BJT voltage follower – Emitter follower

Schematic diagram for a NPN bipolar junction transistor emitter follower aka common collector circuit by electronzap
Schematic diagram for a NPN bipolar junction transistor emitter follower aka common collector circuit by electronzap

Voltage followers take the voltage of a weak signal (input), and outputs that same voltage. It must be kept in mind however, that Bipolar Junction Transistor (BJT) emitter followers, have a 0.6V difference between input and output. There’s no way to get the same voltage out as the voltage in with a single BJT, but they can be combined (cascaded) to do so.

NPN BJT emitter follower will output 0.6V less than the signals, whereas a PNP BJT emitter follower will output 0.6V more than the signal.

It’s preferred to take almost no current at all from the signal source, as it will likely drop it’s voltage if it is a weak signal. But, the BJT does actually need a little bit of current, which should still be no problem for most weak signals to provide.

The extra emitter resistor (10k seems to work well) going to ground, helps ensure that the output voltage is correct. Some loads may throw it off by themselves.

Voltage divider amplified with NPN BJT emitter follower Electronzap learning electronics lesson 0016
Voltage divider amplified with NPN BJT emitter follower Electronzap learning electronics lesson 0016

Voltage dividers can’t provide much current because their output voltage quickly drops. Adding a voltage follower, such as the NPN BJT emitter in the diagram above, can provide a lot more current while maintaining the voltage divider voltage, but minus 0.6V when using a NPN BJT.

This is not free power of course. Ultimately the output power is provided by the power source. The transistor simply controls how much of the supply voltage is applied at the output.

NPN BJT emitter follower voltage regulator using a zener diode:

NPN BJT emitter follower circuit made with 2N3904 bipolar junction transistor schematic and pictorial diagram by electronzap
NPN BJT emitter follower circuit made with 2N3904 bipolar junction transistor schematic and pictorial diagram by electronzap

Zener diodes also have a lot of voltage output limitations. Being diodes, they need to have current limited though them.

With just a zener and current limiting resistor, you would have to pick a resistor that allows enough current to set the zener voltage, plus the current that the load will need at that zener voltage.

You would have to pick a resistor that probably only works for one load. Changing the load with just a Zener and current limiting resistor will probably require changing the resistor as well. That makes for a poor voltage regulator circuit.

A BJT makes a good voltage follower as long as you remember the 0.6V difference of input versus output. Again, it important to remember that in relation to the signal, NPN BJTs lower the output voltage by about 0.6V, whereas PNP BJTs raise the output by about 0.6V.

5.6V is a commonly available Zener voltage. Which is nice if you want to use a Zener and NPN BJT to output a fairly reliable regulated 5V.

Cascading PNP and NPN BJTs to get the same voltage out as in:

Cascading PNP and NPN emitter followers cancels base diode offset learning electronics lesson 0018
Cascading PNP and NPN emitter followers cancels base diode offset learning electronics lesson 0018

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