Maintaining a Vbe Multiplier's bias value

Jon Kirwan · 2010-02-09T01:39:49+00:00

I think this fits in not .basics.I'd like to consider the Vbe multiplier often used in audioamplifiers to maintain a bias voltage for the output stage.The purpose is to better mitigate against ripple in theunregulated power supply rails and against the the VASvoltage output resulting from amplified signal voltages.(The only active device under consideration is a BJT, though.No JFETs or MOSFETS or opamps or other ICs.)The basic starting form for a Vbe multiplier is shown in Fig.1 and the bias voltage output is indicated there. Assume Q1is thermally coupled in some magic way, for now, in just theright way so that if the current through the Vbe multiplierwere perfectly stable, that the bias voltage would track justas needed (The 'Eg' of Q1 is exactly what's needed for theoutput stage's temperature tracking in some nice way and thevalues of R1 and R2 are set correctly and the thermalcoupling and location is somehow where it needs to be.) Thefocus is on the Vbe multiplier's variation of bias in theface of changes in sourcing current at the top of Fig. 1.If I use a resistor as the load for the VAS, it's obvious tome that the Vbe multiplier will need to cope with varyingcurrents. But even if I use a BJT (or two) to make a currentsource sitting above the Vbe multiplier, it's still not goingto hold entirely still with +V ripple and with varying VASdrive voltages. That variation will ultimately manifestitself in a varying Vbe bias voltage. That will change theoperating point for the output stage.If it is class-A, I suppose it doesn't matter that much. ButI don't want to be forced into class-A operation. Nor do Iwant to be forced into regulated rails. So it becomes alittle more important, I think, to get this nailed downbetter.There's the problem, anyway.To quantify how bad all this really is, I tried my hand atfiguring out the small signal analysis of the Vbe multiplier.If I got a first approximation about right, it is basedsquarely upon the small-re of the BJT. The...

Feb 09, 2010 200 Replies

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Jim Thompson 16 years ago

That's not the way it "works", it snaps around (saturation of the appropriate device) to effect current steering. Seems to work really good with extremely high betas ;-) ...Jim Thompson

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Maintaining a Vbe Multiplier's bias value

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