imp for mid semister examinations

1. Digital Logic Design
Introduction to Analog devices and circuits

2. Content
• Diode
• Transistor
• FET
• Applications

3. Transistors
• Transistor, semiconductor device for amplifying, controlling, and
generating electrical signals.
• Transistors are the active components of integrated circuits, or
“microchips,”
• They are the nerve cells of the information age.
• There are typically three electrical leads in a transistor, called the
emitter, the collector, and the base.

4. BJT- Bipolar Junction Transistor
Biased npn Transistor
IE = IB + IC

5. Biased npn Transistor
• Electrons begin to inject into the emitter region - forward applied voltage VBE.
• The electrons in this region have sufficient energy to overcome the emitter-base
junction's barrier potential and reach the base region.
• base region is very thin and doped lightly.
• So, only a few electrons combine with the holes once they reach their
destination.
• electrons begin to drift at the collector region due to the very thin base region
and the reverse voltage at the collector-base junction.
• The electrons begin to move towards the collector as recombined holes and
electrons become separated from one another.
• A very small base current also flows through the device as a result of this
movement.

6. Applications of npn Transistors
• High-frequency applications
• Switching applications are where NPN transistors are most commonly
used.
• To amplify weak signals, it's used in Darlington pair circuits.
• NPN transistors are used in applications where a current sink is
required.
• Some classic amplifier circuits, such as ‘push-pull' amplifier circuits,
make use of this component.
• In temperature sensors,
• In logarithmic converters, this variable is used.

7. Transistor Configurations – CB
When the emitter voltage is applied, as it is forward biased, the electrons from the negative terminal repel the
emitter electrons and current flows through the emitter and base to the collector to contribute collector
current. The collector voltage VCB is kept constant throughout this.
In the CB configuration, the input current is the emitter current IE and the output current is the collector
current IC.
The ratio of change in collector
current $ΔIC$ to the change in
emitter current $ΔIE$ when
collector voltage VCB is kept
constant, is called as Current
amplification factor.
It is denoted by α.
α=ΔIC/ΔIE at constant VCB

8. Applications of CB configuration
• This Configuration provides good stability against increase in
temperature.
• The CB configuration is used for high frequency applications.

9. Transistor Configuration – CE
• Just as in CB configuration, the emitter junction is forward biased and the collector junction is
reverse biased.
• The flow of electrons is controlled in the same manner.
• The input current is the base current IB and the output current is the collector current IC here.
• In CE configuration, by keeping the base current IB constant, if VCE is varied, IC increases nearly to
1v of VCE and stays constant thereafter. This value of VCE up to which collector current IC changes
with VCE is called the Knee Voltage.
The ratio of change in collector
current $ΔIC$ to the change in base
current $ΔIB$ is known as Base
Current Amplification Factor.
It is denoted by β
β=ΔIC / ΔIB

10. Application of CE configuration
• CE configuration is usually used for bias stabilization methods and
audio frequency applications.

11. Transistor Configuration - CC Configuration
• the emitter junction is forward biased and the collector junction is reverse biased.
• The flow of electrons is controlled in the same manner.
• The input current is the base current IB and the output current is the emitter current IE here.
• This configuration works as non-inverting amplifier output.
The ratio of change in emitter current ΔIE to the change in
base current ΔIB is known as Current Amplification factor in
common collector CC configuration.
It is denoted by γ.
γ=ΔIE / ΔIB

12. Operating Characteristics of Transistor
• Cut Off Region −
• Both the junctions of BJT are reverse biased
• operating conditions of the transistor are as follows
• − input base current (IB) is equal zero, hence the zero output collector current (IC).
• The collector – emitter voltage (VCE) is maximum.
• This results in a large depletion layer on the junctions of the transistor and no current
can flow through the device.
• transistor operates as Open Switch i.e. fully – off.

13. Operating condition of Transistor
• Saturation Region −
• Both the junctions of the BJT are forward biased,
• The base current can be applied to its maximum value which results in
maximum collector current.
• Due to forward biased junctions the width of depletion layer is as small as
possible causing minimum collector – emitter voltage drop.
• Therefore current flowing through the transistor having maximum value, thus
the transistor is operated as Closed Switch i.e. fully – ON.

14. Transistor as a Switch
• If the transistor is operated in the saturation region then it acts as
closed switch and when it is operated in the cut off region then it
behaves as an open switch.
• When a zero input signal applied to the base of the transistor, it acts
as an open switch. If a positive signal applied at the input terminal
then it acts like a closed switch.

15. Field Effect Transistor
• Unipolar Transistor
• Used as a fast operating switch
• some important points to remember about FET −
• Gate − By using diffusion or alloying technique, both sides of N type bar are
heavily doped to create PN junction. These doped regions are called gate
(G).
• Source − It is the entry point for majority carriers through which they enter
into the semiconductor bar.
• Drain − It is the exit point for majority carriers through which they leave
the semiconductor bar.
• Channel − It is the area of N type material through which majority carriers
pass from the source to drain.

16. FET Classification

17. N – Channel JFET
• It has a thin layer of N type material formed on P type substrate.
• Then the gate is formed on top of the N channel with P type material.
• At the end of the channel and the gate, lead wires are attached and
the substrate has no connection.

18. P- Channel JFET
• It has a thin layer of P type material formed on N type substrate.
• The following figure shows the crystal structure and schematic symbol
of an N-channel JFET.
• The gate is formed on top of the P channel with N type material.
• At the end of the channel and the gate, lead wires are attached.

19. Working of N- Channel JFET
• When no voltage is applied across the gate terminal, the
channel becomes a wide-open path for electrons to flow.
• Therefore maximum current flows from the source to the
drain terminal.
• when a negative voltage is applied to the gate terminal
with respect to the source terminal, making the P-N
junction reverse biased.
• A depletion region is created in the channel that makes
the channel narrower, increasing the channel resistance
between the source and drain, and the current flow
becomes less

20. Applications of FET’s
• FETs are extensively used in Integrated Circuits (ICs) due to their
compact size and significantly lower power consumption.
• FETs are also used in high power switching applications,
• as voltage-variable resistors (VVRs) in operational amplifiers (Op-Amps),
• tone controls,
• for mixer operation on FM and TV receivers
• logic circuits.