Prajakta Kelapure
Prajakta Kelapure
Branch Unspecified
12 Jan 2018

Introduction to Amplifiers - Difference between Power Amplifier and Voltage Amplifier

Many of the input signals have desired frequency, waveform but this signals cannot be used because of its small amplitude. This is because the circuits or the devices need desired levels of input to operate. Thus amplification or in other words strengthening of the input signal is a must. The device that amplifies the input signal is called the Amplifier.

For example, the Radio receivers amplify very weak signal until they are strong enough to make the signal audible. So an amplifier may be defined as a device that increases the current, voltage, or power of the input signal with the help of transistor by furnishing the additional power from the separate source of supply. Thus when only one transistor with an associated circuit is used for increasing the strength of the weak signal, then the circuit is known as Single stage amplifier. Practically multiple stages of an amplifier are used called as the multistage amplifier.

The transistor is connected with some passive components such as resistors, capacitors and biasing supply so as to perform the function of amplification. If in a properly biased transistor circuit, when an ac signal is applied between the base and the emitter, a small base current started flowing through it. Because of the transistor action, a much larger ac current flows through the collector resistor. Since the value of the collector resistor is high, a large voltage appears across Rc. Thus a weak signal applied between the base and the emitter appears in the amplified form between the collector and the emitter. This explains the Amplifying action of the transistor.

When the input signal is very small or weak and produces small fluctuations in the collector current, then they are called as the small signal amplifier or the voltage amplifier. Voltage amplifiers are used as the first stage amplifiers used in radio, tape recorders, TV receivers etc.

When the fluctuations produced in the collector current are large then such amplifiers are called as large signal amplifiers or Power Amplifiers. The transistors used to amplify power are called as power amplifiers. Or it can be defined as a device that converts the DC power system to AC power and its action depends on the input signal.
While I was studying, I always encountered this questions….what are power amplifiers and voltage amplifiers? Are they same…. Lots of similar equations… operations… etc…. So, searched n found the answer….

Firstly let see how these amplifiers differ from normal transistors.

1. In power transistors (amplifiers) the base is made thicker so that it can handle the large current, i.e. current gain β should be small.

2. Collector region is made comparatively larger to dissipate the heat developed during transistor operation. Also, heat sinks are used in power amplifiers.

3. The other two regions, i.e. the emitter and the base are heavily doped. Emitter-base contact and collector base contact are in the ring like form so as to increase the area. With this construction, the ohmic resistance between the emitter and base is reduced and so small power is required at an input.

Difference between Power Amplifier and Voltage Amplifier

The primary function of the voltage amplifier is to raise the voltage level of the signal. In order to achieve high voltage amplification, the transistor should have a thin base, input resistance should be low as compared to the collector resistance and the transistor should be able to take or permit high collector load. The current gain β is high more than 100.The voltage amplifiers are always operated at the low collector current of the order of 1mA. In voltage Amplifiers, RC coupling is preferred because of its smaller size, less weight and cheap at cost. These amplifiers are basically used for high gain, so only less power is available at the output.

In power amplifier, the main function is to deliver a large amount of power and handle a large current. In order to achieve this high power amplification the transistor used are of a large size so as to dissipate the heat produced, its base should be thick so as to carry a large current of the order of 100mA. The current gain β is low in between 20 to 50. Here transformer coupling is preferred for impedance matching. The collector resistance is low, so as to reduce the power losses and voltage drop.

Let’s see in brief the block diagram that explains the amplifier system.

Block Diagram: The Amplifier System

An amplifier system consists of the following blocks:
1. Input Transducer: a transducer is a device used to convert one form of energy to another. The input transducer produces a small electrical signal. An example of a transducer is a microphone which converts the acoustic energy into electrical energy.

2. Voltage Amplifier: the function of the voltage amplifier is to provide high resistance to the input transducer so as to minimize the loading effect (error signal). Also to provide a large voltage signal to the power amplifier stages to operate the output devices.

3. Power Amplifier: the large signal amplifier or power amplifier should be capable of handling large power. The factors that power amplifiers look into are the power efficiency of the circuit, the maximum amount of power that the circuit is capable of handling and impedance matching with the output transducer.

4. Output transducer: The output transducer operates on such large power signal and gives the required output. Example of output transducer is the loudspeaker which converts the electrical energy back to sound signal.

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