:34 PM   #3
sam9 is offline sam9  United States
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Both my hand calculations and LT Spice say there ought to be enough current. However, I'm going to try today to lower in steps the resistors that control current in the CCs. By the way, the RE resistors for the current mirrors and diff pair are already fairly low.
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Old 9th June 2004, 02:50 PM   #4
azira is offline azira  United States
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Question: Why can't you measure the voltage accross your collector resistor(s) and find out how much quiescent current is flowing? Even if you don't have any emitter resistors (which you should) you have to have atleast some kind of collector load...
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Danny
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Old 9th June 2004, 03:45 PM   #5
sam9 is offline sam9  United States
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I did make the measurement and the currenty is what I think it should be. I think the question may be whether I've calculated the correctly what the desired current is. This isn't entirely new as I've not bumped into this with a single long tailed pair. This is my first crack a a symetic topology other than just copying a cookbook design.
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Old 11th June 2004, 06:52 PM   #6
sam9 is offline sam9  United States
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Default MANY THANKS, FOLKS!

Reducing the resitors that set the tail current took care of the problem. Changing the diode(s) to increase the voltage drop and bias would work too, I suposse.

Appearently, in calculating the required tail current for symetric topology you can't treat each half (top&bottom, pos&neg) just like for a single diff pair -- a little more is needed. Either that or the Miller caps just happened to be at the upper end of their tolerance - I don't feel like taking thenm out to measure just yet.

Anyway, I'm going to update my crib notes on tail current to be sure that using them for a symetric topology comes up with the solution that agrees with actual practice.

Thanks again for the input.
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