Biasing Techniques for CE Amplifiers

In output circuit, load equation can be given as

V_CE = V_CC- I_C R_C

This equation includes 2 unknown V_CE and I_C  and hence cannot be solved. To solve this equation output characteristic ( I_C vs V_CE) is used.

The load equation is equation of the straight line and given by the 2 points: I_C= 0, V_CE = V_CC & V_CE = 0, I_C= V_CC / R_C

The intersection of this line which is called as DC load line and the characteristic gives the operating point Q as shown in the figure 1.

Figure 1

The point at which load line intersects with I_B = 0 characteristic is called as cut off point. At this point the base current is zero and collector current is negligibly small. At cut off emitter diode comes out of the forward bias and normal transistor action is lost. To the 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 come 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 minimum current needed to operate transistor in the saturation region. If the I_B is less than I_B (sat), transistor will operate in the active region. If I_B > I_B (sat) it always operates in the saturation region.

If the transistor operates at the saturation or cut off points and nowhere else then it is operating as a switch which is shown in the figure 2.

Fig.2

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 the saturation, If I_B = 0, then it operates at the cut off.

If the transistor is operating as an amplifier then Q point should be selected carefully. However we can select operating point anywhere in the active region by selecting different values of R _B & R_C but various transistor ratings like maximum collector dissipation P_C(max) maximum collector voltage V_C(max) and I_C(max) & V_BE(max) limit operating range.

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

If Q-point is not selected properly then output waveform will not be exactly the input waveform. That is, it may be clipped from one side or both the sides and it can be distorted one.

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