Brief PNP BJT Switch Circuit Bipolar Junction Transistor

Simple PNP bipolar junction transistor BJT switch circuit schematic diagram by electronzap electronzapdotcom
Simple PNP bipolar junction transistor BJT switch circuit schematic diagram by electronzap electronzapdotcom

Bipolar Junction Transistors (BJTs) wired as switches use a weak signal (low voltage and/or current) to turn a higher voltage/current load on or off.

PNP BJTs have their emitter connected to the more positive side of the power supply. For the switch circuit, the PNP emitter is directly connected to +Vcc. There can be no components between Emitter and +Vcc when a PNP BJT is wired as a switch.

A small amount of current from emitter to base allows many times more current to flow from emitter to collector.

If a PNP transistor allows over 100 times as much current to flow from emitter to collector as is passing through emitter to base, then it is said to have a gain of over 100.

NPN PNP Bipolar Junction Transistor BJT gain or beta current flow illustrated by electronzap
NPN PNP Bipolar Junction Transistor BJT gain or beta current flow illustrated by electronzap
  • Most BJTs have a gain over 100. Consult the datasheet of the BJT you are using to learn more about it’s gain.
  • The actual gain of a transistor varies a bit by how much current is flowing, and  number of other factors.
  • Switch circuits need enough base current that the collector will allow more current to flow than what the load sets. In that case, the load is fully on. The BJT is saturated, when it is on.
  • No base current, means the transistor is fully off. There can be no collector current when there is no base current.
    1. Cutoff: When there is no BJT collector current.

Always remember! The load sets the collector current of a BJT switch circuit. There just needs to be enough base current, based on the gain, that the collector doesn’t limit current below what the load will set. 

Schematic symbols for NPN PNP BJTs N channel P channel JFETs and common MOSFET variations diagram by electronzap electronzapdotcom
Schematic symbols for NPN PNP BJTs N channel P channel JFETs and common MOSFET variations diagram by electronzap electronzapdotcom

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