Single-ended and triode class-A amplifiers[ edit ] Some hobbyists who prefer class-A amplifiers also prefer the use of thermionic valve tube designs instead of transistors, for several reasons: A classic application for a pair of class-A devices is the long-tailed pairwhich is exceptionally linear, and forms the basis of many more complex circuits, including many audio amplifiers and almost all op-amps.
Often, bias voltage applied to set this quiescent current must be adjusted with the temperature of the output transistors. At idle no inputthe power consumption is essentially the same as at high output volume.
For example, in the circuit at the beginning of the article, the diodes this circuit diagram is missing would be mounted physically close to the output transistors, and specified to have a matched temperature coefficient.
This characteristic prevents distortion of amplitude-modulated or frequency-modulated signals passing through the amplifier. Any residual harmonics can be removed using a further filter. Distortion is high and practical use requires a tuned circuit as load. This, however, incurs higher signal distortion[ citation needed ].
In one common arrangement the resistor shown in the circuit above is replaced with a parallel-tuned circuit consisting of an inductor and capacitor in parallel, whose components are chosen to resonate the frequency of the input signal.
By this means, power dissipation in the active device is minimised, and efficiency increased.
In practical class-C amplifiers a tuned load is invariably used. Transistors are much less expensive than tubes so more elaborate designs that use more parts are still less expensive to manufacture than tube designs.
The crossover distortion can be reduced further by using negative feedback. Class-A power amplifier designs have largely been superseded by more efficient designs, though their simplicity makes them popular with some hobbyists. This is why tuned operation is sometimes called a clamper.
The signal bandwidth of the amplifier is limited by the Q-factor of the tuned circuit but this is not a serious limitation. An improvement is to bias the devices so they are not completely off when they are not in use.
Inefficiency comes from the standing current that must be roughly half the maximum output current, and a large part of the power supply voltage is present across the output device at low signal levels. Disadvantage of class-A amplifiers[ edit ] Class-A amplifiers are inefficient.
Suffix numbers for vacuum tube amplifiers[ edit ] A vacuum tube amplifier design will sometimes have an additional suffix number for the class, for example, class B1. In practice, distortion occurs near the crossover point.
They are sometimes used as medium-power, low-efficiency, and high-cost audio power amplifiers. The exact choice of quiescent current the standing current through both devices when there is no signal makes a large difference to the level of distortion and to the risk of thermal runawaythat may damage the devices.
A class-A amplifier is distinguished by the output stage devices being biased for class A operation. The tuned circuit resonates at one frequency, the fixed carrier frequency, and so the unwanted frequencies are suppressed, and the wanted full signal sine wave is extracted by the tuned load.
When the proper load e. This is called untuned operation, and the analysis of the waveforms shows the massive distortion that appears in the signal. For every watt delivered to the loadthe amplifier itself, at best, uses an extra watt. The power consumption is unrelated to the output power.
There is a market for expensive high fidelity class-A amps considered a "cult item" among audiophiles  mainly for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion. The angle of flow is closely related to the amplifier power efficiency.
The average voltage at the collector is then equal to the supply voltage, and the signal voltage appearing across the tuned circuit varies from near zero to near twice the supply voltage during the RF cycle.Amplifier, class-F, efficiency.
I. INTRODUCTION ower amplifiers (PAs) efficiency is enhanced by operating the transistor in different classes.
Class-F PAs offer high power capabilities and high efficiency with a limited number of controlled harmonics .
In an ideal class-F PA the.
Tunable Resonant Network. This thesis presents a high-efficiency Class-F power amplifier (PA) operating from MHz through the use of an electronically tunable resonant network.
UNIVERSITY OF CALIFORNIA, SAN DIEGO CMOS Power Ampliﬁers for Wireless Communications A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy. Graduate Theses and Dissertations Iowa State University Capstones, Theses and Dissertations Design of a class-F power amplifier with reconfigurable output harmonic termination in.
Class-F Power Amplifier with Maximized PAE A Master Thesis presented to the Faculty of California Polytechnic State University, San Luis Obispo. In a class D amplifier, power field-effect transistors (FETs) are driven to produce an output square-wave that switches between a high and low level at a frequency outside the range of human hearing.Download