NPN BJT emitter follower circuit – transferring a voltage minus a diode drop

The bipolar junction transistor (BJT) circuit we are going to look at is typically called an emitter follower. But is also commonly called a common collector.

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Circuit basics:

Simple NPN bipolar junction transistor BJT emitter follower set by trimpot divider schematic diagram by electronzap electronzapdotcom
Simple NPN bipolar junction transistor BJT emitter follower set by trimpot divider schematic diagram by electronzap electronzapdotcom

An emitter follower power a load at the signal voltage with an approx. 0.6V base to emitter diode drop for NPN BJT. the transistor provides virtually all of the needed current to power that load so that the signal voltage isn’t thrown off.

As you can see in the diagram, the load is on the emitter side of the BJT transistor. The arrow indicates the emitter and it is pointing in the direction of how conventional current flows through the component, which is out of the transistor for the NPN BJT.

Since the base current goes to the emitter and heads towards ground, a load in it’s way from emitter to ground will build up a voltage. The base current is usually far too weak to maintain that voltage alone, but a very small base current gets the collector conducting as much current as is needed from the power supply to emitter and through the load to maintain the base signal voltage minus the 0.6 base to emitter diode drop.

The extra emitter resistor, commonly 10kΩ value, helps hold the expected voltage across the load. Some loads will throw off the voltage.

2N3904 pin layout:

2N3904 NPN BJT pin layout schematic symbol and current paths diagram by electronzap electronzapdotcom
2N3904 NPN BJT pin layout schematic symbol and current paths diagram by electronzap electronzapdotcom

I use the 2N3904 for the basic NPN BJT in my recent video because they are one of the most common NPN BJTs, and they have a complementary PNP version called the 2N3906.

I stands for current in electronics formulas, and the arrows next to them are showing the direction of conventional current flow (positive to negative), which is how we imagine current flowing through a circuit, even though it was discovered that in reality, electrons are flowing from negative to positive.

Demonstration circuit:

NPN bipolar junction transistor emitter follower set with trimpot demo circuit schematic diagram by electronzap
NPN bipolar junction transistor emitter follower set with trimpot demo circuit schematic diagram by electronzap

Quick NPN BJT emitter follower set with trimpot voltage divider using 2N3904 transistor

If you’d rather just look at these circuits in a quicker diagram/video format, then go to the following page which has related topics linked in it.

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