1.What is BJT – Bipolar Junction Transistor?
Bipolar junction transistor (BJT) is a bidirectional device that uses
both electrons and holes as charge carriers. While Unipolar transistor i.e. field effect
transistor uses only one type of charge carrier. BJT is a current controlled device. The
current flows from emitter to collector or from collector to emitter depending on the type
of connection. This main current is controlled by a very small current at the base
terminal.
2. Constructional Details
3. Why Transistor is called BJT ?
Bipolar transistors are a type of transistor composed of pn junctions, which are
also called bipolar junction transistors (BJTs). Whereas a field-effect transistor
is a unipolar device, a bipolar transistor is so named because its operation
involves two kinds of charge carriers, holes and electrons.
4. Sketch and explain various conduction regions of Bipolar Transistor?
The transistor can be operated in three modes:
1. Cut-off mode
2. Saturation mode
3. Active mode
In order to operate transistor in one of these regions, we have to supply dc
voltage to the npn or pnp transistor. Based on the polarity of the applied
dc voltage, the transistor operates in any one of these regions.
Applying dc voltage to the transistor is nothing but the biasing of transistor.
Active Mode
In the active mode, one junction (emitter to base) is forward biased and another
junction (collector to base) is reverse biased. In other words, if we assume two p-
n junctions as two p-n junction diodes, one diode will be forward biased and
another diode will be reverse biased. The active mode of operation is used for the
amplification of current.
In the cutoff mode, both the junctions of the transistor (emitter to base and
collector to base) are reverse biased. In other words, if we assume two p-n
junctions as two p-n junction diodes, both the diodes are reverse biased in
cutoff mode. We know that in reverse bias condition, no current flows
through the device. Hence, no current flows through the transistor.
Therefore, the transistor is in off state and acts like an open switch.
Cut off Mode
The cutoff mode of the transistor is used in switching operation for switch
OFF application.
Saturation Mode
In saturation mode, free electrons (charge carriers) flows from emitter to
base as well as from collector to base. As a result, a huge current will flow
to the base of transistor.
Therefore, the transistor in saturation mode will be in on state and acts like
a closed switch.
5. What is biasing of Transistor ?
The proper flow of zero signal collector current and the maintenance of proper
collectoremitter voltage during the passage of signal is known as Transistor
Biasing. The circuit which provides transistor biasing is called as Biasing Circuit.
Biasing is the process of providing DC voltage which helps in the functioning of
the circuit. A transistor is based in order to make the emitter base junction
forward biased and collector base junction reverse biased, so that it maintains in
active region, to work as an amplifier.
6. Sketch and explain Transistor working as Amplifier?
When a weak a.c. signal is applied to the base of the transistor, a small base
current starts flowing in the input circuit. Due to transistor action, a much larger
(β times the base current) a.c. current flows through the the load Rc in the output
circuit. Since the value of load resistance Rc is very high, a large voltage will drop
across it. Thus, a weak signal applied in the base circuit appears in amplified form
in the collector circuit. In this way the transistor acts as an amplifier.
Operation. During the positive half-cycle of the signal, the forward bias across the
emitter-base junction is increased. Therefore, more electrons flow from the
emitter to the collector via the base. This causes an increase in collector current.
The increased collector current produces a greater voltage drop across the
collector load resistance RC. However, during the negative half-cycle of the signal,
the forward bias across emitter-base junction is decreased. Therefore, collector
current decreases. This results in the decreased output voltage (in the opposite
direction). Hence, an amplified output is obtained across the load.
7. What is DC Load Line? What is significance of DC Load Line?
When a value for the maximum possible collector current is considered, that
point will be present on the Y-axis, which is nothing but the 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 the cutoff point.
When a line is drawn joining these two points, such a line can be called as Load
line.
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 as DC condition. Here there will
be no amplification as the signal is absent.
In BJT analysis, a load line is a line drawn over collector curves to show every
possible operating point of a transistor.
The DC load line contains possible DC values of IC and VCE for a given amplifier.
8. In the circuit diagram shown if VCC = 12V and RC = 6 kΩ, draw the d.c. load
line. What will be the Q point if zero signal base current is 20μA and β = 50 ?
The collector-emitter voltage VCE is given by :
When IC = 0, VCE = VCC = 12 V. This locates the point B of the load line.
When VCE = 0, IC = VCC / RC = 12 V/6 kΩ = 2 mA.
This locates the point A of the load line. By joining these two points, load line AB
is constructed as shown in 15.
9. What is Q point or Operating Point ? What is significance of Q Point ?
The point which is obtained from the values of the IC (collector current) or
VCE (collector-emitter voltage) when no signal is given to the input is known as
the operating point or Q-point in a transistor. It is called operating point because
variations of IC (collector current) and VCE (collector-emitter voltage) takes place
around this point when no signal is applied to the input.
The position of the Q-point depends on the applications of the transistor. If the
transistor is used as a switch then for open switch the Q-point is in the cutoff
region, and for the close switch, the Q-point is in the saturation region. The Q-
point lies in the middle of the line for the transistor which operates as an
amplifier.
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 amplification 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.
10. Draw a single stage Common Emitter amplifier? Explain working?
C2 & C3 are coupling capacitors prevent the source and Load affecting from dc
biasing of Transistor. It provide the AC coupling and DC isolation between the
stages of an amplifier .By pass capacitor it provide the AC ground at the emitter
terminals of the transistor. This is effect of increasing the circuit voltage gain.R3 &
R2 are voltage divider circuit to fed the voltage to Base of the transistor.RE is
increase linearity & stability.R4 is the output resistance or connected the load
resistance. Emitter Bypass Capacitor (CE) : An emitter bypass capacitor of value
100 μF is used in parallel with RE to provide a low reactance path to the amplified
a.c. signal. If this capacitor is not connected in the output circuit then the
amplified a.c. signal will flow through RE and cause a voltage drop across it,
thereby reducing the output voltage.
11. What are the applications of Transistor?
If the circuit uses the Transistor as a Switch, then the biasing is arranged to
operate in the output characteristics curves seen previously in the areas known as
the "Saturation" and "Cut-off" regions
It can be used as amplification device to amplify current or voltage.
It can be used as switching device in analog or digital circuits.
It is used as regulator for voltage or current or power.
It is used as building blocks of ICs.
It is used as buffer or impedance matching purpose between mismatched circuits.
BJT transistor is used as oscillator, switch and amplifier. It is also used in logic
circuits. It is found in mobile phones, TV, radio transmitters, industrial control etc.
12. Explain various configuration of NPN Transistor?
As the Bipolar Transistor is a three terminal device, there are basically three
possible ways to connect it within an electronic circuit with one terminal being
common to both the input and output signals. Each method of connection
responding differently to its input signal within a circuit as the static
characteristics of the transistor vary with each circuit arrangement.
Common Base Configuration – has Voltage Gain but no Current Gain.
Common Emitter Configuration – has both Current and Voltage Gain.
Common Collector Configuration – has Current Gain but no Voltage Gain.
13. What is UJT ?
Unijunction Transistor is a semiconductor switching device having 2 layers and
3 terminals and is abbreviated as UJT. It has the ability to limit large power with
a small input signal . Its structure is almost similar to an N-channel JFET. UJT
consists of a lightly doped N-type silicon bar in which a P-type material is
diffused thus producing PN junction. Due to the existence of a single PN
junction, it is termed as a Unijunction device.
14. Sketch and explain working of UJT ? Draw the Characteristic Curve?
Initially when VE is zero, some reverse current IE flows until, the value of VE
reaches a point at which
VE=ηVBBVE=ηVBB
This is the point where the curve touches the Y-axis.
When VE reaches a voltage where
VE=ηVBB+VD At this point, the diode gets forward biased.
The voltage at this point is called as VP (Peak Voltage) and the current at this
point is called as IP (Peak Current). The portion in the graph till now, is termed
as Cut off region as the UJT was in OFF state.
Now, when VE is further increased, the resistance RB1 and then the voltage
V1 also decreases, but the current through it increases. This is the Negative
resistance property and hence this region is called as Negative resistance
region.
Now, the voltage VE reaches a certain point where further increase leads to the
increase in voltage across RB1. The voltage at this point is called as VV (Valley
Voltage) and the current at this point is called as IV (Valley Current). The region
after this is termed as Saturation region.
15. List the applications of UJT ?
UJT can be used to build simple oscillators like relaxation oscillators.
UJT can also be used as trigger device for SCR (Silicon control rectifier) and
TRIACS (Triode for Alternating current).
Also used as non-sinusoidal oscillators, saw tooth generators, phase control
and timing circuit.
16. Draw and explain UJT as Relaxation Oscillator?
17. Draw and explain UJT as Triggering circuit?
18. Sketch and Explain working of JFET ?
When a reverse voltage (VGS ) is applied between the gate and source, the
width of the depletion layers is increased. This reduces the width of conducting
channel, thereby increasing the resistance of n-type bar. Constantly, the
current from source to drain is decreased. On the other hand, if the reverse
voltage on the gate is decreased, the width of depletion layers also decreases.
This increases the width of conduction channel and hence source to drain
current. Thus, it is obvious that the current from source to drain can be
controlled by the application of potential on the gate. That is why, the device
is known as field effect transistor (FET).
19. Draw and explain JFET as Common Emitter Amplifier?
The weak signal is applied between gate and source and
amplified output is obtained in the drain-source circuit. For
the proper operation of JFET, the gate must be negative
w.r.t. source i.e., input circuit should always be reverse biased.
In the present case, we are providing biasing by the battery
VGG.
A small change in the reverse bias on the gate produces a
large change in drain current. This fact makes JFET capable of
raising the strength of a weak signal. During the positive half
of signal, the reverse bias on the gate decreases. This
increases the channel width and hence the drain current.
During the negative half-cycle of the signal, the reverse voltage
on the gate increases. Consequently, the drain current
decreases. The result is that a small change in voltage at the
gate produces a large change in drain current. These large
variations in drain current produce large output across the
load RL. In this way, JFET acts as an amplifier.
JFET as Amplifier
20. List the difference between JFET and BJT ?
iv. A BJT uses a current in to base to control a large current between collector and
emitter where as JFET uses voltage on the gate terminal to control the current
between Drain and source.
21. Draw and explain N channel MOSFET?
When no voltage is applied between gate and source, some current flows due to
the voltage between drain and source. Let some positive voltage is applied at VGG.
Then the minority carriers i.e. holes, get repelled and the majority carriers i.e.
electrons gets attracted towards the SiO2 layer. With some amount of positive
potential at VGG a certain amount of drain current ID flows through source to
drain. When this positive potential is further increased, the current ID increases
due to the flow of electrons from source and these are pushed further due to the
voltage applied at VGG. Hence the more positive the applied VGG, the more the
value of drain current ID will be. The current flow gets enhanced due to the
increase in electron flow better than in depletion mode. Hence this mode is
termed as Enhanced Mode MOSFET.
22. What are Applications of MOSFET?
Amplifiers
Regulation for DC Motors
Constructions of Chopper Amplifiers
Switching and Amplifying Signals
Due to high switching speed, it can be used in chopper circuits.
23. Draw a DC Motor speed control by MOSFET?
24. What is difference between Enhancement & Depletion MOSFET ?
There is not much more difference in the construction. You can see in the above
diagram, the depletion MOSFET has a permanent fabricated channel between the
Source and Drain but in the enhancement MOSFET, there is no channel. It is the
only constructional difference between D-MOSFET and E-MOSFET.
25. What is Gain of amplifier ?
The gain of an amplifier is defined as the measure this ability of amplifying a
signal. Gain is a ratio so it does not have any unit. If an amplifier has an input
signal of 0.1 Vpp (peak-peak volts) and has an output signal of 10 Vpp, the voltage
gain is: Δv = 10 Vpp / 0.1 Vpp = 100.
26.Determine the Voltage, Current and Power Gain of an amplifier that has an
input signal of 1mA at 10mV and a corresponding output signal of 10mA at 1V.
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