Circuits Analysis Study Notes for GATE EE Exam

By Yash Bansal|Updated : August 17th, 2021

Circuits Analysis Study Notes for GATE EE Exam: In this article, you can find the Study Notes on Basics of Electronic Devices, Circuits Analysis and Applications of Diodes and BJT which will cover the topic as Circuit analysis and applications of Diodes and Transistors and their Characteristics.

Circuit analysis and applications of Diodes and Transistors and their Characteristics

• An ideal diode may be considered as a most fundamental nonlinear element.
• An ideal diode is simply a PN junction where the change from p-type to the n-type material is assumed to occur instantaneously.
• Ideal Diode:
• Silicon and germanium diodes exhibit a cut-in voltage of 0.6 V and 0.2 V respectively in their characteristic curves and thus approximate closely the ideal diode in this respect.
• The Peak Inverse Voltage (PIV) is the highest reverse voltage that the diode can withstand before breaking down and permitting current to flow in the reverse direction.
•  Semiconductor Diode:
• The two types of semiconductor materials n-type and p-type are chemically combined to form a p-n junction.
• A region near the junction is without any free charge particles called depletion.
• Biasing of a Diode:
• The electric field across the junction has a constant polarity called barrier potential.
• A popular semiconductor device is formed using a p-n junction called a p-n junction diode.
• No Applied Bias (VD = 0 V ): In absence of applied bias voltage, the net flow of charge in any direction for a semiconductor diode is zero.
• Forward Bias (VD > 0V ): In forward biased condition, when applied voltage approaches barrier potential, the depletion region reduces as forwarding bias increases.

• Reverse Bias: While in reverse biased condition, the depletion region widens and minority carriers carry the current called reverse saturation current denoted as I0.

• The breakdown in Diode: If reverse-biased voltage increases, a particular voltage breakdown occurs due to accelerated minority charged particles. This is called the avalanche effect. For a heavily doped diode, the electric field across the depletion region is extremely intense to pull the electrons out of valence bands. This effect is known as the Zener effect.

Circuit Analysis of Diodes

There are three different techniques that are used to analyse circuits that contain diodes:

• Load line analysis should be used only when  you have an I-V characteristic for the diode
• Piece-wise models are used to estimate the diode current and voltage
• The accuracy of this depends on the region of operation and the use of the series resistors, Rs and Rz

Applications of Diodes

• Radio demodulation:  an AM signal consists of alternating positive and negative peaks of voltage, whose amplitude or “envelope” is proportional to the original audio signal, but whose average value is zero. The diode rectifies the AM signal, leaving a signal whose average amplitude is the desired audio signal.
• Power conversion: This type of rectifier are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC). Automotive alternators are an example, where the diode provides better performance than the commutator of the earlier dynamo.
• Over-voltage protection: Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors. Specialised diodes are operated to protect from over-voltages at higher power.
• Ionising radiation detectors, Temperature measuring, Current steering etc.

Junction Transistor

• Both the electrons and holes take part in the conduction process for bipolar devices.
• BJT consists of two p-n junctions manufactured in a special way and connected in series, back to back.
• The transistor is generally a 3-terminal device with emitter, base and collector terminals.
• From the physical structure, BJTs can be divided into 2 groups: NPN and PNP transistors.

• Modes of operation:

The transistor consists of two p-n junctions, the emitter-base junction (EBJ) and the collector-base junction (CBJ). Depending on the bias condition (forward or reverse) of each of these junctions, different modes of operation of BJT are obtained, as shown in the below table.

• Active Mode:

When the emitter-base junction of the transistor is forward biased and the collector-base junction is reverse biased, the transistor operates inactive region. In this mode, the transistor is used as an amplifier. This bias configuration is shown below the figure for n-p-n and p-n-p transistors.

• Saturation Region

When both the emitter-base junction and collector-base junction are forward biased, the transistor operates in the saturation region. Transistor has a large current in saturation mode. In the saturation mode, the transistor is used as a closed switch.

• Cut-off region

When both the emitter-base and collector-base junctions are reverse biased, the transistor operates in the cut-off region. In cut-off mode, the current through the transistor is zero(ideally). In this case, the transistor is operated as an open switch. Both n-p-n and p-n-p transistors are biased in cut-off mode as shown in the below figure.

• Reverse Active Region or Inverse Region

When the emitter-base junction of the transistor is reverse biased and the collector-base junction is forward biased, the transistor is said to be in reverse active mode. This mode of operation is not often used. In the below figure, transistors are biased in reverse active mode.

Current Relationship in BJT

Example:

A transistor has IB = 25 μA, β = 100,ICBO = 100 nA. Calculate a, IC, IE and ICEO.

Solution:

Transistor Configuration

The transistors can be connected in the following three different configurations, depending upon the terminals which are used as a common terminal to the input and output terminals.

• Common Base Configuration
• Common Emitter Configuration
• Common Collector Configuration

a) Common Base Configuration:

• In this configuration, the base terminal is connected to a common terminal.
• The input signal is applied between the emitter and base terminals.
• The output signal is taken between the collector and base terminals.

Properties of CB configuration:

• Lowest input resistance (Ri < 100 Ω)
• Highest output resistance (R0 > 1 Ω)
• Lowest current gain (α < 1)
• Highest voltage gain
• Medium power gain (Typical value 68)
• Output and input voltages are in phase i.e. phase shift is 0°.
• In CB amplifier current gain is less and therefore bandwidth is large and hence CB amplifier is widely used as a high-frequency amplifier.

Applications of CB configuration:

• As a non-inverting voltage amplifier
• As a high-frequency amplifier
• As an impedance matching device between low resistance to high resistance.

b) Common Emitter Configuration:

• In this configuration, the emitter terminal is connected to a common terminal
• The input signal is applied between the emitter and base terminals.
• The output signal is taken between the collector and base terminals

Properties of CE Configuration:

• Moderate input resistance (around 1 kΩ).
• Moderate output resistance (50 kΩ to 500 kΩ).
• Moderate current gain (Typical value 49).
• Moderate voltage gain.
• Highest power gain (Typical value 4226).
• Output and input voltages are out of phase i.e. phase shift = 180°.

Application of CE Configuration:

• It is the most common and frequently used amplifier circuit.

c) Common Collector Configuration:

• In this configuration, the collector terminal is connected to a common terminal.
• The input is applied between the base and collector terminals.
• The output is taken between the emitter and collector terminals.

Properties of CC Configuration:

• Highest input resistance (50 kΩ to 500 kΩ).
• Lowest output resistance (< 100 Ω).
• Highest current gain.
• Lowest voltage gain.
• Voltage gain is less than 1 or very close to 1.
• Lowest power gain (Typical value 48).
• Output and input voltages are in phase i.e. phase shift is 0°.
• Common collector configuration is also known as an emitter follower.
• Emitter follower is basically a Current Controlled Voltage Source (CCVS).

Applications of CC Configuration:

• As an audio frequency power amplifier.
• As a butter (impedance matching device between high resistance to low resistance).
• In designing voltage sweep circuits.
• As a high input resistance device.
• As a “Bootstrap emitter follower”.

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