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Old 20th April 2013, 06:35 AM   #1
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Default help explain the current source behavior

I have built the small signal portion of the simple amplifier on page 42 of Cordell's book on a breadboard. It is just the IPS and VAS. I used a 100uf coupling cap , and a 20k ground reference resistor for my input circuit. I did not include the bias spreader, and my resistor values are not exactly his, but well within tolerances. I have a 300pf miller cap, and I take the negative feedback from the VAS collector. I also included a 100uf cap in the grounded portion of the feedback network to have no gain at DC. Simple resistive load on the IPS.

It works as it should, feeding it with a sine wave from my function generator, and watching it on the scope. There is one behavior that I don't understand though.

I am monitoring the VAS current source (a feedback pair source) by watching the voltage across the 68ohm resistor that sets the current. Under normal signal conditions, it says about 600mV, suggesting 8mA with my 18 volt rails. It is right around there until I turn up the source signal to the point that the VAS clips. Then the current from the current-source drops quickly as I turn up the signal more.

Why is this?
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Old 20th April 2013, 07:11 AM   #2
Mooly is offline Mooly  United Kingdom
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I think what you are observing is simply the VAS getting cut off during clipping and under those conditions the current source has nowhere to feed its current and so it to is "cut off".
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