Transistor Biasing

Transistor biasing

Biasing of a transistor means the establishment of the Q- point by connecting the transistor to external voltage sources via appropriate circuit.

Factors for which the Q- point may shift-

Q-point of a transistor may shift due to the following factors :

i) due to change in temperature

ii) due to change in VBE

iii) due to change in  leakage current (Ico)

iv) due to change in β.

Due to change in temperature thermal runaway may occur  as other may be adjusted. But the character leakage current is beyond our control.

Stability factor

The ratio of the change in collector current to the corresponding change in leakage current is known as Stability factor. i.e.

There are also another two stability factors,

Remember : A low value of stability factor (s) offers good stability to the Q- point of a transistor.

There are three types of biasing arrangement :

(1) Fixed bias

(2) Collector to base bias

(3) Self bias

Fixed bias

The above figure shows the fixed bios arrangement for an n-p-n transistor in CE mode. The source voltage Vcc makes the emitter- base junction forward biased and the collector- base junction reverse biased.

The power supply Vcc together wilt Rc and RB provide Suitable operating point. The capacitors C1 and C2 are used as  bypass action.

Stability factor calculation –

Apply KVL to the input loop,

As

therefore

i.e. base current of this circuit is primarily fixed as Vcc and RB is fixed , hence this circuit is called fixed a biased circuit.

Thus, collector current,

Differentiating ,

So, Stability factor,  S = β+1

1) Biasing circuit is very simple since its requires only one resistance.

2) By suitably changing the value of RB, the operating point can be fixed at any desired part of the active region.

1) The circuit is unstable , hence there is a finite possibility of thermal runaway.

Example:

i.e. Ic increases 101 times of Iço, due to a small change in Ico, hence thermal runaway may occur.

2) When transistor is replaced by another of the same type, operating point of the circuit shifts.

3) Stability is very low.

Collector to Base bias

The power supply together wilt RB and Rc provide suitable operating point. C1 and C2 are the bypass capacitor. To improve the stability of the fixed bias circuit, an additional resistance RB is connected between the collector and base of the transistor, hence the name collector to base bias circuit.

Stability factor calculation –

Apply KVL to the input circuit,

Collector current,

Differentiating with respect to Ico,

S < β+1 i.e. Stability factor of this circuit is less than that for fixed bias circuit.

Stability discussion as-

Suppose lc increases due to increase in temperature. The voltage drop across Rc increases consequently. Since Vcc is constant, VCE decreases which diminish the base current IB. Hence collector current lc decreases. Thus the circuit tries to counterbalance an increase in Ic and stabilise.

Though S < β+1  but  as RB >> RC . The improvement is marginal.

Self bias ( Emitter bias)

The power supply voltage Vcc along with R1, R2, RE and Rc provide the forward bias of the emitter- base junction and reverse bias of the collector- base junctions. C1, C2, C3 act as the bypass capacitors.

Stability factor calculation –

To make simple the above circuit, Thevenin’s theorem can be applied.

Then, Thevenin voltage,

Thevenin resistance,

Apply KVL to the input loop,

Thus collector current,

Differentiating with respect to Ico,

S << β+1 i.e. stability is high

Stability discussion as –

Due to rise in temperature if Ico increases, then Ic increases which enhances the current across RE Hence voltage drop across RE increases which decrease the forward bias of emitter junction. As a result the base current IB is lowered. This is in turn decreases the collector current Ic. Thus stability is increased.

For better stability

Emitter follower circuit

The voltage gain of the above circuit is close to unity. So the output voltage is approximately equal to the input voltage. The emitter voltage follows the input base voltage,hence the circuit is known as emitter follower circuit. The emitter follower circuit is common- collector amplifier circuit. Using hybrid model, we get –

Current gain –

and voltage gain

In terms of CE h- parameters :

Uses of emitter follower circuit :

1. The emitter follower as a CC amplifier is used as an impedance matching.

2. The emitter follower is used as buffer between high impedance source and a low impedance load with voltage gain of almost unity.