By the end of this chapter, you will be able to:
Mastering these skills will help you design and troubleshoot electronic devices effectively, making you a valuable technician in the field of analogue electronics.
Compiled by: Joan Jerotich
Amplifiers are classified in several ways at once, because the same amplifier is described by how many stages it has, how the stages are joined, and what range of frequencies it handles. A technician who reads these three descriptions on a data sheet or a circuit diagram can tell at once what the amplifier is for, where its limits are and what to look for when it fails. An amplifier in a radio receiver, for instance, is a multistage amplifier, its stages are coupled by capacitors and transformers, and it works at audio, intermediate and radio frequencies in different parts of the circuit.
A stage is one amplifying device (a transistor or an op-amp) with the components that bias it and connect it to the signal. One stage seldom gives enough gain, so stages are connected one after another, in cascade, so that the output of one is the input of the next.
The overall voltage gain of a cascade is the product of the gains of the stages, provided that each stage is not loaded by the next. In decibels, the gains add.
Example: three stages have voltage gains of 20, 15 and 10.
The stages of a multistage amplifier have different jobs. The first stage handles a very small signal and must add little noise. The middle stages supply most of the voltage gain. The last stage, the output or power stage, delivers the current needed to drive the load. Adding stages raises gain but narrows the bandwidth, increases the risk of noise and unwanted oscillation, and makes the circuit more complex.
Coupling is the way the output of one stage is connected to the input of the next. The coupling method has to pass the signal while keeping the direct (bias) voltages of one stage from disturbing the bias of the next, and it largely decides the frequency response, the cost and the size of the amplifier.
| Feature | Capacitive (RC) coupling | Transformer coupling | Direct coupling |
|---|---|---|---|
| Coupling part | Capacitor and resistors | Transformer | A direct connection, or resistors only |
| Passes DC | No | No | Yes |
| Low-frequency response | Limited by the capacitor | Poor, limited by the core | Excellent, down to zero hertz |
| Impedance matching | Poor | Good | Poor |
| Size and cost | Small and cheap | Large and costly | Small, but harder to design |
| Typical use | Audio and general amplifiers | Tuned and power stages, isolation | Op-amps and DC amplifiers |
A capacitor between the stages passes the changing signal and blocks the steady bias voltage, so each stage is biased independently. The capacitor and the input resistance of the next stage form a high-pass filter, and the low-frequency cut-off is f = 1 ÷ (2πRC). Example: a 4.7 µF coupling capacitor that feeds a stage whose total resistance is 2.2 kΩ gives f = 1 ÷ (2π × 2200 × 4.7 × 10⁻⁶) = 15.4 Hz, so frequencies below about 15 Hz are reduced.
Capacitive coupling is cheap and compact, and is widely used in audio amplifiers. Its limits are that it cannot amplify a steady (DC) signal, and that an ageing electrolytic coupling capacitor can lose capacity, which weakens the bass. A leaking coupling capacitor passes some DC and upsets the bias of the next stage.
A transformer passes the signal from the collector circuit of one stage to the input of the next through its windings. It isolates the stages electrically and can match the high output impedance of one stage to the low input impedance of the next, so that the maximum power is transferred. Because the response of a transformer falls at low frequency, and because it is large, heavy and costly, it is used less in modern audio equipment. It is still found in tuned radio-frequency stages and in output stages that must be isolated from the load.
In direct coupling the output of one stage is connected to the input of the next with a wire or with resistors, without any capacitor or transformer. It passes DC as well as AC, so it is the usual method for amplifying very slow signals, such as the output of a temperature sensor or a thermocouple. It uses fewer parts and gives good performance at low frequencies. Its weakness is that any small change in the bias of the first stage, caused by temperature or by supply variation, is amplified by all following stages and shows up at the output as drift. The circuits are designed carefully, with complementary transistors or differential pairs, to reduce this drift. Op-amps are direct-coupled inside.
Amplifiers are also classified by the range of frequencies they are designed to amplify, because the parts, the layout and the design method change with frequency.
| Type | Approximate range | Examples |
|---|---|---|
| Audio-frequency (AF) amplifier | 20 Hz to 20 kHz | Public address systems, radio and television sound stages, musical instruments |
| Video amplifier | From a few hertz to several megahertz | Television and monitor video circuits |
| Intermediate-frequency (IF) amplifier | A fixed frequency, such as 455 kHz in AM radio and 10.7 MHz in FM radio | Radio and television receivers |
| Radio-frequency (RF) amplifier | From about 20 kHz up to many gigahertz | Transmitters, receivers, mobile base stations |
These cover the range that the human ear hears. They are designed for a flat response across the band, low distortion and low noise, and the output stage drives a loudspeaker. The preamplifier raises a weak microphone signal, and the power amplifier drives the speakers of an outdoor public address system.
Radio-frequency amplifiers amplify signals from the antenna or in the transmitter. They often use tuned circuits so that they amplify only a chosen band and reject others. Short leads, careful layout and screening are essential, because at high frequency stray capacitance and inductance change how the circuit behaves.
In a superheterodyne receiver the incoming signal is converted to a fixed intermediate frequency, and the IF amplifier provides most of the gain and the selectivity at that frequency. Because the frequency is fixed, the tuned circuits in the stage can be set once at the factory or in the workshop.
A different classification is by the portion of the input cycle during which the output device conducts. It affects the efficiency and the distortion.
| Class | Conducts for | Features |
|---|---|---|
| A | The whole cycle | Low distortion and low efficiency, and the device runs hot even with no signal |
| B | Half the cycle | Higher efficiency but crossover distortion, so two devices are used in push-pull |
| AB | More than half but less than the whole cycle | A compromise between A and B, widely used in audio power stages |
| C | Less than half the cycle | High efficiency but a distorted output, so it is used with a tuned load in RF stages |
The detail of efficiency and of power stages is dealt with in the topic on power amplifiers.
Create a free account to open more of this chapter.
Free: practical guides, quick cards, workplace scenarios and more.
Create a free accountThis chapter explored the classification of amplifiers based on coupling methods and frequency response, highlighting how these factors influence amplifier design and application. Various types of amplifiers were examined, including RC coupled amplifiers, power amplifiers, and tuned amplifiers, each serving distinct functions in electronic circuits. The principle of feedback was introduced, with a clear distinction between positive and negative feedback and their effects on amplifier performance. Operational amplifiers were discussed in detail, covering their internal construction, the differences between ideal and practical characteristics, and a range of configurations such as inverting, non-inverting, voltage follower, summing, differential, instrumentation, integrator, differentiator, comparator, and Schmitt trigger. The chapter concluded with an overview of the practical applications of amplifiers in electronic systems, demonstrating their critical role in signal processing across various fields.
At the start of this chapter we promised you would be able to:
Tick each one you can genuinely do.
So, are you there yet?
You're competent when you can confidently do 50% or more of what this chapter promised.
Sign in to record how you're doing.