active collector loads (audio freq)

seanbroderick20003 · 2004-12-26T11:13:04+00:00

just wondering if there is a reference out there that goes into lotsadetail about using BJT current sources as collecor loads in analogamps. why can't i look at the output of an actively loaded gain stage(open loop) with a scope? if it's a "current output," shouldn't thescope's input Z turn the current into a voltage? the authors brushover this in Art of Electronics, though there is plenty of detail aboutcurrent sources and mirrors themselves. in the AoE, it is stated thatan active load should not be heavily loaded, or much of the gain willdisappear, but if it's a current output, shouldn't the next stage below Z for best current transfer? i'm so confused.one more thing about differential stages loaded with a current mirror:with a differential amp, i thought the differential signal wasrepresented by the collector current difference between the twotransistors (before conversion to a voltage). but the mirror forcesthe collector currents to be largely equal. why doesn't this eliminatethe output signal?if one wants to actively load a differential stage, and have adifferential output at the same time, how does one go about this? thecurrent mirror restricts you to a single ended output, since the gainon the diode-connected BJT side is low.please help! thanks.Sean B

Dec 26, 2004 40 Replies

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John Woodgate 21 years ago

I read in sci.electronics.design that Stephan Goldstein wrote (in ) about 'active collector loads (audio freq)', on Fri, 31 Dec 2004:

My goal is to attain guru status, so I'm cooking everything with dried sage.

Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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