Power amplifiers,Class A amplifiers , Integrated circuit amplifiers and Frequency selective amplifiers.

Power amplifiers

A power amplifier is designed to deliver power to a load. Usually the term is applied to the  put stage of an audio amplifier, but similar design techniques are used in many other applications.

Power amplifiers are classified according to the bias condition of the output stage. The simplest arrangement is known as Class A and is shown in (a), while its typical characteristic is shown in (b).

Class A amplifiers

In Class A operation a transistor is biased such that collector current flows at all times. The stage has low distortion, but poor efficiency -at best 50%. Class A is therefore best suited for low power amplifiers for portable radios and similar circuits.

 

The human ear has a very nonlinear response, and at low volumes it is less sensitive to low frequencies. On some amplifiers a loudness control is used in place of a simple attenuator volume control. This provides progressive bass lift as volume is reduced, to compensate for the response of the ear. Hi-fi purists tend to decry the loudness control and prefer a simple volume control.

Power amplifiers

A power amplifier is designed to deliver power to a load. Usually the term is applied to the 0\ltput stage of an audio amplifier, but similar design techniques are used in many other applications.

Power amplifiers are classified according to the bias condition of the output stage. The simplest arrangement is known as Class A and is shown in (a), while its typical characteristic is shown in (b).

Class A amplifiers

In Class A operation a transistor is biased such that collector current flows at all times. The stage has low distortion, but poor efficiency -at best 50%. Class A is therefore best suited for low power amplifiers for portable radios and similar circuits.

Class B amplifiers

A Class B amplifier, shown below in (a), is biased at cut-off as shown in (b). By its very nature, Class B assumes push-pull operation, with one transistor operating during positive half-cycles and one transistor operating during negative half-cycles. The trimming resistor RV 1 sets bias for transistors TR 1 and TR2, while diodes D 1 and D2 provide compensation for variation in Vbe of the transistors with temperature. Class B amplifiers have high efficiency because quiescent current is small.

Class AB amplifiers

The nonlinear characteristic of a Class B amplifier causes severe distortion at low volume levels. This is known as crossover distortion and is particularly irritating to the ear. Consequently, pure Class B is seldom used in audio circuits, and it is usual to arrange a small standing current.

This mode is known as Class AB, and a typical characteristic is shown in (c). Here, both transistors are biased just beyond the nonlinear region, thereby reducing distortion considerably at the expense of just a small decrease in power efficiency.

Class C amplifiers

Class C amplifiers are only found in RF applications. The output stage is biased well beyond cut-off and only conducts on the peaks of the input signal. This produces the highly distorted waveform shown, which is useless for audio circuits.

In RF circuits, on the other hand, the use of a tuned load restores the correct shape. Class C amplifiers are very efficient and so are widely used in RF applications.

Class D amplifiers

The final type of amplifier is Class D, which uses the output transistors as simple switches. They are therefore either on or off as shown, and the output is achieved by varying the mark-space ratio of a square wave. Dissipation of the output transistors is low, as they are either on (low volts, high current) or off (high volts, zero current).

Class D amplifiers are very efficient, but few practical designs have yet been produced.

All power amplifiers dissipate a fair amount of heat, the majority of it coming from the output transistors. Heat sinks are used to remove the heat and ensure that the transistors operate at a safe temperature.

Video amplifiers

An amplifier whose bandwidth extends from a frequency in the low audio range to a frequency in the megahertz range is generally termed a video amplifier, regardless of use. The term originates from television circuits where wideband amplifiers are needed, although video amplifiers are used in other equipment such as radar and ultrasonics.

The design of video amplifiers is superficially similar to audio amplifiers. The bandwidth is limited at high frequencies by stray capacitance and the fall of hfe in the transistors. The lower limit is determined by the rising impedance of interstage coupling capacitors and emitter decoupling capacitors.

Stray capacitance is reduced by careful layout and choice of components, but it can never be completely removed. Stray capacitance of 20 pF has an impedance of about 4 kQ at 2 MHz, and I kQ at 5 MHz. This means that very low values of resistance would need to be used to swamp the stray capacitance by the rest of the circuit. Low values of collector resistance, however, give low gain; so this approach is not very practical.

A practical solution, on the other hand, is to use a series inductor and resistor as a collector load. The value of inductor L is chosen with a knowledge of the stray capacitance, such that together they form a resonant circuit at the maximum required frequency (usually around 5 MHz). As the value of resistor R is quite high, the circuit is highly damped and gain is fairly constant over the required frequency range.

Gain at low frequencies is largely determined by interstage coupling. Capacitance coupling and transformer coupling both have poor low frequency characteristics, and it is usual for video amplifiers to employ DC coupling.

A typical video stage is shown . This stage amplifies a video signal from a few volts to around 60 V to drive a TV tube. Note the use of DC coupling and the compensating inductor in the collector circuit.

Frequency selective amplifiers

In RF amplifiers, a circuit is often required to amplify a very narrow band of frequencies and reject all others. Frequency selective amplifiers (often called tuned amplifiers, or simply RF amplifiers) usually use LC circuits to provide the necessary tuning.

The circuit of (a) is known as a series tuned circuit. The inductor and capacitor have exactly opposite phase effects, so resonance occurs when their reactances are equal, that is:

Gain at low frequencies is largely determined by interstage coupling. Capacitance coupling and transformer coupling both have poor low frequency characteristics, and it is usual for video amplifiers to employ DC coupling.

At resonance, impedance is determined solely by r. The circuit thus exhibits a low impedance at resonance.

The circuit of (b) is known as a parallel tuned circuit, and is more common in RF amplifiers. The circuit again exhibits resonance when the reactance of the inductor and capacitor are equal, and the formulae above apply.

The parallel tuned circuit exhibits a very high impedance at resonance. It is in theory, infinite, but practical components exhibit series resistance, denoted by r in (b).

If a parallel tuned circuit is used as the collector load of a transistor amplifier, gain varies with the impedance of the tuned circuit, being a maximum at resonance.

The maximum gain is determined mainly by the series resistance of the coil. A 'magnification factor', denoted by Q, is defined for a coil at resonance, where:

where w = 2rt/ and f is the resonant frequency. The higher the value of Q, the higher will be the gain of the amplifier. Typical values for Q are in the range 100--500.

At resonance the parallel LC circuit looks like a pure resistance, R, given by any of the formulae:

It might be thought that Q should be designed to be as high as possible, as an RF amplifier is used typically to select just one particular radio station at a time. This is not quite the case, however, as a radio signal does not occupy just one specific frequency. It consists of a centre carrier frequency and a band of side frequencies. An RF amplifier is thus required to amplify a fairly narrow band of frequencies, and the ideal response would be that of (a).

Amplifier response in radio receivers: (a) ideal response; (b) single stage with Q = 100; (c) single stage with Q = 500; (d) multistage, each stage Q = 100

The higher the value of Q in a tuned circuit, the narrower the band of frequencies that can be amplified. Figures (b) and (c) compare the response of identical circuits with Q of 100 and 500. The high Q circuit has a very narrow response, and would not amplify the required band of frequencies. The low Q circuit passes too large a range of frequencies. The shape of a resonance curve, in fact, means that there is no value of Q which gives a reasonable approximation to (a) in a single-stage amplifier.

If, however, a multistage amplifier is constructed, and each stage has a Q of 100, we get the response of (d). This has the required width and reasonably sharp sides, approximating fairly well to (a). In RF amplifiers, multistages are used primarily to improve selectivity -not necessarily for more gain.

A transistor RF amplifier usually has a high output impedance and a fairly low input impedance. Transformers are therefore used to obtain the correct matching between stages, as shown. A tapped primary is used to allow L and C to have convenient values.

Techniques described here allow RF amplifiers to be constructed up to the bottom of the VHF band (about 40 MHz). Above this frequency, special techniques are needed which are somewhat beyond the scope of this book.

Integrated circuit amplifiers

Integrated circuits are, of course, used as AC amplifiers- but not quite to the same extent they are used as DC amplifiers and logic circuits. Initially integrated circuits were used for audio amplifiers, and today simple integrated circuit high power audio amplifiers can be built with a minimum of external components. A typical circuit, complete with tone control, is shown. The majority of television and 'middle market' music centres use integrated circuit amplifiers. Hi-fi purists tend to prefer transistor pre-amplifiers, and there is some justification for this view as the noise performance of most integrated circuit pre-amplifiers does not yet match good transistor circuits.

Integrated circuits are also used in RF and IF circuits, although the component count is not reduced as dramatically as for audio amplifiers. Tuning components still have to be provided, and this is likely to remain unchanged. Many integrated circuits are being designed for specific domestic applications, and the majority of modern colour TVs use integrated circuits for the IF, sound detector and colour demodulator. A unique single-chip TRF radio receiver is available from one enterprising manufacturer.

Integrated circuit AC amplifiers will doubtless increase in versatility and the day will probably come when circuit designers select an AC amplifier from a catalogue in much the same way as they select DC amplifiers and logic gates.