DOC-20241227-WA0002.

Lecture Notes on Analog Electronics Circuit

Page 1

• Prepared by Debasish Mohanta

• Assistant Professor, Department of Electrical Engineering, GCE, Keonjhar

Page 2

• Topic: Biasing of BJTs

• Prepared by: Debasish Mohanta

• References:

  1. "Principles of Electronics" by VK Mehta

  2. "Electronic Devices and Circuit Theory" by Robert L. Boylestad and L. Nashelsky

Page 3

Bipolar Junction Transistor (Introduction)

• Transistors are solid-state devices based on electric charge carriers in solids.

• Functions: Capable of amplification. Analogous to vacuum triodes but differs as:

  • Transistors: Current-controlled devices

  • Vacuum Triodes: Voltage-controlled devices

• Advantages of Transistors:

  • Compact size

  • Lightweight

  • Rugged construction

  • Shock-resistant

  • Instant operation (no heating)

  • Low operating voltage

  • High efficiency (minimal heat loss)

  • Long lifespan

• Drawbacks:

  • Loud hum noise

  • Limited operational temperature (up to 75°C)

  • Limited operating frequency (a few MHz)

Page 4

Characteristics of CE Transistor

• Input Characteristics:

  • Curve between base current (IB) and base-emitter voltage (VBE) at constant collector-emitter voltage (VCE).

  • Similar to forward-biased diode due to its base-emitter region being diode-like.

• Output Characteristics:

  • Curve between collector current (IC) and collector-emitter voltage (VCE) for a fixed base current (IB).

  • A family of curves displayed for varying IB.

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Points Regarding Output Characteristics

• IC varies with VCE when 0 < VCE < 1V then becomes constant.

• CE configuration has output characteristics with slope, while common base configuration shows horizontal characteristics.

• In active region:

  • Collector junction reverse biased, emitter junction forward biased.

  • For small IB, collector voltage effect on IC is minor; for high IC, this effect increases.

  • In cutoff region: Small collector current flows when IB = 0 (IBE0).

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Faithful Amplification

• Function of transistors for amplification ensures signal magnitude increase without shape change.

• Conditions for faithful amplification:

  1. Proper zero-signal collector current

  2. Minimum base-emitter voltage (VBE) at any instant

  3. Minimum collector-emitter voltage (VCE) at any instant

  • Faithful amplification necessitates keeping base-emitter junction forward biased and collector-base junction reverse biased during signal application.

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Conditions Satisfaction

• (i) and (ii) guarantee forward bias of base-emitter junction during all signal parts.

• (iii) ensures reverse bias of collector-base junction at all times.

• Proper zero-signal collector current's relationship:

  • Positive signal (base positive w.r.t. emitter) causes collector current increase.

  • Negative cycle (base reverse biased) results in zero output, which is unfaithful amplification.

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Minimum VBE and VCE

• VBE should stay above 0.5V (Ge) and 0.7V (Si) for faithful amplification; else, gain diminishes causing distortion.

• Minimum VCE stays above 0.5V for Ge, 1V for Si to maintain reverse bias of collector-base junction.

Page 9

Numerical Example (Collector Circuit Calculation)

• Example of given circuit:

  • RB = 10KΩ, RC = 1KΩ estimates VBB for active mode VCE = 5V, edge of saturation, deep saturation, with forced beta = 10.

• Solutions: a) IC = (10V-5V)/1KΩ → 5mA; IB = IC/beta → 0.1mA; VBB = IB*RB + VBE = 1.7Vb) For saturation at VCE ≈ 0.3V, calculate IB and VBB → 2.64V.c) Deeply in saturation, VCE ≈ 0.2V yields IB = 0.98mA and VBB = 10.5V.

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Operating Point Definition

• DC current/voltage establish an operating point (quiescent point) for amplification.

• Point A: No bias - transistor off

  • Point B: Amplifier operates symmetrically around quiescent values.

  • Point C: Collector current and voltage allowed to vary but clipped at negative peaks.

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Bias Stabilization

• Changes in temperature/transistor variations can rapidly change the collector current (IC); hence stabilization is needed.

• Process of ensuring constant operating point independent of temperature or device variation is called bias stabilization.

• Clear understanding of maintaining operating points during temperature variations or when components are replaced is vital.

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Need for Stabilization

• Temperature change affects collector leakage current (ICO).

• Collector current IC = beta * IB + (beta + 1)ICO: Requires stabilization.

• Small changes in temperature can significantly impact operation.

Page 13

Thermal Runaway Mechanism

• Excessive collector current can lead transistor heating, further raising IC due to increased ICO, leading to very high IC, damaging device.

• Effective design aims to stabilize IC even as temperature and gain changes; circuit modifications can later include these compensations.

Page 14

Stability Factor Definition

• Stability Factor (S): Measures how well the circuit maintains stability regarding variations in leakage current, input bias, and gain.

• For equilibrium, stability factor defined for various biases, resulting in enhanced thermal stability.

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Transistor Biasing

• Achieves operational fidelity and stabilizes performance amidst variations.

• Methods include:

  1. Fixed Bias Method

  2. Emitter Bias Method

  3. Biasing with Collector-Feedback Resistor

  4. Voltage-Divider Bias...

Page 18

Feedback and Oscillators

• Includes feedback problem-solving basics detailing feedback's effects, amplifier's linearity, and output prediction. ---