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.

Page 5

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).

Page 6

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.

Page 8

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.

Page 10

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.

Page 11

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.

Page 12

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. ---