Electronics - Bipolar Junction Transistors (BJT) - Discussion

Re provides negative feedback hence the stability increase.

Addition of Re to fixed bias called emitter bias

Re increases stability it can be explained easily by mathematically :
Ib =(Vcc-Vbe)\(Rb+Re(1+Beta))

Ic=Beta*Ib

Choose different values of Beta variation in Ic and Vcc less compared to FIXED BIAS: where Beta is temp sensitive

By adding an emitter resistance reduces the changes occurred to the parameters like Ic, Vce due to the change in beta with temperature.

Suppose without emitter resistance, with change in temperature beta changes from 100 to 200 and due to this change Ic changes from 1.2 mA to 3.2 mA.

But when emitter resistance is added the change in Ic is less say from 1.2 mA to 1.5 mA.

Similarly with the other parameters like Ib, Vce etc. Also, the higher the value of emitter resistance, the lesser is the change and hence it stabilizes the transistor circuit.

When emitter resistance is there in the circuit. We will say circuit in stable. Because if temperature changes the operation of bjt also change it happens when emitter resistance is not there. To the bjt operation should be same at any time. We will use the emitter resistance.

The emitter resistor provides stable biasing to the BJT. It also decreases gain and increases the input impedance.