EARLY EFFECT

It is the variation in the effective width of the base in

a bipolar junction transistor (BJT) due to a variation in the

applied base-to-collector voltage. A greater reverse bias

across the collector–base junction, for example, increases the

collector–base depletion width, thereby decreasing the width

of the charge carrier portion of the base.

Under increased collector–base reverse bias, the lower panel of

Figure 1 shows a widening of the depletion region in the base

and the associated narrowing of the neutral base region.

The collector depletion region also increases under reverse

bias, more than does that of the base, because the collector is

more heavily doped relative to the base. The principle

governing these two widths is charge neutrality. The

narrowing of the collector does not have a significant effect as the collector

is much longer than the base. The emitter–base junction is unchanged because the emitter–base

voltage is the same.

Base-narrowing has two consequences that affect the current:

There is a lesser chance for recombination within the "smaller" base region.

The charge gradient is increased across the base, and consequently, the current of minority

carriers injected across the emitter junction increases.

TRANSISTOR BIASING AND STABILIZATION

What is the need for biasing?

In order to produce distortion free output in amplifier circuits, the supply voltages and resistances

establish a set of dc voltage VCEQ and ICQ to operate the transistor in the active region. These voltages

and currents are called quiescent values which determine the operating point or Q-point for the

transistor. The process of giving proper supply voltages and resistances for obtaining the desired Q-

Point is called Biasing. The circuits used for getting the desired and proper operating point are known

as biasing circuits. To establish the operating point in the active region biasing is required for

transistors to be used as an amplifier. For analog circuit operation, the Q-point is placed so the

transistor stays in active mode (does not shift to operation in the saturation region or cut -off region)

when input is applied. For digital operation, the Q-point is placed so the transistor does the contrary

- switches from "on" to "off" state. Often, Q-point is established near the center of active region of

transistor characteristic to allow similar signal swings in positive and negative directions. Q-point

should be stable. In particular, it should be insensitive to variations in transistor parameters (for

example, should not shift if transistor is replaced by another of the same type), variations in

temperature, variations in power supply voltage and so forth. The circuit must be practical: easily

implemented and cost -effective.

DC OPERATING POINT (OR) QUIESCENT POINT:

Application of DC voltages (bias) establishes a fixed level of current and voltage. For

transistor amplifiers the resulting DC current and voltage establish an operating point on the

characteristics that define the region that will be employed for amplification of the applied signal.

Since the operating point is a fixed point on the characteristics, it is called as Quiescent point (Q -

Point).

DC LOAD LINE

The Fig 3.5shows the output

characteristics of a common emitter

configuration with points A and B,

and line drawn between them.The

line drawn between points A and B

is called d.c. load line. The 'd.c.'

word indicates that only d.c.

conditions are considered, i.e. input

signal is assumed to be zero.

The d.c. load line is a plot

of IC versus VCE. For a given value

of RC and a given level of VCC

Significance of Q point:

The intersection of curves of

different values of IB with d.c. load

line gives different operating points.

For different values of IBIB, we

have different intersection points (quiescent point or Q point) such as P, Q and R. The Q-point tends

to shift its position due to any or all of the following three main factors:

Reverse saturation current, ICO, which doubles for every 10 °C increase in temperature.

Base-emitter voltage, VBE, which decreases by 2.5 mV per °C.

Transistor current gain, β i.e., hfe which increases with temperature.

AC load line:

When no signal is applied, the transistor voltage and current conditions are

as indicated at the quiescent point (Q point) on the dc load line. When an

ac signal is applied, the transistor voltage and current vary above and below

point Q.

Therefore, point Q is common to both the ac and the dc load lines. Starting

from the Q point, the ac load line is drawn by taking a convenient collector

current change

Consider the dc equivalent circuit of common emitter amplifier fig. 1.

When considering the ac equivalent circuit, the output

impedance becomes RC || RL which is less than RC .

In the absence of ac signal, this load line passes through Q

point. Therefore ac load line is a line of slope (-1 / ( RC ||

RL) ) passing through Q point. Therefore, the output

voltage fluctuations will now be corresponding to ac load

line as shown in fig. 2. Under this condition, Q-point is

not in the middle of load line, therefore Q-point is selected

slightly upward, means slightly shifted to saturation side.

SELECTION OF OPERATING POINT FOR A.C

INPUT SIGNAL:

The operating point can be selected at three different positions on the DC load line:

-off region

NEAR SATURATION REGION:

If the biasing circuit is designed to fix the Q – point very near to saturation region, the

collector current is clipped at the positive half cycle.

Thus because of the distortions present at the collector current, point P is not the suitable

operating point.

NEAR CUT-OFF REGION:

If the biasing circuit is designed to fix the Q – point near cut-off region, the collector current

is clipped at the negative half cycle.

Thus Q-point is also not a suitable operating point.

AT ACTIVE REGION:

If the Q – point is fixed at the centre of the active region, the output signal is sinusoidal

waveform without any distortion. Thus the point Q is the best operating point.

THERMAL RUNAWAY.

THERMAL RUN AWAY:

Collector current causes junction temperature to rise, which in term rises ICBO rise in Ic. This

cumulative process leads to collector current to increase further and transistor may be

destroyed. This phenomenon is called thermal Run away.

There are several approaches to mitigate bipolar transistor thermal runaway. For example,

Negative feedback can be built into the biasing circuit so that increased collector current

leads to decreased base current. Hence, the increasing collector current throttles its source.

Heat sinks can be used that carry away extra heat and prevent the base–emitter temperature

from rising.

The transistor can be biased so that its collector is normally less than half of the power supply

voltage, which implies that collector–emitter power dissipation is at its maximum value.

Runaway is then impossible because increasing collector current leads to a decrease in

dissipated power.

STABILITY FACTOR (S)

The extent to which the collector current IC is stabilized with varying Ico is measured by stability

factor S.

It is defined as the rate of change of collector current to the change in Ico, keeping IB and β as

constant.

FIXED BIAS

COLLECTOR EMITTER FEEDBACK BIAS

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1+𝛽(𝑅𝑐+𝑅𝑒

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