Electronics - Bipolar Junction Transistors (BJT) - Discussion

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

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.

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.

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

Re provides negative feedback hence the stability increase.

The cause of the emitter resistor is that it stabilize the current and gain against temperature rise, also against aging, and replacement, that if the temperature include so Ic will increase according to a relation directly proportional to T^ (3/2) , then after analyzing the emitter base junction (taking a kvl) we'll find that Ib dec so Ic = beta*Ib so it will then decrease so we have a variable current gain.

THe stability of the amplifier is achieved when a negative feedback in given to limit the gain of the amplifier. RE gives a negative feedback and hence it stabilises the circuit.

Emitter resistor provides stabilization against temperature or due to transistor parameters.

In general, we stabilise the model with the help of self biasing technique, the other method is to place a resistor Re, in the feedback path.

TO bypass the ac signal we use capacitor, to bias the collector we have to give a signal to base, to increase gain we have to modified the tang circuit. So there is last option is to stabilization.