What is BJT transistor – Introduction

There are 2 main types of transitor, namely Bipolar Junction Transistor (BJT) and Field Effect Transistor (FET). Basically both type perform the same function ie. control current flow but their behaviour are slightly different.

Transistors are Active electronic component which means by using an input signal, they can control current flow. Active components require power source.Drawing power and current control make transistors a power amplifier (when in active mode) , just like Op-Amp.

This post is all about BJT.

The voltage and current entering the base(for NPN) and the voltage and current leaving the base(for PNP) control the amount of current coming in from the collector(for NPN) and the amount of current leaving the collector(for PNP).

So the base (B) is like a water tap which control the flow of water(equilivent to current) into a hose.

NPN and PNP can operate in 3 modes:

Cut-off mode:

• For NPN: No current flow into base(B), V_BE < 0.7 V, therefore no current flow from C to E. The junction CE is like a open circuit
• For PNP: No current flow out of the base(B),V_BE > -0.7 V , therefore no current flow from E to C. The junction EC is like a open circuit.

Saturation mode:

• For NPN: Current flow into base(B), V_BE approximately = 0.7 V, therefore current flow from C to E. The junction CE is like a short circuit. In reality, there is a slight voltage drop between C and E of approximately = 0.2 V.
• For PNP: Current flow out of the base(B),V_BE approximately = -0.7 V , therefore current flow from E to C. The junction EC is like a short circuit. In reality, there is a slight voltage drop between E and C of approximately = 0.2 V.

Active mode (amplifier):

• The transistor is like partial ‘On’ state.
• For NPN: Current flow into base(B), V_BE approximately = 0.7 V, therefore current flow from C to E. The difference with saturation mode is that during active mode I_B is smaller and also the relationships of the voltages at the 3 terminals are also different between the 2 modes.
• For PNP: Current flow out of the base(B),V_BE approximately = -0.7 V , therefore current flow from E to C. The difference with saturation mode is that during active mode I_B is smaller and also the relationships of the voltages at the 3 terminals are also different between the 2 modes.

The table below shows the voltages relationship for the 3 modes:

Mode	NPN	PNP
cut- off	VB < VE VB < VC	VB > VE VB > VC
saturation	VB > VE VB > VC	VB < VE VB < VC
active	VB > VE VB < VC	VB < VE VB > VC

Active mode current amplication:

Ok, now I will try to demo the transistor in active mode.