Semiconductor Electronics Notes

Transistor and Amplifier

Premium coaching notes on transistor structure, p-n-p and n-p-n action, CB/CE/CC configurations, current gains, characteristics, amplifier action, switching, oscillator basics, numericals and PYQs.

NCERT Style DiagramsReference diagrams are embedded from the supplied source images for scientific accuracy.

Exam FormulasClear α, β, gain, resistance and transconductance formula boxes.

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1. Transistor Structure

A junction transistor is a three-layer, two-junction semiconductor device. It has three terminals: emitter E, base B and collector C.

Emitter

Emitter is heavily doped and injects charge carriers into the base.

Base

Base is very thin and lightly doped. Only a small fraction of carriers recombine in the base.

Collector

Collector is moderately doped and physically larger. It collects most carriers coming from emitter.

Fig. 14.60 n-p-n transistor and its circuit symbol.

Fig. 14.61 p-n-p transistor and its circuit symbol.

Arrow rule: In n-p-n transistor, emitter arrow points out. In p-n-p transistor, emitter arrow points in.

2. PNP and NPN Transistor Action

For normal transistor action, emitter-base junction is forward biased and collector-base junction is reverse biased.

NPN Transistor

Electrons are majority carriers. Electrons are injected from n-type emitter into the thin p-type base. A small part recombines in the base producing I_B, while most electrons reach the collector and produce I_C.

PNP Transistor

Holes are majority carriers. In p-n-p transistor, holes are injected from emitter into base. A small percentage recombines in base and produces base current. Most carriers reach collector and produce collector current.

Current relation: I_E = I_B + I_C

3. CB, CE and CC Configurations

A transistor can be connected in three basic configurations depending on which terminal is common to input and output circuits.

Fig. 14.65 Common-base, common-emitter and common-collector connections for n-p-n transistor.

Configuration	Input Terminals	Output Terminals	Main Feature
Common-base (CB)	Emitter-base	Collector-base	Low input resistance, voltage gain possible, current gain less than 1.
Common-emitter (CE)	Base-emitter	Collector-emitter	High current and voltage gain; output is phase reversed.
Common-collector (CC)	Base-collector	Emitter-collector	High input resistance; used as emitter follower.

4. Current Gain and Relation between α and β

Common-base Current Gain α

alpha formula box — α formula at constant V_CB.

α = ΔI_C / ΔI_E at constant V_CB

Since I_C is slightly less than I_E, α is less than 1. Typical value is 0.95 to 0.99.

Common-emitter Current Gain β

beta formula box — β formula at constant V_CE.

β = ΔI_C / ΔI_B at constant V_CE

β is large because a small base current controls a much larger collector current.

Relation between alpha and beta — Relation between α and β.

α = β / (1 + β)

β = α / (1 - α)

5. CE Transistor Characteristics

In common-emitter configuration, base-emitter side is input and collector-emitter side is output.

Circuit for studying CE characteristics — Fig. 14.66 Circuit for studying the common emitter characteristics of an n-p-n transistor.

CE Input Characteristic

Graph between base current I_B and base-emitter voltage V_BE at constant V_CE.

Fig. 14.67 Input characteristics of CE n-p-n transistor.

Input resistance r_i = ΔV_BE / ΔI_B at constant V_CE

CE Output Characteristic

Graph between collector current I_C and collector-emitter voltage V_CE at constant base current I_B.

Fig. 14.68 Output characteristic of CE n-p-n transistor.

Output resistance r_o = ΔV_CE / ΔI_C at constant I_B

Regions: cut-off region, active region and saturation region are identified using transistor characteristics.

6. Transistor as Amplifier

A transistor amplifier uses a small input signal to control a larger output signal. In active region, small change in input current produces a large change in collector current.

1. Common-base PNP Amplifier

p-n-p transistor as a common base amplifier.

Input is applied in the emitter-base circuit and output is taken from the collector-base circuit. Current gain is less than 1, but voltage gain may be high.

2. Common-base NPN Amplifier

Fig. 14.72 n-p-n transistor as a common base amplifier.

Base is common to input and output. This configuration has low input resistance and high output resistance.

3. Common-emitter PNP Amplifier

p-n-p transistor as a common emitter amplifier.

Emitter is common to input and output. It gives current gain, voltage gain and power gain; polarities are according to p-n-p operation.

4. Common-emitter NPN Amplifier

Fig. 14.74 n-p-n transistor as a common emitter amplifier.

Input is applied between base and emitter; output is taken between collector and emitter. CE amplifier gives high current gain, high voltage gain and 180° phase reversal.

Gains

Current gain A_i = ΔI_out / ΔI_in. Voltage gain A_v = ΔV_out / ΔV_in. Power gain = A_i × A_v.

Resistances

Input resistance r_i = ΔV_in / ΔI_in. Output resistance r_o = ΔV_out / ΔI_out.

Phase Reversal

In CE amplifier, output voltage is 180° out of phase with the input signal.

Transconductance g_m = ΔI_C / ΔV_BE

7. Transistor as Switch and Logic Applications

A transistor can behave like an electronic switch. It is OFF in cut-off region and ON in saturation region.