What is a PNP transistor, what is its working principle, what are its components, and how can we design a circuit with it?
A PNP transistor has the same legs as an NPN transistor which are:
A PNP transistor is “turned on” when there is a small current going from the emitter to the base of the transistor.
When we say “on,” we mean that the transistor will open a channel between the emitter and collector, and this channel can carry a very large current.
To operate the current from the emitter to the base, you need a potential difference of about 0.7 volts, since the current goes from the emitter to the base, the base must be 0.7 volts less than the emitter.
By setting the base voltage of the PNP transistor to 0.7 volts below the emitter, the transistor can be “turned on” and allow current to flow from the emitter to the collector.
It may seem a little confusing, so read on to see how you can design a circuit with a PNP transistor.
PNP Transistor circuit
Let’s see how to create a simple PNP transistor circuit. Using this circuit, an LED can be powered.
Step 1: Emitter
To power the PNP transistor, you need the voltage on the base to be less than the emitter.
For a simple circuit like this, it is common practice to connect the emitter to the positive voltage from the power source, this way you know what voltage you have on the emitter.
Step 2: What You Want to Control
When the transistor is turned on, the current can flow from the emitter to the collector.
So, let’s connect what we want to control like an LED. Since it’s an LED it must always have a resistor in series with it.
And you can replace the LED and the resistor with what you want to control.
Step 3: Input transistor
To turn on the LED, you need to turn on the transistor so that the channel from the emitter to the collector is opened.
To power the transistor, the voltage on the base must be 0.7 volts less than the emitter, i.e.: 9 volts – 0.7 volts = 8.3 volts.
For example, you can now power an LED with a photoresistor and a standard resistor set up as a voltage divider.
The voltage on the base will not operate exactly as the voltage divider formula tells you because the transistor affects the voltage as well.
But in general, when the value of the photoresistor is large (no light), the voltage will be very close to 8.3V and the transistor is on
And when the value of the photoresistor is low (there is a lot of light), the voltage will be close to 9V and the transistor will turn off (which turns off the LED)
What controls the base voltage?
You may be wondering, “How did the photoresistor and the resistor on the base create the correct voltage of 8.3 volts when it was dark?”
This is because the emitter and base make up the diode, and the diode always tries to get the same diode voltage.
This special diode has a diode voltage of about 0.7 volts, and 8.3 volts i.e. 0.7 volts less than 9 volts.
But, this is also because the value of the photo resistance and the resistance on the base set the voltage to be within the correct range.