Table of Contents
Bipolar Junction Transistor (BJT) Emitter followers look at a weak signal voltage. Then they power other circuitry at a voltage that is approx. 0.6V less than the signal voltage.
Trimpots are great for outputting an easily adjustable fraction of a larger voltage.
However, trimpots use resistance to get that fraction of a voltage, so they can’t provide much current. They should provide as close to no current as possible in order to be effective.
With resistance and/or a load between the NPN BJT emitter and ground, the emitter will build up a voltage that is 0.6V less than the voltage applied at the base.
When making the same circuit with PNP BJTs, polarities are reversed.
PNP BJTs wired as emitter followers will provide 0.6V higher than the base voltage. But, never as high as the positive supply voltage.
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The resistance/load is between the +Vcc and the emitter of the PNP BJT.
The PNP collector passes current directly from the negative supply (ground), as needed to maintain the voltage set at the base, with the 0.6V offset. That collector current also flows through the emitter resistance/load.
The emitter resistance/load sets the current based on the voltage across it.
Some loads may throw off the emitter voltage. So, it is best to always include an additional resistor from emitter to ground (Vcc if it is a PNP BJT). 10k works well, but a wide range of values can be used.
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