upside down bjts

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In sample/hold circuits from bygone times, it was common to find a bjt upside-down. If, as an example, there was a capacitor that needed 'resetting', the bjt across it had, for an NPN, collector to GND, emitter to the positive end of the capacitor, and base used as usual.

When you do this, accidentally or on purpose, you find that a typical BC547 has a beta of about 12, instead of 300.

Why was this done?

I can imagine saturation recovery time being radically different, but it might also have quite different saturation voltage.

/Kasper

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Perhaps not worry, a small fet may be a better the way today.

It all starts with a mental vision.

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Just offhand, one reason could be that the BJT has a lower saturation voltage when used upside down, albeit with higher drive levels. Nowdays a FET might be preferred.

Klave

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I am not building integrate-and-dump circuits. At least not at the moment.

My desire is to know if there is some benefit in the (mis)use that could be applied to other things too.

(MOSFETs have nasty features too; One of them is that a small MOSFET is useful as a varactor. And I have used them as such.)

/Kasper

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Yes, Kasper, I have seen this unusual topology but I cannot remember where...

I am just speculating, but, could that be because of the lower reverse leakage current and/or capacitance of the BE junction against the forward BC ones?

-George

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)

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Second to Klave:

Quote:
Inverted operation is often used in analog switch applications. Due to the doping of ordinary transistors, the emitter-collector saturation voltage is almost negligible, a few milivolts vs. 20-40mV. This makes inverted BJTs competitive with JFETs for analog switches, as long as the low hFE can be addressed.

Plus - more stable current against heating; lower output capacitance.
More exactly this topology is known as "inverse".