Load line , Operating point and Transistor biasing

Load line

In graphical analysis , a load line is a line drawn on the characteristic curve, a graph of the current vs the voltage in a nonlinear device such as a diode or a transistor. It describes the constraint implied on the voltage and current in the nonlinear device by the external circuit. The load line, usually a straight line, presents the response of the linear part of the circuit, connected to the nonlinear device. The points where the characteristic curve and the load line intersect are the  operating point. (Q points) of the circuit; at these points the current and voltage parameters of both parts of the circuit match properly.

Operating point

The operating point is a particular point within the operation characteristic of a technical device. This point will be engaged by virtue of the system and the outside influences and parameters. In electronic engineering establishing an operating point is known as the biasing.

Transistor biasing

Transistor Biasing is the process of setting a transistor as DC operating voltage or current conditions to the exact level so that any AC input signal can be amplified correctly by the transistor.

What do you mean by  d.c. load line ? Draw the d.c. load line for the CE circuit.

Ans-  For drawing the d.c. load line of a transistor, we have to know only its cut-off and saturation point. It is a straight line that is joining these two points.

To draw the d.c. load line we take the transistor amplifier circuit in CE configuration with d.c. operation :

Applying KVL to the output circuit –

…………. 1

It is the linear equation similar to

Thus the graph of equation 1 is a Straight line whose slope

Consider the two following particular cases –

cut -off point.

saturation point.

Thus load line is a straight line passing two points Vcc at VCE ordinate and VCC/RL ordinate. Also load line can be drawn if only VCC and RL are known.

 

What do you by Quiescent point or Q-point or Operating point?

Quiescent point is a point on the d.c. load line, which represents the values of Ic, and VCE that exist in a transition circuit when no input signal is applied.

Usually, the Q-point – RL is chosen in the active region of a transistor near the centre of the load line i.e.

The intersecting point by a d.c. load line on the output characteristics curve of a transistor are Q-points.

 

Significance of Q-point –

The Q point is needful to the overall component and circuit functionality. It confirms that non-linear components (diodes) operate at their optimal current and voltage throughout the operating range. This also promotes increased functionality, reliability, and life cycle of any electronic circuits.

Position of lithe Q-point on the d.c. load line deter the maximum undistorted output.

Explanation –

i) When Q-point is located near cut off point signal first starts to clip at A. It is called cut-off- clipping because the +ve half cycle of the signal drives the transistor to cut-off.

ii) When Q-point is located near saturation point, then clipping first starts at point B. It is caused by saturation.

iii) When Q-point is located at the centre of the load line, we get maximum possible output signal.

What do you mean by transistor biasing ?

Transistor biasing –

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

The choice of Q- point is determined by —

I) The available supply voltage.

II) The load resistance of the amplifier.

III) The amplitude of the signal that have to be amplified.

IV) The allowed distortion of the output signal.

 

Why biasing of a a transistor is necessary?

For normal operation of a transistor amplifier circuit, it is essentials that –

a) Forward bias on the emitter-base junction.

b) Reverse bias on the collector- base junction.

Amount of bias required which is very important for establishing the Q- point which is dictated by the mode of operation needed.

If the transistor is not biased correctly then it would

i) work inefficiently.

ii) produce distortion in the output signal.

What are the factors that affect the bias stability of transistor ?

The Q- point and hence, bias stability of a transistor may shift due to the following factors –

a) due to change in temperature.

b) due to change in VBE.

c) due to change in VCE.

d) due to change in β.

 

Define stability factors ?

Stability factors –

Stability factor is defined as ‘the rate at which collector current changes when Base to emitter voltage changes, keeping base current constant’. i.e.

The ratio of the change in collector current to the corresponding change in leakage current when β and IB held constant i.e.

The stability factor

Originally, a low value of stability factor, offers good stability of a point offers good stability of Q- point.

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ELECTRONICS | Transistor Biasing