Most electronic devices require at least one stage of amplification. So, amplifiers can be seen in almost all electronic systems. Mostly, Amplifiers are used to increase the amplitude of the input signal. The Amplifiers can increase only the amplitude of the device and the other parameters such as frequency and shape remain as it is.
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What are the types of amplifiers?
There are three types of amplifiers depending on the property of their output that are as follows:
- Voltage Amplifier
- Current Amplifier
- Power Amplifier
Let us look into these amplifiers in brief.
1. Voltage Amplifier
A voltage amplifier in the simplest form of a circuit that puts out a higher voltage than the input voltage. When you are forced to work with a set amount of voltage, these amplifiers can be commonly used to increase the voltage and thus the amount of power coming out of a circuit. This is useful for reading and adapting small signals such as boosting or increasing an audio signal before sending it on its way to speakers. A voltage amplifier is a form of the common emitter amplifier, which based on the transistor. The amplification of voltage is dependent on the ratio of resistors on the collector and emitter of this transistor.
2. Current amplifier
A current amplifier amplifies the input current by a fixed factor and sends it to the succeeding circuit. A current amplifier is similar to a voltage buffer but the difference is that an ideal voltage buffer will try to deliver whatever current required by the load while keeping the input and output voltages the same, where a current amplifier supplies the succeeding stage with a current that is a fixed multiple of the input current. A current amplifier can be realized using transistors. The schematic of a current amplifier circuit using transistors is shown in the figure below. Two transistors are used in this circuit. β1 and β2 are the current gains of transistors Q1 and Q2 respectively. Iin is the input current, Iout is the output current and+Vcc is the transistor T2’s collector voltage The equation for the output current is Iout = β1 β2 Iin .
3. Power Amplifier
Power amplifiers are generally used to enhance the power level of the input signal. It is also called large-signal amplifiers, as in order to get large power at the output, input signal voltage required must also be large.
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Differences between voltage amplifier, current amplifier, and power amplifier:
| Parameter | Voltage Amplifier | Current Amplifier | Power Amplifier |
|---|---|---|---|
| Amplifies | Voltage | Current | Power (Voltage x Current) |
| Input Impedance | High | Low | Variable |
| Output Impedance | Low | High | Low |
| Purpose | Increase voltage | Increase current | Increase power |
| Load | High impedance | Low impedance | Variable |
| Efficiency | Typically lower | Typically higher | Moderate to high |
| Applications | Audio amplifiers, RF amplifiers | Current sources, transimpedance amplifiers | Audio power amplifiers, RF power amplifiers |
Classification of Power amplifiers
Power amplifiers are classified as different classes which states the variation of the output signal with respect to the applied input signal.
a. Class A
In this class A power amplifiers, the output current flows for overall i.e., 360﮲ of the input cycle. It only operates on the linear region of the load as the operating point is so selected which gives us exact output as that of input.
Here, the maximum possible efficiency is of 50%. This class is used when we simply want a distortion-free output.
Let’s look at the circuit diagram of class A amplifier:
b. Class B
In this class B Amplifier, the transistor is biased in such a way that the current flows only during the positive half input cycle. In this two complementary transistors are used which receive the input signal of equal magnitude but opposite in phase.
When the input is applied at the center-tapped secondary of transformer it generates two identical signals of opposite phase and the two transistors are driven by these two input signals.
c. Class AB
It is a combination of class A and class B type of power amplifiers. This class was basically introduced to eliminate crossover distortion that occurs in class B.
In this type, the angle of conduction lies somewhere between 180﮲ to 360﮲. Here, the transistor biasing is done in such a way that operating point Q lies near cut-off voltage.
d. Class C
Class C power amplifier is designed to provide the highest efficiency of about 80%. These are biased in such a way that it operates for less than 180﮲ of the input signal but provides the full output signal in case of resonant frequency tuned circuits.
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