Transistor Audio Power Amplifier
A transistor amplifier which raises the power level of the signals that have audio frequency range is know as transistor audio power amplifier. In general, the last stage of a multistage amplifier is the power stage.
The power amplifier differs from all the previous stages in that here a concentrated effort is make to obtain maximum output power.
A transistor that is suitable for power amplification is generally call a power
transistor. It differs from other transistors mostly in size ; it is considerably larger to provide for handling the great amount of power.
Audio power amplifiers are use to deliver a large amount of power to a low resistance load.
Typical load values range from 300Ω (for transmission antennas) to
8Ω (for loudspeakers). Although these load values do not
cover every possibility, they do illustrate the fact that audio
power amplifiers usually drive low-resistance loads. The
typical power output rating of a power amplifier is 1W or
Small-Signal and Large-Signal Amplifiers
The input signal to a multistage amplifier is generally small (a few mV from a cassette or CD or a few μV from an antenna). Therefore, the first few stages of a multistage amplifier handle small signals and
have the function of only voltage amplification. However, the last stage handles a large signal and its
job is to produce a large amount of power in order to operate the output device (e.g. speaker).
(i) Small-signal amplifiers:
Those amplifiers which handle small input a.c. signals (a few μV
or a few mV) are called small-signal amplifiers. Voltage amplifiers generally fall in this class. The small-signal amplifiers are designed to operate over the linear portion of the output characteristics.
Therefore, the transistor parameters such as current gain, input impedance, output impedance etc. do not change as the amplitude of the signal changes. Such amplifiers amplify the signal with little or no distortion.
(ii) Large-signal amplifiers:
Those amplifiers which handle large input a.c. signals (a few volts) are call large-signal amplifiers. Power amplifiers fall in this class. The large-signal amplifiers are designed to provide a large amount of a.c. power output so that they can operate the output device e.g. a speaker.
The main features of a large-signal amplifier or power amplifier are the circuit’s
power efficiency, the maximum amount of power that the circuit is capable of handling and the impedance matching to the output device.
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