Electronic Devices & Circuits

Definition: A BJT is a three-terminal semiconductor device consisting of two PN junctions. It has three regions: Emitter (E), Base (B), and Collector (C). Current flow involves both electrons and holes (bipolar).

Three basic configurations based on which terminal is common:

Configuration	Input	Output	Current Gain	Voltage Gain	Applications
Common Emitter (CE)	Base-Emitter	Collector-Emitter	High (β)	High	Amplifiers, Switching
Common Base (CB)	Emitter-Base	Collector-Base	Low (<1)	High	High-frequency amplifiers
Common Collector (CC)	Base-Collector	Emitter-Collector	High	Low (<1)	Impedance matching, Buffers

BJT has two junctions: Emitter-Base (EB) and Collector-Base (CB). Four operating modes:

Operating Mode	EB Junction	CB Junction	Application
Active/Linear	Forward Biased	Reverse Biased	Amplification
Saturation	Forward Biased	Forward Biased	ON switch (Digital)
Cutoff	Reverse Biased	Reverse Biased	OFF switch (Digital)
Inverse Active	Reverse Biased	Forward Biased	Rarely used

• BJT (Bipolar Junction Transistor):
  • NPN Transistor
  • PNP Transistor
• FET (Field Effect Transistor):
  • JFET (Junction FET) - N-channel, P-channel
  • MOSFET (Metal Oxide Semiconductor FET)
    • Enhancement Mode (E-MOSFET)
    • Depletion Mode (D-MOSFET)

Key Characteristics:

• Current-controlled device: IC depends on IB
• Low input impedance: Few hundred ohms
• High output impedance: Several kΩ
• Temperature sensitive: Performance varies with temperature
• Power consumption: Higher than FET
• Switching speed: Moderate (slower than MOSFET)
• Gain (β): Typically 50-200

Three Main Characteristics:

• Input Characteristics: IB vs VBE (VCE constant)
• Output Characteristics: IC vs VCE (IB constant)
• Transfer Characteristics: IC vs IB (VCE constant)

• Amplification: Audio, RF, and power amplifiers
• Switching: Digital circuits, logic gates
• Oscillators: Signal generation
• Voltage Regulation: Power supplies
• Signal Processing: Modulation, demodulation
• Motor Control: Speed and direction control

Definition: MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a voltage-controlled three-terminal device with Gate (G), Drain (D), and Source (S). Current flow is controlled by electric field.

Parameter	BJT	MOSFET
Control Type	Current-controlled (IB controls IC)	Voltage-controlled (VGS controls ID)
Terminals	Emitter, Base, Collector	Source, Gate, Drain
Input Impedance	Low (few hundred Ω)	Very High (MΩ range)
Current Flow	Bipolar (electrons & holes)	Unipolar (electrons or holes)
Switching Speed	Moderate	Fast
Power Consumption	Higher	Lower
Temperature Stability	Less stable	More stable
Noise	Higher	Lower
Size	Larger	Smaller (IC applications)
Cost	Lower	Higher
Applications	Audio amplifiers, linear circuits	Digital circuits, switching, VLSI

Given:

• Amplification factor (α) = 0.95
• Collector current (IC) = 1.9 mA

Formulas:

Solution:

Step 1: Find Current Gain (β)

Step 2: Find Base Current (IB)

Step 3: Find Emitter Current (IE)

Given: β = 50, VCC = 12V, RB = 240 kΩ, RC = 2.2 kΩ

Assume: Silicon transistor ⇒ VBE = 0.7V, VE = 0

Step 1: Base Current (IB)

Step 2: Collector Current (IC)

Step 3: Collector Voltage (VC)

Step 4: Collector-Emitter Voltage (VCE)

Step 5: Base Voltage (VB)

Step 6: Base-Collector Voltage (VBC)

Current Relations:

Typical Values:

• α: 0.95 to 0.99
• β: 20 to 200
• VBE (Silicon): 0.7V
• VBE (Germanium): 0.3V

In active region, small changes in base current cause large changes in collector current.

• Voltage Gain (AV): AV = Vout / Vin = -β × (RC / Rin)
• Current Gain (AI): AI = IC / IB = β
• Power Gain (AP): AP = AV × AI

CE configuration provides both voltage and current gain, making it most popular for amplification.

Transistor operates between cutoff (OFF) and saturation (ON):

• Cutoff Mode (OFF): IB = 0, IC ≈ 0, VCE ≈ VCC (Open switch)
• Saturation Mode (ON): IB is large, IC = VCC/RC, VCE ≈ 0.2V (Closed switch)

Applications: Digital logic gates, relay drivers, LED drivers, motor control

• Fixed Bias: Simple but temperature unstable
• Voltage Divider Bias: Most stable, widely used
• Emitter Bias: Good stability with negative supply
• Collector Feedback Bias: Moderate stability

BJT Advantages:

• High current handling capacity
• Better for analog/linear applications
• Lower cost
• Higher transconductance

BJT Disadvantages:

• Lower input impedance
• Temperature sensitive
• Slower switching speed
• Requires continuous base current