Vidyarthi Academy

Home NCERT Solutions Chapter Notes Test Papers Contact Us

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 14 SEMICONDUCTORS

Types of substances on the basis of conductivity

Metals on the basis of conductivity

Semiconductors on the basis of conductivity

Insulators on the basis of conductivity

Energy bands in solids

Valence band

Conduction band

Forbidden band

Types of substances on the basis of energy bands

Metals on the basis of energy bands

Insulators on the basis of energy bands

Semiconductors on the basis of energy bands

Types of semiconductors

Elemental semiconductors

Compound semiconductors

Types of semiconductors based on purity

Intrinsic semiconductors

Effect of temperature on conductivity of semiconductors

Extrinsic semiconductor

n-type semiconductor

p-type semiconductor

Conductivity of extrinsic semiconductor

p-n junction

Diffusion of charge

Diffusion current

Depletion region

Drift of charge carriers

Drift current

Potential barrier across p-n junction

Semiconductor diode

Forward bias of p-n junction

Reverse bias of p-n junction

V-I characteristics of a diode

Threshold voltage or cut-in voltage

Dynamic resistance of diode

Application of junction diode as a rectifier

Half wave rectifier

Full-wave rectifier

Centre-tap transformer

Electric filter

Role of capacitor in the filter

Some special type of diodes

Zener diode

Zener diode as voltage regulator

Photodiode

Light emitting diodes (LED)

Photovoltaic devices (Solar cells)

Junction transistor

n-p-n transistor

p-n-p transistor

Transistor emitter

Transistor base

Transistor collector

Transistor in saturation region

Transistor in cut-off region

Transistor in active region

Basic transistor circuit configurations and transistor characteristics

Transistor in common base configuration

Transistor in common emitter configuration

Common emitter transistor characteristics

Input resistance of transistor

Output resistance of transistor

Current amplification factor

Transistor as a device

Transistor as a switch - base-biased CE configuration

Transistor as an amplifier

Amplification of dc voltage

Amplification of ac signal

Feedback amplifier

Transistor oscillator

Working of feedback amplifier

Tank circuit

Digital electronics

Analog signal

Digital signal

Logic gates

NOT gate

OR gate

AND gate

NAND gate

NOR gate

Integrated circuits

Linear or analogue ICs

Digital ICs

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 14 SEMICONDUCTORS

Transistor as a device

  1. Transistor as an amplifier

    Amplification of dc voltage: In the active region transistor behaves like an amplifier. For Vo versus Vi curve, the slope of the linear part of the curve represents the rate of change of the output with the input. It is negative because the output is VCC – ICRC and not ICRC. Hence, the output voltage of the CE amplifier decreases as input voltage increases. In this case the output is said to be out of phase with the input. If we consider ΔVo and ΔVi as small changes in the output and input voltages then ΔVoΔVi is called the small signal voltage gain AV of the amplifier.

    If the VBB voltage has a fixed value corresponding to the midpoint of the active region, the circuit will behave as a CE amplifier. The dc base current IB would be constant and corresponding collector current IC will also be constant.

    The voltage gain AV can be expressed in terms of the resistors in the circuit and the current gain of the transistor as follows.

    We have, Vo = VCC – ICRC

    Therefore, ΔVo = 0 – RC Δ IC

    Similarly, from Vi = IBRB + VBE

    ΔVi = RB ΔIB + ΔVBE

    But ΔVBE is negligibly small in comparison to ΔIBRB in this circuit.

    Or, AV =  RC Δ ICRB ΔIB= βacRCRB

    Also, the dc voltage VCE = VCC - ICRC would remain constant. The operating values of VCE and IB determine the operating point, of the amplifier.

    Amplification of ac signal

    If a small sinusoidal voltage with amplitude vs is superposed on the dc base bias by connecting the source of that signal in series with the VBB supply, then the base current will have sinusoidal variations superimposed on the value of IB. As a consequence the collector current also will have sinusoidal variations superimposed on the value of IC, producing in turn corresponding change in the value of VO. We can measure the ac variations across the input and output terminals by blocking the dc voltages by large capacitors.

    ???

    Let us superimpose an ac input signal vi, to be amplified, on the bias VBB (dc). The output is taken between the collector and the ground.

    To start with let us assume that vi = 0. Then applying Kirchhoff’s law to the output loop, we get,

    Vcc = VCE + ICRC

    And from the input loop, we get,

    VBB = VBE + IB RB

    When vi is not zero, we get,

    VBE + vi = VBE + IB RB + ΔIB (RB + ri)

    Or vi = ΔIB (RB + ri) = r ΔIB

    ac = ΔICΔIBVCE=icib

    The power gain Ap can be expressed as the product of the current gain and voltage gain. Mathematically

    Ap = βac × AV

    • The transistor is not a power generating device. The energy for the higher ac power at the output is supplied by the battery.
Previous
Next