In the output circuit, the load equation can be written as

V_CE = V_CC- I_C R_C

This equation involves two unknown V_CE and I_C and therefore can not be solved. To solve this equation output characteristic ( I_Cvs V_CE) is used.

The load equation is the equation of a straight line and given by two points:

I_C= 0,         V_CE = V_CC

&            V_CE = 0,        I_C= V_CC / R_C

The intersection of this line which is also called dc load line and the characteristic gives the operating point Q as shown in fig. 3.

Fig. 3

The point at which the load line intersects with I_B = 0 characteristic is known as cut off point. At this point base current is zero and collector current is almost negligibly small. At cut off the emitter diode comes out of forward bias and normal transistor action is lost. To a close approximation.

V_CE ( cut off) » V_CC (approximately).

The intersection of the load line and I_B = I_B(max) characteristic is known as saturation point . At this point I_B= I_B(max), I_C= I_C(sat). At this point collector diodes comes out of reverse bias and again transistor action is lost. To a close approximation,

I_C(sat) » V_CC / R_C(approximately ).

The I_B(sat) is the minimum current required to operate the transistor in saturation region. If the I_B is less than I_{B (sat)}, the transistor will operate in active region. If I_B > I_{B (sat)} it always operates in saturation region.

If the transistor operates at saturation or cut off points and no where else then it is operating as a switch is shown in fig. 4.

Fig. 4

V_BB = I_B R_B+ V_BE

I_B = (V_BB – V_BE ) / R_B

If I_B> I_B(sat), then it operates at saturation, If I_B = 0, then it operates at cut off.

If a transistor is operating as an amplifier then Q point must be selected carefully. Although we can select the operating point any where in the active region by choosing different values of R_B & R_C but the various transistor ratings such as maximum collector dissipation P_C(max) maximum collector voltage V_C(max) and I_C(max) & V_BE(max) limit the operating range.

Once the Q point is established an ac input is connected. Due to this the ac source the base current varies. As a result of this collector current and collector voltage also varies and the amplified output is obtained.

If the Q-point is not selected properly then the output waveform will not be exactly the input waveform. i.e. It may be clipped from one side or both sides or it may be distorted one.

GOTO >> 1 || 2 || 3|| Home