An amplifier is an electronic device that is used to enhance the strength of a signal in terms of voltage, current, or power. It receives a weak electrical signal and produces stronger signal at the output by using an external power source. Small-signal amplifiers are generally referred to as ‘voltage amplifiers’ as they convert a small input voltage (a few µV or a few mV) into a much larger voltage. At times an amplifier is needed to feed a loudspeaker or drive a motor, and for these kinds of applications where we require high switching currents, large-signal amplifiers i.e. power amplifiers are used. A power amplifier can be defined an electronic device designed to increase the magnitude of power of a given input signal. The power of the input signal is usually increased to a level high enough to drive loads at the output such as loudspeakers as aforementioned, RF transmitters, etc. The main function of power amplifiers is to deliver power (product of the voltage and current) to the load. The power amplifier works on the basic principle of converting the DC power drawn from the power supply into an AC voltage signal delivered to the load.
How Power Amplifiers differs from Voltage Amplifiers
A voltage amplifier is designed to achieve maximum voltage amplification; however it is not essential to raise the power level, whereas a power amplifier is designed to obtain maximum output power. Additional differences between the voltage and power amplifiers have been detailed in the table below:
| Parameter | Voltage Amplifier | Power Amplifier |
| Voltage input | The input voltage is usually very low, a few µV or mV. | The input voltage is relatively high, a few volts (2-4 V). |
| Output voltage | High | Low |
| Power output | Low | High |
| β of a transistor | Transistor with high β (>100) is used i.e. transistors used have thin base. | A transistor with low β used (5-20) i.e. the base is made thicker to handle large currents. |
| Load resistance (RC) | High (4-10 kΩ) | Low (5-20 Ω) |
| Output impedance | Typically high ≈ 12 kΩ | Low (200 Ω) |
| Collector current | Low ≈ 1 mA | High (>100 mA) |
| Coupling | Typically R-C coupling | Invariably transformer coupling (this permits impedance matching, resulting in the transference of maximum power to the load). |
| Transistor size | The size of transistors used is small. | The size of power transistors used is made considerably larger to dissipate the heat that is produced in the transistor during operation. |
Classification of Power Amplifiers
Audio power amplifiers may be classified according to their mode of operation i.e. the portion of the input cycle during which the collector current is expected to flow. On this basis, they are classified as:
- Class A power amplifier
- Class B power amplifier
- Class AB power amplifier
- Class C power amplifier
These different classes of operation range from a linear output but with low efficiency to a non-linear output but with a high efficiency.
Power amplifiers designed to amplify PWM digital signals come under classes D, E, F, and so on.
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Class A Power Amplifier
This is the simplest form of power amplifier that utilizes a single switching transistor in the standard common emitter circuit configuration to produce an inverted output. A single transistor is used to amplify both the positive and negative halves of the waveform. This simple design makes class A amplifiers the most widely used power amplifiers.
Class A Power Amplifier Operation
The entire input signal waveform is faithfully reproduced at the amplifiers output as the transistor is perfectly biased within its active region, thereby never reaching either the Cut-off or Saturation regions. As a result, the AC input signal is perfectly centered between the amplifier upper and lower limits as illustrated below:
The operating point Q is chosen such that the collector current flows at all times throughout the full cycle of the applied signal. Since the output wave shape is exactly similar to the input wave shape, such amplifiers have minimum distortion; however they have the disadvantage of low output and low collector efficiency (around 33%). The transistor in this case, is in use all the time even if there is no input signal. This generates a lot of heat and contributes to the low efficiency.
Class B Power Amplifier
If the collector current flows only during the positive half-cycle of the input signal, it is referred to as class B power amplifier.
Class B Power Amplifier Operation
Unlike the class A amplifier above that utilizes a single transistor for its output stage, the class B amplifier uses two complimentary transistors (an NPN and a PNP) for each half of the output waveform. This means that each transistor spends half of its time in the Active region and half its time in the Cut-off region. Class B amplifier operation has no DC bias voltage instead the transistor only conducts when the input signal is greater than the base-emitter voltage which is 0.7v for silicon devices. Thus, at zero input there is zero output and no power is being consumed. This means that the actual Q-point of class B amplifier is on the Vce part of the load line as shown below:
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The Q-point is at Cut-off, as shown above, the class B amplifier is biased at the Cut-off point so that it is brought out of cut-off and operates in its linear region, when the input drives the transistor into conduction.
The efficiency of class B amplifiers is enhanced much well than class A amplifiers because of the two transistor design.
As the output transistors for each half of the waveform, both positive and negative, requires a base emitter voltage greater than the 0.7v required for the bipolar transistor to start conducting, the lower part of the output waveform which is below this 0.7v window will not be reproduced accurately resulting in a distorted area of the output waveform as one transistor turns ‘off’ waiting for the other transistor to turn back ‘on’. This distortion is called crossover distortion. Class AB amplifiers, discussed in the next section help reduce this signal distortion.
Class AB Power Amplifier
The class AB amplifier is a compromise between the class A and class B configurations.
Class AB Power Amplifier Operation
Even though class AB operation still uses two complementary transistors in its output stage, a very small biasing voltage is applied to the base of the transistor to bias it close to the Cut-off region when no input signal is present. An input signal waveform will cause the transistor to operate as normal in their active region thereby eliminating any crossover distortion present in pure class B amplifier designs however there is a little drop in efficiency. A small collector current will flow when there is no input signal but it is much less than that for the class A amplifier configuration. This means that the transistor will be ‘on’ for more than half a cycle of the waveform. This type of amplifier configuration enhances both the efficiency and linearity of the amplifier circuit compared to class A.
Class C Amplifier
If the collector current flows for less than half-cycle of the input signal, it is called class C power amplifier. It has the greatest efficiency but the poorest linearity compared with classes A, B, or AB.
In class C amplifier, the base is given some negative bias so that collector current doesn’t flow just when the positive half-cycle of the signal begins.
In this type of amplifier, the active element conducts only when the input voltage is above a certain threshold, which reduces power dissipation hence enhancing efficiency.
The conduction angle for the transistor is significantly less than 180° and is generally around 90° area. The efficiency is around 80%, however it introduces a very heavy distortion of the output signal, therefore not appropriate for audio amplifier. They are used in high frequency oscillators and amplification of radio frequency signals.
Other Classes of Power Amplifiers
We have other classes of power amplifiers such as classes D, E, F, G, and so forth. Let’s look at some of them briefly:
Class D Power Amplifier
Class D power amplifier is a non-linear amplifier or pulse-width modulation (PWM) amplifier. Theoretically they can reach 100% efficiency, as there is no period during a cycle where the voltage and current waveforms overlap as current is drawn only through the transistor that is ON.
Class E Power Amplifier
This is a version of class D amplifiers that comes with a more complex circuitry and better efficiency.
Class F Power Amplifier
Class F amplifiers boost both efficiency and output by using harmonic resonators in the output network that shape the output waveform into a square wave. Amplifiers under this class are capable of high efficiencies of more than 90% if infinite tuning is employed.
Class G Power Amplifier
Class G amplifiers provides enhancements to the basic class AB amplifier design. They utilize multiple power supply rails of various voltages and automatically switch between these supply rails as the input signal changes. This constant switching reduces the average power consumption, and thus power loss caused by wasted heat.
Class I Power Amplifier
Amplifiers under this class have two sets of complementary output switching devices arranged in a parallel push-pull configuration with both sets of switching devices sampling the same input waveform. One device switches the positive half of the waveform, while the other switches the negative half in the same way as class B amplifier. With no input applied, or when a signal reaches the zero crossing point, the switching devices are both turned ON and OFF at the same time with a 50% PWM duty cycle canceling out any high frequency signals.
Class S Power Amplifier
Class S power amplifier is basically a non-linear switching mode amplifier similar in operation to the class D amplifier. It converts analog input signals into digital square wave pulses by a delta-sigma modulator, and amplifies them to enhance the output power before finally being demodulated by a band pass filter. Since the digital signal of this switching amplifier is always either fully ‘on’ or fully ‘off’, efficiencies reaching 100% can be achieved (theoretically zero power dissipation).
Application of Power Amplifiers
Power amplifiers are used in various fields that range from telecommunication systems, medical equipment to scientific instrumentation. These amplifiers play a critical role in those fields by ensuring that the input signals are sufficiently enhanced for transmission, processing or analysis depending on the specific requirements of each application.
Power amplifiers are used in audio systems such as loudspeaker to deliver high power audio signals.
Power amplifiers are used in radio frequency applications to boost signals to ensure reliable transmission over long distances. For wireless transmissions such as FM broadcasting, antennas require input signals at thousands of kilowatts of power. RF power amplifiers are used to boost the magnitude of modulated waves to a level high enough for reaching the needed transmission distance.
Power amplifiers are also used in industrial instrumentation to amplify signals for measurement and control purposes. DC power amplifiers are used to amplify the power of a PWM signals. They increase the power of the input signal from microcontroller systems, and feed the amplified signal to an actuator such as dc motor.
Class B power amplifiers are suitable for applications requiring high efficiency and moderate to high power output e.g. audio amplifiers in battery powered devices or particular radio frequency applications where efficiency is key.
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