Topics: Amplifier, Electronic amplifier, Transistor Pages: 9 (1098 words) Published: October 16, 2014
An amplifier is an electronic device increases the amplitude of a small signal to a useful level, even maintaining small signal’s detail. This is known as Linearity. FIGURES OF MERIT
1) Gain: Ratio between the magnitude of output and input signals. 2) Bandwidth: The Width of the useful frequency range.
3) Efficiency: The ratio between the power of the output and total power consumption. 4) Linearity: The degree of proportionality between input and output. 5) Noise: A measure of undesired noise mixed into the output. 6) Output Dynamic Range: The ratio of the largest and the smallest useful output levels. 7) Slew Rate: The maximum rate of change of the output.

8) Rise Time: Characterize the step response.
9) Stability: The ability to avoid self-oscillation.

The measure of how much an amplifier “amplifies” the input signal, found by observing the difference between the input and output signals is known as the Gain of amplifier. Amplifier gain can be calculated by the ratio of output divided-by the input.

Audio power amplifiers are classified in an alphabetical order according to their circuit configurations and the mode of operation. These different amplifier classes range from a non-linear output with low efficacy to a non-linear output with high efficacy. CLASS A AMPLIFIER

The general property of class A amplifier is that the output transistor always has the significant current flow hence have a large quiescent current. Quiescent current is defined as the current level in the amplifier even if there is no input signal. Class A amplifiers are made more efficient by employing Double-ended arrangement, employs two complimentary transistors. Some form of heat sink is required to maintain the heat develops in the output transistor of class A amplifier. Hence class A amplifier are inefficient as most of the DC power is converted to heat.

Class A Power Amplifier

Class A Bias
The general property of class B amplifier is that the transistor only conducts when the input signal is greater than the base-emitter voltage. In this configuration, each transistor conducts half of its time in active region and half of its time in cut-off region thereby amplifying only 50% of the input signal.

Class B Bias
In class B arrangement, whenever the signal waveform changes polarity and have to turn “ON” one transistor when other is turned “OFF”. This result in a small part of the output waveform at the Zero voltage cross-over point which will be distorted. This type of distortion is called Crossover distortion.

Class C amplifier operates for less than half of one wave cycle[100° to 150°], these are most efficient amplifier but not used for audio application because of their excessive distortion at output stage. If the input signal amplitude overcomes the dc operating point, the transistor goes into saturation and cut-off region. Then it is called as an over-driving amplifier. These amplifiers are used as Radio- Frequency amplifiers in transmitters.

Class C Bias

Class AB amplifiers also uses two complimentary transistors in which the transistor will be “ON” state for more than half a cycle of waveform. A small biasing voltage applied as input signal cause transistor to operate in its active region thereby eliminating cross-over distortion. The significance of class AB amplifiers are it has improved efficiency and linearity when compared to Class A amplifier.

Class AB Bias
Class D amplifiers are active devices- Power FET’s which are driven as switches. A square wave with higher frequency component than input waveform drives amplifier between saturation and cut-off. At the amplifier output, a low-pass filter attenuates the switching frequency and its harmonics, leaving only the amplified version of input waveform. Hence class D amplifier achieves...
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