Download 1.Explain about fixed bias of BJT and derive the

EC6304-Electronics Circuits-I
Part-B-Unit-I
1.Explain about fixed bias of BJT and derive the expression for stability factor .
This form of biasing is also called base bias. In the example image on the right, the single power
source (for example, a battery) is used for both collector and base of a transistor, although
separate batteries can also be used.
In the given circuit,
Vcc = IBRB + Vbe
Therefore,
IB = (Vcc - Vbe)/RB
For a given transistor, Vbe does not vary significantly during use. As Vcc is of fixed value, on
selection of RB, the base current IB is fixed. Therefore this type is called fixed bias type of circuit.
Also for given circuit,
Vcc = ICRC + Vce
Therefore,
Vce = Vcc - ICRC
The common-emitter current gain of a transistor is an important parameter in circuit design, and
is specified on the data sheet for a particular transistor. It is denoted as β on this page.
Because
IC = βIB
we can obtain IC as well. In this manner, operating point given as (Vce,IC) can be set for given
transistor.
Merits:
 It is simple to shift the operating point anywhere in the active region by merely changing
the base resistor (RB).
 A very small number of components are required.
Demerits:
 The collector current does not remain constant with variation in temperature or power
supply voltage. Therefore the operating point is unstable.
 Changes in Vbe will change IB and thus cause IE to change. This in turn will alter the gain
of the stage.
 When the transistor is replaced with another one, considerable change in the value of β
can be expected. Due to this change the operating point will shift.
 For small-signal transistors (e.g., not power transistors) with relatively high values of β
(i.e., between 100 and 200), this configuration will be prone to thermal runaway. In
particular, the stability factor, which is a measure of the change in collector current with
changes in reverse saturation current, is approximately β+1. To ensure absolute stability
of the amplifier, a stability factor of less than 25 is preferred, and so small-signal
transistors have large stability factors.
2.Explain about voltage divider bias of BJT and derive the expression for stability factor.
Voltage divider bias is the most popular and used way to bias a transistor. It uses a few
resistors to make sure that voltage is divided and distributed into the transistor at correct
levels. One resistor, the emitter resistor, RE also helps provide stability against variations in β
that may exist from transistor to transistor.
The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forward
biases the emitter junction. By proper selection of resistors R1 and R2, the operating point of the
transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage
fixed independent of base current provided the divider current is large compared to the base
current. However, even with a fixed base voltage, collector current varies with temperature (for
example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with
emitter resistor.
In this circuit the base voltage is given by:
voltage across
provided
.
Also
For the given circuit,
Merits:



Unlike above circuits, only one dc supply is necessary.
Operating point is almost independent of β variation.
Operating point stabilized against shift in temperature.
Demerits:

In this circuit, to keep IC independent of β the following condition must be met:
which is approximately the case if
where R1 || R2 denotes the equivalent resistance of R1 and R2 connected in parallel.

As β-value is fixed for a given transistor, this relation can be satisfied either by keeping
RE fairly large, or making R1||R2 very low.
o If RE is of large value, high VCC is necessary. This increases cost as well as
precautions necessary while handling.
o If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1 raises
VB closer to VC, reducing the available swing in collector voltage, and limiting
how large RC can be made without driving the transistor out of active mode. A
low R2 lowers Vbe, reducing the allowed collector current. Lowering both resistor
values draws more current
3.Explain about the compensation technique to stabilize Q point using diode and thermistor.
Compensation techniques refer to the use of temperature sensitive devices such as diodes, transistors,
thermistors etc. which provide compensating voltages and currents to maintain the operating point
constant. In comparison, stabilization technique use only resistive biasing circuits.
Due to bias circuit, feedback is there and it reduce drastically the amplification of the signal. If this loss in
signal gain is intolerable in particular application, it is often possible to use compensating techniques to
reduce the drift of the operating point.
Diode compensation for
is shown.
: A circuit utilizing self bias stabilizing technique and diode compensation
The diode is kept biased in the forward direction by the source
same material and type of transistor, the voltage
coefficient as the base to emitter voltage
and resistance
. If the diode is of
across the diode will have the same temperature
. If KVL is applied around the base circuit then,
Since
tracks
compensation of
w.r.t. temperature, then
will be insensitive to variations in
. In practice, the
is not like this exact, but it is sufficiently effective to take care of great part of
transistor drift due to variations in
Diode Compensation for
: The changes of
with temperature contribute significantly to
changes in collector current of Si transistors. On the other hand, for Ge transistors changes in
with
temperature play the more important role in collector current stability. The compensation circuit is
shown.
It offers stabilization against variations of
If diode and the transistor are of same type and material, The reverse saturation current
of the diode
will increase with temperature at the same rate as the transistor collector saturation current
. Now
Since the diode is reverse biased by an amount
through D is
. The base current
.
for Ge devices, it follows that the current
If
> > 1 and if D and
of transistor track each other over the desired temperature range, then
remains essentially constant.
4.Write the factors that affect the stability of Q point and explain how self bias circuit acts as a
constant current circuit?
It is desirable that once selected, the operating (or Q) point should remain stable i.e. the
operating point should not shift its position owing to change in temperature etc. Unfortunately it
is not possible in practice unless special efforts are made to achieve it. The maintenance of the
operating point stable is called the stabilization.
The stabilization of operating point is essential because of
a) Temperature dependence of IC
(b) Individual variations and
(c) Thermal runaway.
With the increase in temperature, the collector leakage current ICO, the current gain ß tend to
increase and VBE required to produce a given collector current IC tends to decrease. Thus increase
in temperature tends to cause increase in IC.
The value of ß and VBE are not exactly the same for any two transistors even of the same type. So
when a transistor is replaced by another one (even of the same type) the operating point (zero
signals IC and VCE) is shifted.
The collector current IC, being equal to ßIB + (1+ ß ) ICO, increases with the increase in
temperature. This leads to increased power dissipation with further increase in temperature.
Being a cumulative process, it can lead to thermal runaway resulting in burn out of the transistor.
However, if by some modification, IC is made to fall with increase in temperature automatically,
then decrease in the term ß can be made to neutralize the increase in the term (1 +ß ) ICO, thereby
keeping IC almost constant. This will achieve thermal stability resulting in bias stability.
The biasing network associated with the transistor should fulfill the requirements of (i) ensuring
proper zero signal collector current, (ii) ensuring VCE not falling below 0.5 V for Ge transistors
and 1 V for Si transistors at any instant and (iii) ensuring stabilization of operating point (zero
signal IC and VCE)
5. Potential divider biasing from
Given
= 15 V is obtained for an npn transistor.
Determine the operating point when
(a)
= 125 and
(b)
is doubled and comment on stability of operating with change in
.
Ans.
6. Determine the resistor
stability factors?
Ans. For fixed bias;
For collector to base bias;
for a fixed bias and collector to base bias and compare the two
The stability factor is lesser using collector to base bias than fixed bias. Hence collector to base bias is
more stable.
7. 3. A transistor with  = 50, Vbe = 0.7V, Vcc = 22.5V & Rc = 5.6K is used in a biasing circuit.
It is designed to establish the quiescent point at Vce = 12V, Ic = 1.5mA, S = 3. Find the
values of RE, R1 & R2.
8. Explain the biasing technique of enhancement type MOSFET