PNP BJT Emitter Follower circuit transferring weak signal voltage with a diode voltage shift

Bipolar Junction Transistor (BJT) Emitter followers set a certain voltage across a load. The transistor drops some of the supply voltage to do so. Emitter followers can also be called “common collectors”.

PNP BJT emitter follower common collector 2N3906 bipolar junction transistor demonstration circuit schematic by electronzap
PNP BJT emitter follower common collector 2N3906 bipolar junction transistor demonstration circuit schematic by electronzap

You can use a signal voltage, such as a trimpot output, to set a voltage at the Base.

A load at the emitter will have the positive supply voltage on one side of it, and the Base voltage plus 0.6V on the other side of it.

That voltage difference across the load is the voltage that powers the load.

In my example circuit diagram, there is….

  • 5V at the base.
  • Therefore there is 5.6V at the emitter.
  • Whichs put the supply voltage of 10V, minus the 5.6V emitter voltage, equals 4.4V across the load.
  • The additional 10k resistor that is parallel to the load, helps ensure that the emitter voltage is 0.6V higher than the signal. Many loads, especially purely resistance loads, will likely be just fine without the 10k emitter resistor, but it is best to just always add it.
  • The voltage across the Emitter to Collector (E-C) and the current flowing through it (set by the load and parallel 10k resistor) determines how hot the PNP BJT gets. A component’s power (heat generation) is voltage across it, times the current through it.  Voltage in volts and current in amps.
    1. TO-92 package 2N3906 has a total wattage rating of  625mW (0.625W). Try to keep it limited to half of that.

When using a trimpot, you can set any voltage that you want.

Remember than NPN BJTs have the opposite polarities of PNP BJTs. So, the emitter and load are on the low side (more negative) of an NPN BJT emitter follower.  That is actually the far more common way an emitter follower circuit is made.

Video:

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