Till now we have discussed different regions of operation for a transistor. But among all these regions, we have found that the transistor operates well in active region and hence it is also called asÂ linear region. The outputs of the transistor are the collector current and collector voltages.

## Output Characteristics

When the output characteristics of a transistor are considered, the curve looks as below for different input values.

In the above figure, the output characteristics are drawn between collector currentÂ ICÂ and collector voltageÂ VCEÂ for different values of base currentÂ IB. These are considered here for different input values to obtain different output curves.

## Operating point

When a value for the maximum possible collector current is considered, that point will be present on the Y-axis, which is nothing but theÂ saturation point. As well, when a value for the maximum possible collector emitter voltage is considered, that point will be present on the X-axis, which is theÂ cutoff point.

When a line is drawn joining these two points, such a line can be called asÂ Load line. This is called so as it symbolizes the output at the load. This line, when drawn over the output characteristic curve, makes contact at a point called asÂ Operating point.

This operating point is also called asÂ quiescent pointÂ or simplyÂ Q-point. There can be many such intersecting points, but the Q-point is selected in such a way that irrespective of AC signal swing, the transistor remains in active region. This can be better understood through the figure below.

The load line has to be drawn in order to obtain the Q-point. A transistor acts as a good amplifier when it is in active region and when it is made to operate at Q-point, faithful amplification is achieved.

Faithful amplificationÂ is the process of obtaining complete portions of input signal by increasing the signal strength. This is done when AC signal is applied at its input. This is discussed in AMPLIFIERS tutorial.

When the transistor is given the bias and no signal is applied at its input, the load line drawn at such condition, can be understood asÂ DCÂ condition. Here there will be no amplification as the signal is absent. The circuit will be as shown below.

The value of collector emitter voltage at any given time will be

VCE=VCCâˆ’ICRCVCE=VCCâˆ’ICRC

As VCCÂ and RCÂ are fixed values, the above one is a first degree equation and hence will be a straight line on the output characteristics. This line is called asÂ D.C. Load line. The figure below shows the DC load line.

To obtain the load line, the two end points of the straight line are to be determined. Let those two points be A and B.

### To obtain A

When collector emitter voltage VCEÂ = 0, the collector current is maximum and is equal to VCC/RC. This gives the maximum value of VCE. This is shown as

VCE=VCCâˆ’ICRCVCE=VCCâˆ’ICRC
0=VCCâˆ’ICRC0=VCCâˆ’ICRC
IC=VCCRCIC=VCCRC

This gives the point A (OA = VCC/RC) on collector current axis, shown in the above figure.

### To obtain B

When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the VCC. This gives the maximum value of IC. This is shown as

VCE=VCCâˆ’ICRCVCE=VCCâˆ’ICRC
=VCC=VCC

(As ICÂ = 0)

This gives the point B, which means (OB = VCC) on the collector emitter voltage axis shown in the above figure.

Hence we got both the saturation and cutoff point determined and learnt that the load line is a straight line. So, a DC load line can be drawn.

The importance of this operating point is further understood when an AC signal is given at the input. This will be discussed in AMPLIFIERS tutorial.