PNP Transistor Common Base question

In a PNP common base configuration circuit, how about if the circuit is set up at reverse configuration. Say no voltage source at emitter pin and just passive load. At collector pin there is voltage supplied. There would be base current at the base pin but will it be current flowing from collector to emitter?

by manuelkw
January 12, 2018

Sounds like zero-zero if I understand you correctly. Can you give actual voltages wrt base?

by mikerogerswsm
January 12, 2018

A bias resistor at base pin is 5K to GND, voltage supplied at C pin is 3V

by manuelkw
January 12, 2018

Sorry, I don't understand. Can you post full schematic surrounding that device?

by mikerogerswsm
January 13, 2018

The circuit looks like this :

Definitely CB has current but I found current flowing at the emitter to the load resistor, not understand why.

by manuelkw
January 13, 2018

From the picture, the bias conditions are following:

VE = 0V

VB = 0V

VC = 3V

Hence, you should observe no currents.

by coolman
January 15, 2018

1 Answer

Answer by mikerogerswsm

If you connect your PNP 'backwards' with reversed polarity, it looks roughly like two diodes, but not quite. You have biased the collector-base diode 'on', which puts the base at + 2.4 volts, with a current of 480 uA. The emitter-base diode is reverse-biased, and, were it just a diode, would pass no current. But there are a lot of carriers floating around and I suspect some are finding their way to the emitter. PNPs are not intended to work backwards. I suggest adding a diode in series with the emitter (or base) to block this effect.

+2 votes
by mikerogerswsm
January 13, 2018

PS - There may also be an emitter-collector diode inside the chip which is normally reverse-biased and which you are turning 'on'. You need to look at the chip structure, is it a vertical or a lateral PNP?

by mikerogerswsm
January 13, 2018

This may be relevant: . I'm just a dull old retired engineer and can't pretend to understand it.

by mikerogerswsm
January 13, 2018

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