Analog Electronics — Complete Notes ECE Sem 3

Semiconductor Diode

A p-n junction diode allows current in one direction only.

V-I Characteristics:

Forward biased (V > 0.7V for Si): conducts, I = Is(e^(V/VT) - 1)
Reverse biased:                   very small leakage current
Breakdown region (Zener):         conducts in reverse

Diode Types:

| Type | Use | |---|---| | Rectifier diode | AC → DC conversion | | Zener diode | Voltage regulation (reverse breakdown) | | LED | Light emission | | Photodiode | Light detection | | Schottky diode | Fast switching, low forward voltage | | Varactor | Variable capacitor (voltage-controlled) |

Rectifiers

Half-wave:   η = 40.6%,  Vout = Vm/π
Full-wave:   η = 81.2%,  Vout = 2Vm/π  (center-tap)
Bridge:      η = 81.2%,  Vout = 2Vm/π  (no center-tap needed)

Filter capacitor reduces ripple. Ripple factor γ = ripple voltage / DC voltage.

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BJT — Bipolar Junction Transistor

Three terminals: Base (B), Collector (C), Emitter (E)
Types: NPN, PNP

Operating Regions:

• Active — normal amplification (EB forward, CB reverse biased)
• Saturation — both junctions forward biased (switch ON)
• Cutoff — both junctions reverse biased (switch OFF)

Configurations:

| Config | Input | Output | Current gain | Voltage gain | Application | |---|---|---|---|---|---| | CB | E | C | < 1 | High | RF amplifier | | CE | B | C | β (high) | High | Audio amplifier | | CC | B | E | β+1 | < 1 | Buffer/emitter follower |

Important relations:

IC = β × IB
IE = IC + IB = (β+1)IB
α = IC/IE = β/(β+1)

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FET — Field Effect Transistor

Three terminals: Gate (G), Drain (D), Source (S)

JFET:

• N-channel: VGS ≤ 0 controls ID
• Pinch-off voltage VP — beyond this, ID saturates
• Transconductance gm = ΔID/ΔVGS

MOSFET (MOST IMPORTANT):

• Enhancement MOSFET — normally OFF, VGS > VTh to turn ON
• Depletion MOSFET — normally ON, negative VGS to turn OFF

Drain current (saturation):
ID = (μn Cox W/2L)(VGS - VTh)²

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Op-Amp — Operational Amplifier

Ideal Op-Amp Properties:

• Open-loop gain A = ∞
• Input impedance Zin = ∞
• Output impedance Zout = 0
• Bandwidth = ∞

Golden Rules (with negative feedback):

1. V+ = V- (virtual short)
2. I into inputs = 0 (virtual open)

Inverting Amplifier

Vout = -(Rf/R1) × Vin
Gain = -Rf/R1  (negative = phase inversion)
Input impedance = R1

Non-Inverting Amplifier

Vout = (1 + Rf/R1) × Vin
Gain = 1 + Rf/R1  (always > 1)
Input impedance = ∞ (ideal)

Other Op-Amp Circuits

| Circuit | Formula | |---|---| | Voltage Follower | Vout = Vin (Gain=1, buffer) | | Summing Amplifier | Vout = -(Rf/R1)V1 - (Rf/R2)V2 | | Difference Amplifier | Vout = (R2/R1)(V2 - V1) | | Integrator | Vout = -(1/RC)∫Vin dt | | Differentiator | Vout = -RC(dVin/dt) | | Comparator | Vout = +Vsat (V+ > V-) or -Vsat |

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Amplifier Frequency Response

Bandwidth BW = fH - fL (typically fL ≈ 0)
Gain-Bandwidth Product (GBP) = Av × BW = constant

At 3dB frequency: Av = Av_max/√2 = 0.707 × Av_max

Bode Plot: Plot of |Av| (dB) vs frequency (log scale)

• -20 dB/decade rolloff for single pole

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Oscillators

Barkhausen Criterion for sustained oscillations:

1. Loop gain |Aβ| = 1
2. Total phase shift = 0° or 360°

| Oscillator | Frequency | Type | |---|---|---| | RC Phase Shift | f = 1/(2π√6 RC) | Low frequency audio | | Colpitts | f = 1/(2π√(LC)) | RF, uses capacitor divider | | Hartley | f = 1/(2π√(LC)) | RF, uses inductor divider | | Crystal | Very stable | Clock circuits | | Wien Bridge | f = 1/(2πRC) | Audio, low distortion |

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Filters

Passive filters (R, L, C only):

| Filter | Pass | Cutoff | |---|---|---| | Low Pass | f < fc | fc = 1/(2πRC) | | High Pass | f > fc | fc = 1/(2πRC) | | Band Pass | f1 < f < f2 | — | | Band Stop (Notch) | f < f1, f > f2 | — |

Active filters use op-amps with R,C — no inductors needed, better performance.

Filter order: Higher order → sharper rolloff (-20n dB/decade for nth order)