Analog Electronics / Chapter 3

Bipolar Junction Transistor (BJT): Topics, Subtopics, Study Flow, and Working Steps

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Chapter 3 / Original Transistor Builder

Bipolar Junction Transistor (BJT)

This chapter treats the BJT as a controllable carrier valve. The base does not carry the main output current; it decides how much of the emitter carrier stream reaches the collector. That one idea connects construction, biasing, characteristics, and small-signal gain.

GATE/PSU Lens

Always identify region of operation, write current relations, fix the Q-point, then use the small-signal model.

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BJT carrier-control visualization

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Working Steps: From Base Signal to Collector Output

Forward bias the emitter-base junction and reverse bias the collector-base junction for active operation.

A small base current controls a much larger collector current.

Choose CE, CB, or CC depending on gain, input resistance, and output resistance needs.

Set a stable Q-point using a bias network before applying the AC signal.

Replace the transistor by its small-signal model to calculate gain and resistance values.

3.1

Main Topic

BJT Basics

A BJT is a three-layer current-control device. Its power comes from a small base action controlling a much larger collector-emitter current path.

3.1.1

BJT Basics

Construction

A BJT is made as either NPN or PNP. The emitter is heavily doped to inject carriers, the base is very thin and lightly doped to let most carriers pass through, and the collector is moderately doped with a larger area so it can collect carriers and handle power.

Step-by-step working

1Emitter is designed as the carrier supplier.
2Base is made thin so injected carriers do not mostly recombine there.
3Collector is designed to collect carriers and withstand reverse voltage.
4In NPN, electrons are the main transported carriers; in PNP, holes are.
5The three regions create two junctions: emitter-base and collector-base.

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Construction visualization

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Remember

Emitter injects, base controls, collector collects.

3.1.2

BJT Basics

Working principle

In active-region NPN operation, the emitter-base junction is forward biased and the collector-base junction is reverse biased. Electrons injected from the emitter cross the thin base, and the collector field sweeps most of them into the collector.

Step-by-step working

1Forward bias at emitter-base junction injects electrons from emitter to base.
2Only a small fraction recombines inside the thin base.
3That recombination creates the small base current.
4Most electrons reach the collector-base depletion region.
5The collector electric field sweeps them into the collector, creating collector current.

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Working principle visualization

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Remember

Small base current exists because a small part of injected carriers recombines in the base.

3.1.3

BJT Basics

Current components

The emitter current splits into collector current and base current. In normal active operation, collector current is much larger than base current, so current gain becomes possible.

Step-by-step working

1Emitter current enters the transistor action as the supplied carrier stream.
2A small portion contributes to base recombination current.
3The larger portion becomes collector current.
4For NPN, conventional current relation is IE = IC + IB.
5Current gain is written as beta = IC / IB in common-emitter analysis.

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Current components visualization

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Remember

Always start BJT current questions from IE = IC + IB and beta = IC / IB.

3.2

Main Topic

BJT Configurations

The same transistor behaves differently depending on which terminal is common to input and output. Configuration decides gain, phase, and impedance behavior.

3.2.1

BJT Configurations

CE configuration

In common-emitter configuration, the emitter is common to input and output. It gives high voltage gain and current gain, but the output is phase inverted with respect to the input.

Step-by-step working

1Input is applied between base and emitter.
2Output is taken between collector and emitter.
3Small base signal controls collector current.
4Collector resistor converts current variation into voltage variation.
5When collector current increases, collector voltage falls, causing phase inversion.

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CE configuration visualization

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Remember

CE is the main voltage amplifier configuration and gives 180 degree phase shift.

3.2.2

BJT Configurations

CB configuration

In common-base configuration, the base is common. It has low input resistance, high output resistance, current gain slightly less than one, and no phase inversion.

Step-by-step working

1Input is applied at the emitter side.
2Output is taken from the collector side.
3Most emitter-injected carriers are collected by the collector.
4Current gain is near but less than unity.
5Voltage gain can be high because output resistance is high.

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CB configuration visualization

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Remember

CB has current gain below one but useful high-frequency behavior.

3.2.3

BJT Configurations

CC configuration

In common-collector configuration, the collector is common and output is taken from the emitter. It is also called emitter follower because output follows input with nearly unity voltage gain.

Step-by-step working

1Input is applied between base and collector reference.
2Output is taken at the emitter.
3Emitter voltage follows base voltage minus the base-emitter drop.
4Voltage gain is close to one.
5Low output resistance makes it useful as a buffer.

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CC configuration visualization

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Remember

CC is used for impedance matching and buffering, not large voltage gain.

3.3

Main Topic

BJT Characteristics

BJT characteristic curves are maps of device behavior. They show how input junction current starts and how collector current responds to collector voltage for different base currents.

3.3.1

BJT Characteristics

Input characteristics

Input characteristics usually plot base current against base-emitter voltage for a CE transistor. The curve resembles a forward-biased diode because the base-emitter junction is forward biased in active operation.

Step-by-step working

1Increase base-emitter voltage gradually.
2Below the practical turn-on region, base current remains small.
3After the junction conducts, base current rises rapidly.
4The curve looks diode-like because the input junction is a PN junction.
5Input resistance is estimated from the local slope of this curve.

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Input characteristics visualization

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Remember

BJT CE input curve behaves like a forward-biased diode curve.

3.3.2

BJT Characteristics

Output characteristics

Output characteristics plot collector current against collector-emitter voltage for different base currents. They reveal cutoff, active, and saturation behavior.

Step-by-step working

1Set a fixed base current.
2Increase collector-emitter voltage and observe collector current.
3At low VCE, the transistor is in saturation and IC depends strongly on VCE.
4In active region, IC is mainly controlled by IB and only slightly by VCE.
5With IB near zero, the transistor is in cutoff except for leakage.

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Output characteristics visualization

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Remember

Use output curves to identify cutoff, active region, and saturation before solving.

3.4

Main Topic

Biasing Circuits

Biasing is the art of placing the transistor at a useful DC operating point before any signal arrives. Without a stable Q-point, amplifier calculations are only decoration.

3.4.1

Biasing Circuits

Fixed bias

Fixed bias uses a resistor from supply to base. It is simple, but Q-point depends strongly on beta, so two transistors with different beta can produce very different collector currents.

Step-by-step working

1Base resistor sets base current approximately from supply voltage.
2Collector current is beta times base current.
3Collector resistor creates collector voltage from that current.
4If beta changes, collector current changes significantly.
5This makes fixed bias weak for stable amplifier design.

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Fixed bias visualization

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Remember

Fixed bias is easy to calculate but poor in stability.

3.4.2

Biasing Circuits

Voltage divider bias

Voltage divider bias uses two resistors to set base voltage and an emitter resistor to provide negative feedback. If collector current rises, emitter voltage rises, reducing effective base-emitter voltage and opposing the change.

Step-by-step working

1Divider resistors establish an approximate base voltage.
2Emitter voltage becomes base voltage minus VBE.
3Emitter resistor sets emitter current from emitter voltage.
4Collector current becomes less dependent on beta.
5The emitter resistor provides self-correction against temperature and beta changes.

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Voltage divider bias visualization

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Remember

Voltage divider bias is preferred because emitter feedback stabilizes Q-point.

3.4.3

Biasing Circuits

Stability factor

Stability factor measures how sensitive collector current is to leakage current, beta, or temperature-related changes. A smaller stability factor means the bias point is less likely to drift.

Step-by-step working

1Temperature rise increases leakage current.
2Leakage can increase collector current.
3Increased collector current can heat the transistor further.
4Good biasing introduces feedback to oppose this drift.
5Stability factor numerically expresses how strongly IC changes.

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Stability factor visualization

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Remember

Lower stability factor means better thermal stability.

3.5

Main Topic

Small Signal Analysis

Small-signal analysis separates the transistor into two lives: DC bias sets the operating point, and AC variations around that point are analyzed with a linear model.

3.5.1

Small Signal Analysis

Hybrid model

The hybrid model represents the transistor by small-signal parameters around the Q-point. It lets us replace the nonlinear transistor with a local linear circuit for gain and impedance calculations.

Step-by-step working

1Find the DC Q-point first.
2Turn DC supplies into AC ground for small-signal analysis.
3Replace coupling capacitors by shorts in midband analysis.
4Replace the BJT by its small-signal hybrid model.
5Solve the resulting linear circuit for gain and resistances.

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Hybrid model visualization

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Remember

Never start small-signal analysis before establishing the DC operating point.

3.5.2

Small Signal Analysis

h-parameters

h-parameters describe input resistance, reverse voltage feedback, forward current gain, and output admittance. They are useful because BJT behavior can be represented as a two-port model for AC analysis.

Step-by-step working

1Treat the transistor as a two-port network.
2Use h11 as input resistance with output shorted.
3Use h21 as forward current gain.
4Use h12 for reverse feedback, often small in simplified analysis.
5Use h22 as output admittance for output resistance estimation.

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h-parameters visualization

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Remember

For CE h-parameter questions, hfe is the familiar small-signal current gain.