Transistors act as electronic switches, capable of turning current on or off. In this issue, let's take a look at how BJT (bipolar junction transistor) works using practical examples.

Transistor (NPN) (Figure 1)

To turn on a BJT, the voltage between the base and emitter needs to be approximately 0.7V. You can directly connect a 0.7V battery across the base and emitter, and the BJT will turn on.

When it's in the on state, current flows from the collector through the transistor to the emitter.

And when the BJT is off (as in Figure 2), no current flows, and the light-emitting diode (LED) is also turned off.

But what if you don't have a 0.7V battery? How can you turn on the transistor?

It's actually quite simple! That's where a series resistor comes in!

The principle between the base and emitter of the BJT is like a diode. A diode has a forward voltage, and it will obtain that forward voltage from the available voltage. When you add a resistor in series, the rest of the voltage will be divided across the resistor.

By increasing this resistor, you'll automatically get around 0.7V.

As current flows from the base to the emitter, the transistor conducts, allowing a larger current to flow from the collector to the emitter. The ratio between these two currents is the transistor's gain.

For commonly used transistors, the gain is approximately 100. This means that if there's a 0.1mA current flowing from the base to the emitter, the current flowing from the collector to the emitter will be around 100mA or more (Figure 3).

Now, the question is, what should be the resistance value of R1 in Figure 3 to obtain a 0.1mA current?

If the battery is 9V and the voltage from the base to the emitter of the transistor is 0.7V, then there's 8.3V across the resistor.

You can use Ohm's Law to find this value:

Therefore, you need a resistor of around 83kΩ.

R2 can then limit the current to the LED. Additionally, if you want to connect the LED and resistor directly to the 9V battery without the transistor, you can choose a resistance value of approximately 1kΩ, and it will work normally without the need for the transistor.

This example is based on an NPN transistor, but there's also a PNP type, which operates on the same principle as NPN, except that all currents are in the opposite direction.

When choosing a transistor, remember to pay attention to the maximum current (collector current) it can withstand.

In the next issue, we'll discuss the working principle of MOSFET with practical examples and device selection.