Amplifiers

Amplifiers increase the amplitude of a signal. Voltage amplifiers deal with small signals, but the final (output) stage must deliver significant power to a load such as a loudspeaker or antenna, so efficiency and distortion become important. This topic covers the classification of output stages (Class A, B, AB, C), biasing of the Class AB stage, power BJTs, transformer-coupled push-pull stages, tuned amplifiers and the operational amplifier (op-amp), which is the most widely used analogue IC.

Key concepts

• Classification by conduction angle (portion of the input cycle for which the transistor conducts):

Class	Conduction angle	Max. efficiency	Distortion	Typical use
A	360°	25% (resistive load), 50% (transformer coupled)	lowest	small-signal, driver stages
B	180°	78.5% ($\pi/4$)	crossover distortion	push-pull audio
AB	slightly > 180°	between A and B (~50–78%)	low	audio power amplifiers
C	< 180°	> 78.5% (up to ~90%)	very high	RF tuned amplifiers

• Class A output stage: The transistor is biased at the middle of the load line and conducts for the whole cycle. Simple and linear, but it dissipates maximum power when there is no signal, so efficiency is poor.
• Class B output stage: Two complementary transistors (npn and pnp), each conducting for one half cycle, are biased at cut-off. No quiescent current, so efficiency is high. Because each transistor needs about 0.7 V before it conducts, there is a dead zone near zero crossing — crossover distortion.
• Class AB output stage: A small forward bias (about $2V_{BE}$ between the two bases) keeps both transistors slightly on at zero signal, removing crossover distortion at a small cost in efficiency.
• Biasing the Class AB stage: Two diodes (or a $V_{BE}$ multiplier circuit) placed between the bases provide the bias. Diodes thermally matched and mounted on the same heat sink also give thermal compensation, preventing thermal runaway; small emitter resistors add stability.
• Power BJTs: Large-area devices that handle high currents and dissipation. Key ratings: $I_{C(max)}$, $V_{CE0}$ (breakdown), $P_{D(max)}$ and the safe operating area (SOA), including the second-breakdown limit. Heat must be removed through a heat sink; junction temperature $T_J = T_A + P_D\,\theta_{JA}$. Power transistors usually have lower β than small-signal types; Darlington pairs are used to raise current gain ($\beta \approx \beta_1\beta_2$).
• Transformer-coupled push-pull stage: A centre-tapped input transformer drives two identical transistors in antiphase; a centre-tapped output transformer combines their currents. Advantages: impedance matching to the load, no DC in the load, even harmonics cancel. Disadvantages: transformers are bulky, costly and limit frequency response.
• Tuned amplifiers: Use an LC tank as load so that they amplify only a narrow band around the resonant frequency $f_r = 1/(2\pi\sqrt{LC})$. Used in radio and TV receivers (IF and RF stages). Single-tuned, double-tuned and stagger-tuned types exist; stagger tuning broadens bandwidth. Class C tuned amplifiers are used in RF transmitters.
• Op-amp: A high-gain DC-coupled differential amplifier. Ideal characteristics: infinite open-loop gain, infinite input impedance, zero output impedance, infinite bandwidth, infinite CMRR, zero offset. With negative feedback the two inputs are at the same potential (virtual short) and draw no current.
• Common op-amp circuits: inverting amplifier, non-inverting amplifier, voltage follower (gain 1), summing amplifier, differential amplifier, integrator, differentiator, comparator. Important specifications: slew rate (V/µs), CMRR, input offset voltage, gain-bandwidth product. The 741 is the classic general-purpose op-amp (slew rate about 0.5 V/µs).

High-yield points

• Maximum efficiency: Class A 25% (50% transformer-coupled), Class B 78.5%.
• Conduction angle: A 360°, B 180°, AB slightly > 180°, C < 180°.
• Class B has crossover distortion; Class AB biasing removes it.
• Push-pull stages cancel even harmonics.
• Ideal op-amp: infinite gain and input impedance, zero output impedance.
• Inverting gain $-R_f/R_{in}$; non-inverting gain $1 + R_f/R_{in}$.

Formulas

$\eta = \frac{P_{ac(out)}}{P_{dc(in)}}\times100\%$

Class B maximum: $\eta_{max} = \dfrac{\pi}{4} = 78.5\%$.

$A_{v,\text{inv}} = -\frac{R_f}{R_{in}}, \qquad A_{v,\text{non-inv}} = 1 + \frac{R_f}{R_{in}}$ $R_f$ = feedback resistor, $R_{in}$ = input resistor (Ω).

$\text{CMRR} = \frac{A_d}{A_{cm}}, \quad \text{in dB: } 20\log_{10}\frac{A_d}{A_{cm}}$ $A_d$ = differential gain, $A_{cm}$ = common-mode gain.

Worked example

An inverting op-amp amplifier has $R_{in} = 10$ kΩ and $R_f = 100$ kΩ. Input is 0.2 V. Find the output.

1. $A_v = -R_f/R_{in} = -100/10 = -10$.
2. $V_o = -10\times0.2 = -2$ V.
3. If the same resistors were used as a non-inverting amplifier: $A_v = 1+10 = 11$, $V_o = 2.2$ V.

Exam tips

• Efficiency order: C > B > AB > A. Linearity order is the reverse.
• Crossover distortion belongs to Class B and is removed by Class AB biasing.
• Class A dissipates most power with no input; Class B dissipates nothing at zero signal.
• Push-pull cancels even harmonics.
• Voltage follower: gain = 1, very high input impedance — used as a buffer.

Exam-style questions

From Calmon's question bank, which has 25 questions on this topic.

1. In a class A output stage, the transistor conducts for:

• A. Less than 180°
• B. The full 360° of the input cycle
• C. 180° of the input cycle
• D. Between 180° and 360°

Answer: B. A class A stage is biased so that collector current flows throughout the whole input cycle (conduction angle 360°).

2. The output impedance of an ideal op-amp is:

• A. Zero
• B. Infinite
• C. 75 Ω
• D. 1 kΩ

Answer: A. An ideal op-amp has infinite open-loop gain and input impedance, and zero output impedance.

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