PDF. 1. ©E. COATES Power Amplifiers. Introduction to Power Amplifiers. Power •The limitations due to the efficiency of class A power amplifiers. •Transformer Test your knowledge and understanding of Power Amplifiers. Power Amplifiers (PA) deliver power to a given load with maximum efficiency while faithfully transferring the modulation from the input to the output. Purpose of a power amplifier. – Generate high output power. – Efficient conversion of DC power to RF power. – Linear amplification. • Generally PAs will be.

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Power amplifiers typically contain bulky components having large surface areas the variations in the output level of a power amplifier occur in the active region . those amplifier circuits used to handle large-voltage signals at moderate to high main features of a large-signal amplifier are the circuit's power efficiency, the. In small-signal amplifiers, the main factors are usually amplification linearity and magnitude of gain. • Large-signal or power amplifiers, on the other hand.

Inductor L1 and capacitor C1 forms a tank circuit which aids in the extraction of the required signal from the pulsed output of the transistor. Actual job of the active element transistor here is to produce a series of current pulses according to the input and make it flow through the resonant circuit.

Values of L1 and C1 are so selected that the resonant circuit oscillates in the frequency of the input signal. Since the resonant circuit oscillates in one frequency generally the carrier frequency all other frequencies are attenuated and the required frequency can be squeezed out using a suitably tuned load.

A coupling transformer can be used for transferring the power to the load. Advantages of Class C power amplifier. High efficiency. Lowest physical size for a given power output. Disadvantages of Class C power amplifier. Lowest linearity. Not suitable in audio applications. Creates a lot of RF interference. It is difficult to obtain ideal inductors and coupling transformers.

Audio power amplifiers are typically used to drive loudspeakers. They will often have two output channels and deliver equal power to each. An RF power amplifier is found in radio transmitter final stages.

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A Servo motor controller : amplifies a control voltage to adjust the speed of a motor, or the position of a motorized system. Operational amplifiers op-amps [ edit ] Main articles: Operational amplifier and Instrumentation amplifier An operational amplifier is an amplifier circuit which typically has very high open loop gain and differential inputs. Op amps have become very widely used as standardized "gain blocks" in circuits due to their versatility; their gain, bandwidth and other characteristics can be controlled by feedback through an external circuit.

Though the term today commonly applies to integrated circuits, the original operational amplifier design used valves, and later designs used discrete transistor circuits. A fully differential amplifier is similar to the operational amplifier, but also has differential outputs.

Main article: Distributed amplifier These use balanced transmission lines to separate individual single stage amplifiers, the outputs of which are summed by the same transmission line.

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The transmission line is a balanced type with the input at one end and on one side only of the balanced transmission line and the output at the opposite end is also the opposite side of the balanced transmission line. The gain of each stage adds linearly to the output rather than multiplies one on the other as in a cascade configuration.

This allows a higher bandwidth to be achieved than could otherwise be realised even with the same gain stage elements. Switched mode amplifiers[ edit ] These nonlinear amplifiers have much higher efficiencies than linear amps, and are used where the power saving justifies the extra complexity. Class-D amplifiers are the main example of this type of amplification. Compared to other types of amplifiers, this "negative resistance amplifier" will only require a tiny amount of power to achieve very high gain, maintaining a good noise figure at the same time.

Certain requirements for step response and overshoot are necessary for an acceptable TV image. They typically can amplify across a broad spectrum of frequencies; however, they are usually not as tunable as klystrons. Klystrons are designed for large scale operations and despite having a narrower bandwidth than TWTAs, they have the advantage of coherently amplifying a reference signal so its output may be precisely controlled in amplitude, frequency and phase.

The maser is a non-electronic microwave amplifier. Musical instrument amplifiers[ edit ] Instrument amplifiers are a range of audio power amplifiers used to increase the sound level of musical instruments, for example guitars, during performances.

Classification of amplifier stages and systems[ edit ] Common terminal[ edit ] One set of classifications for amplifiers is based on which device terminal is common to both the input and the output circuit.

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In the case of bipolar junction transistors , the three classes are common emitter, common base, and common collector.

For field-effect transistors , the corresponding configurations are common source, common gate, and common drain; for vacuum tubes , common cathode, common grid, and common plate.

The common emitter or common source, common cathode, etc. The common collector arrangement applies the input voltage between base and collector, and to take the output voltage between emitter and collector. This causes negative feedback, and the output voltage tends to follow the input voltage.

This arrangement is also used as the input presents a high impedance and does not load the signal source, though the voltage amplification is less than one.

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The common-collector circuit is, therefore, better known as an emitter follower, source follower, or cathode follower. Unilateral or bilateral[ edit ] An amplifier whose output exhibits no feedback to its input side is described as 'unilateral'. The input impedance of a unilateral amplifier is independent of load, and output impedance is independent of signal source impedance.

Bilateral amplifier input impedance depends on the load, and output impedance on the signal source impedance. All amplifiers are bilateral to some degree; however they may often be modeled as unilateral under operating conditions where feedback is small enough to neglect for most purposes, simplifying analysis see the common base article for an example. Inverting or non-inverting[ edit ] Another way to classify amplifiers is by the phase relationship of the input signal to the output signal.

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An 'inverting' amplifier produces an output degrees out of phase with the input signal that is, a polarity inversion or mirror image of the input as seen on an oscilloscope. A 'non-inverting' amplifier maintains the phase of the input signal waveforms. An emitter follower is a type of non-inverting amplifier, indicating that the signal at the emitter of a transistor is following that is, matching with unity gain but perhaps an offset the input signal.

Voltage follower is also non inverting type of amplifier having unity gain. This description can apply to a single stage of an amplifier, or to a complete amplifier system. Function[ edit ] Other amplifiers may be classified by their function or output characteristics.

These functional descriptions usually apply to complete amplifier systems or sub-systems and rarely to individual stages. A servo amplifier indicates an integrated feedback loop to actively control the output at some desired level.

A DC servo indicates use at frequencies down to DC levels, where the rapid fluctuations of an audio or RF signal do not occur. These are often used in mechanical actuators, or devices such as DC motors that must maintain a constant speed or torque.

An AC servo amp. A linear amplifier responds to different frequency components independently, and does not generate harmonic distortion or intermodulation distortion. No amplifier can provide perfect linearity even the most linear amplifier has some nonlinearities, since the amplifying devices— transistors or vacuum tubes —follow nonlinear power laws such as square-laws and rely on circuitry techniques to reduce those effects.

A nonlinear amplifier generates significant distortion and so changes the harmonic content; there are situations where this is useful. Amplifier circuits intentionally providing a non-linear transfer function include: a device like a silicon controlled rectifier or a transistor used as a switch may be employed to turn either fully on or off a load such as a lamp based on a threshold in a continuously variable input.

Following such an amplifier with a so-called tank tuned circuit can reduce unwanted harmonics distortion sufficiently to make it useful in transmitters , or some desired harmonic may be selected by setting the resonant frequency of the tuned circuit to a higher frequency rather than fundamental frequency in frequency multiplier circuits.

Automatic gain control circuits require an amplifier's gain be controlled by the time-averaged amplitude so that the output amplitude varies little when weak stations are being received.


The non-linearities are assumed arranged so the relatively small signal amplitude suffers from little distortion cross-channel interference or intermodulation yet is still modulated by the relatively large gain-control DC voltage. Operational amplifier comparator and detector circuits. A wideband amplifier has a precise amplification factor over a wide frequency range, and is often used to boost signals for relay in communications systems. A narrowband amp amplifies a specific narrow range of frequencies, to the exclusion of other frequencies.

An RF amplifier amplifies signals in the radio frequency range of the electromagnetic spectrum , and is often used to increase the sensitivity of a receiver or the output power of a transmitter.

This category subdivides into small signal amplification, and power amps that are optimised to driving speakers , sometimes with multiple amps grouped together as separate or bridgeable channels to accommodate different audio reproduction requirements. Frequently used terms within audio amplifiers include: Power amplifier normally drives loudspeakers , headphone amplifiers, and public address amplifiers. Stereo amplifiers imply two channels of output left and right , though the term simply means "solid" sound referring to three-dimensional —so quadraphonic stereo was used for amplifiers with four channels.

Buffer amplifiers , which may include emitter followers , provide a high impedance input for a device perhaps another amplifier, or perhaps an energy-hungry load such as lights that would otherwise draw too much current from the source. Line drivers are a type of buffer that feeds long or interference-prone interconnect cables, possibly with differential outputs through twisted pair cables.

Interstage coupling method[ edit ] See also: multistage amplifiers Amplifiers are sometimes classified by the coupling method of the signal at the input, output, or between stages. Different types of these include: Resistive-capacitive RC coupled amplifier, using a network of resistors and capacitors By design these amplifiers cannot amplify DC signals as the capacitors block the DC component of the input signal.

RC-coupled amplifiers were used very often in circuits with vacuum tubes or discrete transistors. In the days of the integrated circuit a few more transistors on a chip are much cheaper and smaller than a capacitor.

Inductive-capacitive LC coupled amplifier, using a network of inductors and capacitors This kind of amplifier is most often used in selective radio-frequency circuits.

Transformer coupled amplifier, using a transformer to match impedances or to decouple parts of the circuits Quite often LC-coupled and transformer-coupled amplifiers cannot be distinguished as a transformer is some kind of inductor.

Direct coupled amplifier , using no impedance and bias matching components This class of amplifier was very uncommon in the vacuum tube days when the anode output voltage was at greater than several hundred volts and the grid input voltage at a few volts minus.Can I set up the amplifier from a network?

PowerMatch amplifiers can directly drive constant voltage loads without the use of external transformers. As compare to voltage amplifier, power amplifiers handle larger signals, therefore, the problem of distortion occurs immediately.

Values of L1 and C1 are so selected that the resonant circuit oscillates in the frequency of the input signal. A radio frequency RF amplifier design typically optimizes impedances for power transfer, while audio and instrumentation amplifier designs normally optimize input and output impedance for least loading and highest signal integrity.

Transformer coupled amplifier, using a transformer to match impedances or to decouple parts of the circuits Quite often LC-coupled and transformer-coupled amplifiers cannot be distinguished as a transformer is some kind of inductor. A DC servo indicates use at frequencies down to DC levels, where the rapid fluctuations of an audio or RF signal do not occur. RF oscillators. For field-effect transistors , the corresponding configurations are common source, common gate, and common drain; for vacuum tubes , common cathode, common grid, and common plate.

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