Analog Electronics / Chapter 4

BJT Amplifiers: Topics, Subtopics, Study Flow, and Working Steps

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Chapter 4 / Original Amplifier Builder

BJT Amplifiers

This chapter explains BJT amplifiers as signal-controlled power transfer. Biasing prepares the transistor, the input signal moves the Q-point slightly, and the collector or emitter network turns that motion into useful output.

GATE/PSU Lens

Separate DC bias, midband gain, cutoff frequencies, loading, and power-class conduction angle.

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BJT amplifier visualization

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Working Steps: From Small Signal to Useful Power

Start with DC bias and fix the transistor in the active region.

Superimpose a small AC input on the bias point.

Collector current variation creates a larger voltage variation across the collector load.

Coupling and bypass capacitors shape the low-frequency response.

Device capacitances reduce high-frequency gain, creating a finite bandwidth.

4.1

Main Topic

Single Stage Amplifiers

A single-stage BJT amplifier is a controlled energy converter: a small input signal changes transistor current, and the DC supply plus load resistor turn that current change into a larger output voltage.

4.1.1

Single Stage Amplifiers

CE amplifier

The common-emitter amplifier is the main voltage-gain stage. The input is applied at the base, the output is taken at the collector, and the emitter is common to both. A small increase in base voltage increases collector current, which increases the drop across the collector resistor and pulls collector voltage downward.

Step-by-step working

1DC bias first places the transistor in active region.
2The AC input slightly changes base-emitter voltage.
3Collector current changes in step with the base-emitter variation.
4The collector resistor converts current variation into voltage variation.
5Output voltage is amplified and inverted because higher collector current lowers collector voltage.

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CE amplifier visualization

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Remember

CE gives high voltage gain with 180 degree phase inversion.

4.1.2

Single Stage Amplifiers

CB amplifier

The common-base amplifier accepts input at the emitter and takes output from the collector. It has low input resistance, high output resistance, no phase inversion, and useful high-frequency behavior because the base is held at AC ground.

Step-by-step working

1Base is kept common for input and output signal reference.
2Emitter input changes emitter current directly.
3Most emitter current reaches the collector.
4Collector load converts collector current change into output voltage.
5Output remains in phase with input for voltage variation.

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CB amplifier visualization

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Remember

CB is not a current-gain stage; it is useful for voltage gain and high-frequency work.

4.1.3

Single Stage Amplifiers

CC amplifier

The common-collector amplifier is also called an emitter follower. Output is taken from the emitter, so it follows the base signal with nearly unity voltage gain, high input resistance, and low output resistance.

Step-by-step working

1Input signal is applied at the base.
2Emitter voltage follows base voltage approximately one VBE below it.
3Voltage gain stays close to one.
4The stage can supply more load current than the signal source alone.
5It is used as a buffer between high-resistance source and low-resistance load.

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CC amplifier visualization

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Remember

CC is a buffer: high input resistance, low output resistance, nearly unity voltage gain.

4.2

Main Topic

Frequency Response

An amplifier does not amplify every frequency equally. Its gain is shaped by coupling capacitors, bypass capacitors, transistor capacitances, and wiring parasitics.

4.2.1

Frequency Response

Low-frequency response

At low frequency, coupling and bypass capacitors do not behave like perfect shorts. They create reactance that reduces signal transfer and lowers gain.

Step-by-step working

1Input coupling capacitor blocks part of the slow-changing signal.
2Output coupling capacitor also loses voltage across its reactance.
3Emitter bypass capacitor may fail to fully bypass emitter resistance.
4Extra emitter degeneration reduces gain.
5Gain rises as frequency increases and capacitive reactance decreases.

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Low-frequency response visualization

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Remember

Low-frequency gain drop is mainly caused by external coupling and bypass capacitors.

4.2.2

Frequency Response

High-frequency response

At high frequency, internal transistor capacitances and stray capacitances become important. They provide unwanted signal paths and reduce effective gain.

Step-by-step working

1Base-emitter and base-collector capacitances begin to conduct AC current.
2Some input signal is shunted through capacitance instead of controlling transistor current.
3The Miller effect can make base-collector capacitance appear larger at the input.
4Gain begins to fall after the upper cutoff frequency.
5Phase shift increases as the amplifier approaches high-frequency limits.

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High-frequency response visualization

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Remember

High-frequency gain drop is mainly caused by internal and stray capacitances.

4.2.3

Frequency Response

Bandwidth

Bandwidth is the useful frequency range between lower cutoff and upper cutoff. Inside this range, gain is approximately flat enough for normal signal amplification.

Step-by-step working

1Find midband gain where capacitors behave ideally for the intended range.
2Locate lower cutoff frequency where gain falls by 3 dB from midband.
3Locate upper cutoff frequency where gain again falls by 3 dB.
4Bandwidth is fH minus fL.
5A wider bandwidth means the amplifier can preserve faster signal variations.

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Bandwidth visualization

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Remember

Bandwidth = fH - fL, measured between the two 3 dB cutoff frequencies.

4.3

Main Topic

Multistage Amplifiers

When one amplifier stage cannot give enough gain, stages are cascaded. The price is loading: each stage becomes the next stage's source.

4.3.1

Multistage Amplifiers

Cascaded gain

In a multistage amplifier, the output of one stage drives the input of the next. Overall voltage gain is the product of individual loaded gains, or the sum of gains when expressed in decibels.

Step-by-step working

1Bias each transistor stage separately.
2Pass the AC signal from one collector or emitter output into the next stage.
3Calculate each stage gain under loaded conditions.
4Multiply voltage gains to get total gain.
5In dB, add stage gains instead of multiplying ratios.

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Cascaded gain visualization

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Remember

Overall gain is product of stage gains, but use loaded gain for each stage.

4.3.2

Multistage Amplifiers

Coupling methods

Coupling decides how signal moves between stages while DC bias points remain controlled. RC coupling is common for voltage amplifiers, transformer coupling is used in some power/RF cases, and direct coupling passes DC as well as AC.

Step-by-step working

1RC coupling passes AC and blocks DC between stages.
2Transformer coupling transfers AC energy magnetically and can match impedance.
3Direct coupling connects stages without a capacitor or transformer.
4Each method affects frequency response and bias interaction.
5Choose coupling from signal frequency, gain need, and bias requirement.

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Coupling methods visualization

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Remember

RC coupling is common in small-signal voltage amplifiers because it isolates DC bias between stages.

4.3.3

Multistage Amplifiers

Loading effect

Loading occurs when the next stage input resistance draws signal current from the previous stage. This reduces the actual output voltage of the previous stage and lowers total gain.

Step-by-step working

1Previous stage has an output resistance.
2Next stage presents a finite input resistance.
3Together they form a voltage divider.
4The signal delivered to the next stage is less than unloaded output.
5Buffer stages are used when loading must be reduced.

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Loading effect visualization

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Remember

Do not multiply unloaded gains when stages load each other.

4.4

Main Topic

Power Amplifiers

A power amplifier is judged less by voltage gain and more by how efficiently it delivers large signal power to the load without unacceptable distortion.

4.4.1

Power Amplifiers

Class A

A Class A amplifier conducts for the full input cycle. It is very linear because the transistor never turns off during normal operation, but it wastes power because current flows even with no input signal.

Step-by-step working

1Bias the transistor near the middle of the active region.
2The device conducts for all 360 degrees of the signal cycle.
3Positive and negative signal swings remain within active operation.
4Distortion is low if swing limits are not crossed.
5Efficiency is low because DC power is continuously consumed.

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Class A visualization

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Remember

Class A: best linearity, poorest efficiency, 360 degree conduction.

4.4.2

Power Amplifiers

Class B

A Class B amplifier conducts for half of the signal cycle. Two complementary devices are usually used in push-pull form so one handles positive half and the other handles negative half.

Step-by-step working

1Each device is biased near cutoff.
2One device conducts during the positive half-cycle.
3The other device conducts during the negative half-cycle.
4Efficiency improves because each device rests for half the cycle.
5Crossover distortion can appear near zero crossing.

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Class B visualization

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Remember

Class B improves efficiency but suffers crossover distortion.

4.4.3

Power Amplifiers

Class AB

Class AB slightly biases both devices on near the zero crossing. This reduces crossover distortion while keeping efficiency better than Class A.

Step-by-step working

1Bias each device slightly above cutoff.
2Both devices conduct a little around zero crossing.
3Positive and negative halves transfer more smoothly.
4Crossover distortion is reduced.
5Efficiency sits between Class A and Class B.

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Class AB visualization

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Remember

Class AB is the practical compromise between Class A linearity and Class B efficiency.

4.4.4

Power Amplifiers

Push-pull amplifier

A push-pull amplifier uses two active devices working on opposite halves of the waveform. Their outputs combine at the load to reconstruct the full waveform with greater power capability.

Step-by-step working

1Input waveform is split into opposite drive signals or applied to complementary devices.
2Upper device supplies current for one half-cycle.
3Lower device supplies current for the other half-cycle.
4Load receives both halves as one complete waveform.
5Balanced operation reduces even-order distortion and improves power delivery.

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Push-pull amplifier visualization

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Remember

Push-pull action lets two devices share the waveform and deliver higher output power.