Amplifiers play a pivotal role in analogue electronics by increasing the amplitude of electrical signals without altering their other characteristics. In Kenya’s electronics engineering sector, whether designing communication systems for apartment blocks or developing audio equipment for local broadcasting stations, understanding how amplifiers function and are classified is fundamental. This chapter focuses on the classification of amplifiers and explores the main types used in professional applications, emphasizing their coupling methods and frequency responses.

6.1 Classification of Amplifiers

An amplifier increases the amplitude of a signal. Amplifiers differ greatly in how they are built and what they are used for, so they are classified. A technician who knows the classes can pick the right amplifier for a task and understand what to expect from it. Three classifications are used in this topic: by the number of stages, by the method of coupling the stages, and by the frequency range that the amplifier handles.

6.1.1 Stages

A stage is one transistor (or other active device) with its bias and load components, and it is the basic unit of an amplifier. A single-stage amplifier has one such stage, and a multistage amplifier has two or more stages connected one after the other (in cascade), with the output of each stage feeding the input of the next.

One stage cannot always give enough gain. A microphone gives a few millivolts, and a loudspeaker needs several volts, so the signal must be raised by a factor of hundreds or thousands. Stages are therefore cascaded: early stages (the pre-amplifier or voltage amplifier stages) increase the voltage and the final stage (the power or output stage) supplies the current needed by the load.

Gain of a multistage amplifier. When the stages do not load each other, the overall voltage gain is the product of the individual gains:

A_total = A₁ × A₂ × A₃ × …

Gains are often expressed in decibels (dB), using

Gain in dB = 20 log₁₀ (voltage gain)

and the gain of cascaded stages in dB is the sum of the gains of the stages in dB.

Example 1: A three-stage amplifier has stage voltage gains of 20, 15 and 10. Find the overall gain as a ratio and in dB, and the output for an input of 2 mV.

Overall gain = 20 × 15 × 10 = 3000

In dB: 20 log₁₀ 20 = 26.02 dB, 20 log₁₀ 15 = 23.52 dB, 20 log₁₀ 10 = 20.00 dB, and the sum is 69.54 dB. Check: 20 log₁₀ 3000 = 69.54 dB.

Output = 3000 × 2 mV = 6000 mV = 6 V

Answer: The overall gain is 3000 (69.54 dB), and a 2 mV input gives 6 V at the output. In a real amplifier the gain is a little lower, because each stage loads the previous one.

Single-stage and multistage amplifiers compare as follows.

6.1.2 Coupling Method

The coupling method is the way the signal is passed from one stage to the next. The coupling must pass the signal and, ideally, keep the bias conditions of each stage separate. The main methods are RC coupling, transformer coupling and direct coupling.

RC coupling

A coupling capacitor in series with the signal path, and a resistor to ground at the next stage input, link the stages. The capacitor blocks the direct bias voltage of one stage from the next but passes the alternating signal. RC coupling is cheap, small and has good frequency response over the audio range, and is the commonest form of coupling in voltage amplifiers. Its drawbacks are that it cannot amplify direct (0 Hz) signals, that low-frequency gain falls because the capacitor reactance rises as frequency falls, and that it wastes some power in the coupling resistors.

The low-frequency cut-off caused by the coupling is

f_L = 1 ÷ (2π R C)

where R is the total resistance seen by the capacitor (the output resistance of one stage plus the input resistance of the next) and C is the coupling capacitance.

Example 2: The coupling capacitor between two stages is 1 µF. The output resistance of the first stage is 4.7 kΩ and the input resistance of the second is 5.3 kΩ. Find the lower cut-off frequency.

R = 4.7 + 5.3 = 10 kΩ

f_L = 1 ÷ (2π × 10 000 × 1 × 10⁻⁶) = 1 ÷ 0.06283 = 15.9 Hz

Answer: The lower cut-off frequency is 15.9 Hz, below the audio range, so low notes pass. A capacitor of 0.1 µF would raise the cut-off to 159 Hz and cut the bass.

Transformer coupling

A transformer links the stages through its windings. It passes alternating signals, isolates the bias of one stage from another, and, importantly, matches the impedance of the source to that of the load, so that the maximum power is transferred. It is used in power amplifier output stages, and between an amplifier and a loudspeaker line. It is bulky, heavy and more costly than RC coupling, and its frequency response is poorer, because the winding inductance and capacitance restrict the ends of the range.

For impedance matching, the turns ratio is

N₁ ÷ N₂ = √(Z₁ ÷ Z₂)

where Z₁ is the impedance seen by the primary winding and Z₂ is the impedance of the load on the secondary.

Example 3: An output stage works best into a load of 800 Ω, and the loudspeaker has an impedance of 8 Ω. Find the turns ratio of the matching transformer.

N₁ ÷ N₂ = √(800 ÷ 8) = √100 = 10

Answer: The turns ratio is 10 : 1 (primary to secondary), which transforms the 8 Ω speaker to look like 800 Ω at the primary.

Direct coupling

In direct coupling the output of one stage is connected to the input of the next with no capacitor or transformer. It amplifies signals from 0 Hz (direct voltages) upward, and uses the fewest components. It is the method used inside operational amplifiers and in amplifiers for slowly changing signals from sensors. The disadvantage is that the bias of each stage affects the next, so a small change in supply or temperature in the first stage is amplified by all the following stages and appears as drift at the output. It needs careful design of the bias.

6.1.3 Frequency

Amplifiers are also classified by the band of frequencies they amplify well. The bandwidth is the range between the lower and upper half-power (−3 dB) frequencies:

BW = f_H − f_L

At each of these frequencies the voltage gain has fallen to 0.707 of its mid-band value.

Frequency classes

1. Direct current (DC) amplifiers amplify from 0 Hz up to a limit, and are used for sensor and instrument signals.
2. Audio frequency (AF) amplifiers work from about 20 Hz to 20 kHz, the range of human hearing, and are used in public address systems, radio receivers and music equipment.
3. Video amplifiers handle a wide band from a few hertz to several megahertz, for pictures and fast pulses.
4. Radio frequency (RF) amplifiers work from some tens of kilohertz to many gigahertz, and are used in transmitters and in the front end of receivers. They are usually tuned, so that they amplify a narrow band around the wanted frequency.
5. Intermediate frequency (IF) amplifiers are tuned amplifiers working at a fixed frequency inside a receiver, for example 455 kHz in an AM radio and 10.7 MHz in an FM radio.

Example 4: An audio amplifier has a mid-band gain of 100, a lower cut-off frequency of 20 Hz and an upper cut-off of 20 kHz. Find the bandwidth and the gain at the cut-off frequencies.

BW = 20 000 − 20 = 19 980 Hz, about 20 kHz

Gain at f_L and at f_H = 0.707 × 100 = 70.7 (which is 3 dB below the mid-band gain of 40 dB, so 37 dB)

Answer: The bandwidth is 19.98 kHz and the gain at each cut-off is 70.7.

In general an amplifier trades gain for bandwidth, which is one reason why a large gain is obtained from several cascaded stages rather than from one stage.

Choosing by frequency

Select the amplifier class to cover the whole range of the signal, with some margin. An audio amplifier used for radio frequencies has far too little gain, and an RF amplifier is not suited to audio signals. A signal of 1 MHz passing through an amplifier whose upper cut-off is 100 kHz is attenuated, and its shape is distorted.

Chapter Summary

This chapter explored the classification of amplifiers based on coupling methods and frequency response, providing a foundational understanding of how amplifiers are categorized in analogue electronics. It detailed various types of amplifiers, including RC coupled amplifiers, power amplifiers, and tuned amplifiers, highlighting their unique characteristics and applications. The principle of feedback was examined with a focus on the differences between positive and negative feedback and their effects on amplifier performance. Operational amplifiers were discussed extensively, covering their construction, the distinctions between ideal and practical models, and a range of configurations such as inverting, non-inverting, voltage follower, and summing amplifiers. Additional specialized op-amp configurations like differential, instrumentation amplifiers, integrators, differentiators, comparators, and Schmitt triggers were also explained. The chapter concluded by addressing the practical applications of amplifiers, emphasizing their role in signal processing and electronic systems. Together, these topics provide a comprehensive overview of amplifiers in analogue electronics and their critical functions in various electronic circuits.