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For common emitter configuration of a PNP transistor why is collector current 0 when emitter-collector voltage is 0? Does'nt the emitter-base voltage affect it?

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  • \$\begingroup\$ Which current, emitter, collector or base current? \$\endgroup\$ – Martin Zabel Nov 18 '15 at 17:16
  • \$\begingroup\$ Collector current @MartinZabel \$\endgroup\$ – user72436 Nov 18 '15 at 17:17
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Zero volts across collector and emitter can only mean zero current into the collector. It's basic ohms law.

There may indeed be base emitter current flowing but that current flows out thru the emitter (NPN) and not the collector.

If collector emitter voltage is held at zero volts and the base voltage is significant then the collector base junction is becoming forward biased and current will flow from the collector so maybe this is what you allude to?

This is the same for both NPN and PNP transistors.

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  • \$\begingroup\$ But in case of common base configuration the collector current is not 0 even when base current is 0.Why is it so? \$\endgroup\$ – user72436 Nov 18 '15 at 17:15
  • \$\begingroup\$ If there is voltage across collector emitter then, at the very least, there will be leakage current. \$\endgroup\$ – Andy aka Nov 18 '15 at 17:18
  • \$\begingroup\$ Could you please explain this:- In common base configuration suppose no collector-base voltage exists is there but a emitter-base voltage exists,still a significant collector current exists.But for a common emitter configuration suppose no collector-emitter voltage exists but a emitter-base voltage exists then no current is present as collector current...why is it so? \$\endgroup\$ – user72436 Nov 18 '15 at 17:27
  • \$\begingroup\$ This is the diagram I was referring to youtube.com/… \$\endgroup\$ – user72436 Nov 18 '15 at 17:28
  • \$\begingroup\$ And this youtube.com/… \$\endgroup\$ – user72436 Nov 18 '15 at 17:28
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I think this is the circuit you are asking about because applying an external short circuit is the only sure way to force the c-e voltage to 0 V.

schematic

simulate this circuit – Schematic created using CircuitLab

In this circuit, the collector current is not necessarily 0. If the base voltage, supplied by V1, goes negative by more than a few 100 mV, then the b-c junction will be forward biased and current will flow from the collector to base, producing a positive collector current (under the passive sign convention).

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  • \$\begingroup\$ This is the diagram I was referring to youtube.com/… \$\endgroup\$ – user72436 Nov 18 '15 at 17:27
  • \$\begingroup\$ And this youtube.com/… \$\endgroup\$ – user72436 Nov 18 '15 at 17:27
  • \$\begingroup\$ Could you explain why is current 0 when voltage is 0 in one and not 0 in the other (in the videos) \$\endgroup\$ – user72436 Nov 18 '15 at 17:31
  • \$\begingroup\$ So according to you its not necessarily 0.Right? \$\endgroup\$ – user72436 Nov 18 '15 at 17:33
  • \$\begingroup\$ The OP actually asked for a PNP transistor, but the principal is the same, just a negative voltage at the base. \$\endgroup\$ – Martin Zabel Nov 18 '15 at 17:33

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