Definition – A transistor amplifier which amplifies the power level of a signal having audio frequency range is known as transistor audio power amplifier.
A practical amplifier always consists of several stages that amplify a weak signal until sufficient power is available to operate a loudspeaker or other output device. In this multistage amplifier the first few stages have only the function of voltage amplification. However, the final stage is designed to provide maximum power. This final stage is know as power stage (You can see the picture below).
In general, the final stage of a multistage amplifier is the power stage. The power amplifier will different from all previous stages in that a concentrated effort is made to obtain maximum output power.
Transistors suitable for power amplification are generally called power transistors. It different from other transistors mostly in size. Another special thing about it is that it is large enough to handle a large amount of power.
Audio power amplifiers are use to deliver large amounts of power to low resistance loads. Typical load values range from 300 Ω (for transmission antennas) to 8 Ω (for loudspeaker).
While these load values don’t cover every possibility, they illustrate the fact that audio power amplifiers typically drive low-resistance loads. A power amplifier has a typical power output rating of 1 W or more.
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Small signal amplifier and large signal amplifier
Typically the input signal to a multistage amplifier is small (a few mV from a cassette or CD or a few μV from an antenna). Therefore, the first few stages of the multistage amplifier handle small signals and perform only voltage amplification.
However, the last stage handles a large signal and its job is to generate a large amount of power to operate an output device (such as a speaker).
What is small signal amplifier
Amplifiers that handle small input A. C. signals (a few μV or a few mV) is know as small-signal amplifiers. Voltage amplifiers usually fall into this category.
Small-signal amplifiers are design to operate on the linear part of the output characteristics. Hence, 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.
What is large signal amplifier
Amplifiers that handle large inputs A. C. signals (such as a few volts) is know as large-signal amplifiers.
Power amplifiers fall in this category. The large-signal amplifiers are designed to provide a large amount of A. C. power output so that they can operate the output device (for example a speaker).
The main characteristics of a large-signal amplifier or power amplifier are the power efficiency of the circuit, the maximum amount of power the circuit is capable of handling, and the impedance matching of the output device.
It may be note that not all large-signal amplifiers are necessarily power amplifiers, but it is safe to say that most are. In general, where the amount of power consumed is 1W or more, the amplifier is call a power amplifier.
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Voltage amplifier and power amplifier Difference
Voltage amplifier
- A voltage amplifier is design to achieve maximum voltage amplification.
- The value of β is very high (>100).
- The value of collector resistance (RC) is high (4 kΩ to 10 kΩ).
- R-C coupling is use in voltage amplifier.
- The input voltage is low (about mv) of the voltage amplifier.
- Collector current is low ( nearly equal 1mA) in voltage amplifier.
- The voltage amplifier has a low power output.
- The voltage amplifier has a high output impedance ( approximately 12 kΩ).
Power amplifiers
- A power amplifier is design to achieve maximum output power.
- The value of β is low (around 5 to 20).
- The value of RC is low (5 Ω to 20 Ω) in power amplifier.
- Transformer coupling is use in power amplifiers.
- The input voltage is high (a few volt) of the power amplifier.
- The collector current in the power amplifier is high (>100 mA).
- Power amplifier has a high power output.
- The power amplifier has a low output impedance (about 200 Ω).
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Performance quantities of power amplifier
As mentioned earlier, the main objective for a power amplifier is to obtain maximum output power. As a transistor, like any other electronic device it has voltage, current and power dissipation limits, therefore, the criteria for a power amplifier are :
- Power efficiency
- Distortion
- Power dissipation capability
What is power efficiency?
The main criterion for a power amplifier is not the power gain but it is maximum A. C. power generation.
Now, an amplifier converts D. C. power from supply into A. C. power generation. Therefore a power amplifier’s ability to convert D. C. power from the supply to A. C. output power is a measure of its effectiveness. This is known as power efficiency and can be defined as follows :
The ratio of A. C. output power to the zero signal power (that is D. C. power) supplied by the power amplifier’s battery is known as power efficiency.
Power efficiency refers to how well an amplifier converts D. C. power from the battery to A. C. output power. For example, if the D. C. power supplied by the battery is 10 W and the A. C. output power is 2 W, then power efficiency will be 20%.
The higher the power efficiency, the larger is the A. C. power output. For power amplifiers it is clear that maximum power efficiency is the desired goal.
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What is distortion ?
The change in output wave shape from the input wave shape of the amplifier is know as distortion. A transistor, like any other electronic device, is essentially a non-linear device.
Therefore, whenever a signal is applied to the input of the transistor, the output signal is not exactly like the input signal that means distortion occurs. Distortion is not a problem for small signals (that is voltage amplifiers) because the transistor is a linear device for small changes about the operating point.
However, a power amplifier handles large amount of signals and hence distortion problems arises immediately. For the comparison of two power amplifiers, the one with less distortion is better.
Power dissipation capability
The ability of a power transistor to dissipate heat is call the power dissipation capability.
As stated before, a power transistor handles large currents and gets hot during operation. Since any temperature change affects the operation of transistor, hence, the transistor must dissipate this heat to its surroundings.
To achieve this, a heat sink (in a metal case) is usually attach to a power transistor case. The increased surface area allows heat to escape easily and keeps the transistor’s case temperature within permissible limits.
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