Module VI: Introduction to Transistors - Solved Questions

Common Emitter Amplifier

BJT operates in active region - small base current change causes large collector current change.

Working

1. Transistor biased in active region (BE forward, CB reverse)
2. Small AC signal applied to base through C_in
3. Base current varies: i_c = β × i_b (β = 50-300)
4. Amplified current through R_C produces amplified voltage
5. Output 180° out of phase with input

Gains

Current Gain: A_i = β = I_C/I_B

Voltage Gain: A_v = -R_C/r_e (where r_e = 25mV/I_E)

Power Gain: A_p = A_v × A_i

CE Amplifier Characteristics

Parameter	Value
Current Gain	High (50-300)
Voltage Gain	High (20-200)
Input Impedance	Medium (1-5kΩ)
Phase Shift	180°

Applications: Audio amplifiers, RF amplifiers, oscillators

Operating Regions

Parameter	Cut-off (OFF)	Saturation (ON)
Vin	0V (LOW)	HIGH (> 0.7V)
IB	0	Maximum
IC	≈ 0	VCC/RL
VCE	VCC	≈ 0.2V
Switch State	OPEN	CLOSED
Load	OFF	ON

Design Equations

I_C(sat) = V_CC/R_L

I_B(min) = I_C(sat)/β

R_B = (V_in - 0.7)/I_B

Circuit

V_in → R_B → Base; Collector → R_L(Load) → V_CC; Emitter → Ground

Applications: LED drivers, relay drivers, motor control, digital logic

Definition

FET = Field Effect Transistor. Unipolar, voltage-controlled device using only one type of charge carrier.

Terminals

Gate (G): Control terminal | Drain (D): Current output | Source (S): Current input

Types

• JFET: Junction FET (N-channel, P-channel) - depletion mode
• MOSFET: Metal Oxide Semiconductor FET - enhancement/depletion

Working

Gate voltage creates electric field → varies channel width → controls drain current

Drain Current (JFET)

I_D = I_DSS(1 - V_GS/V_P)²

Characteristics

Parameter	Value
Input Impedance	Very High (10⁹-10¹²Ω)
Control	Voltage (VGS)
Noise	Low
Thermal Stability	Better than BJT

Applications: Digital ICs (CMOS), analog switches, amplifiers, oscillators

Parameter	BJT	FET
Full Form	Bipolar Junction Transistor	Field Effect Transistor
Charge Carriers	Bipolar (electrons + holes)	Unipolar (electrons OR holes)
Control	Current controlled	Voltage controlled
Terminals	E, B, C	S, G, D
Input Impedance	Low (1-5kΩ)	Very High (10⁹-10¹²Ω)
Gain Parameter	β (current gain)	gm (transconductance)
Noise	Higher	Lower
Thermal Stability	Less	More (negative temp coeff)
Switching Speed	Moderate	Faster
Size	Larger	Smaller (easier IC)
Input Current	Significant	Negligible
Applications	Audio/power amplifiers	Digital ICs, low noise amp

When to Use

BJT: High current gain, cost-sensitive, power switching

FET: High input impedance, low noise, digital circuits, high density ICs