Difference between Voltage Amplifier and Power Amplifier

An amplifier is an electronic circuit that is used to increase the strength of a signal in terms of voltage, current, power, etc. It accepts a weak signal at the input terminal and produces a raised signal at the output, though the output signal is identical to the input signal. The function that an amplifier performs is known as amplification. The amount of amplification performed by the amplifier is determined by a factor known as gain of the amplifier.

Depending on the applied input signal and generated output signal, the amplifiers are broadly classified into two major types namely,

• Voltage Amplifier
• Power Amplifier

In this article, we will highlight all the major differences between voltage amplifier and power amplifier by considering different parameters such as basic function, transistor size, transistor gain, load resistance, etc. But, before discussing the differences, let's start with some basics of voltage amplifier and power amplifier so that it becomes easier to understand the differences between them.

What is a Voltage Amplifier?

A voltage amplifier is the one that produces an output signal with increased voltage level when a low voltage signal is input to it. A voltage amplifier is used in such applications where we need signal transmission at higher voltage through a long wire. Therefore, the voltage amplifier is similar to a transformer in the sense that it can increase the voltage level of a signal.

A typical circuit of a voltage amplifier is shown in Figure-1. It consists of one BJT (Bipolar Junction Transistor) and two resistors. Basically, a voltage amplifier is designed to achieve the highest possible voltage gain. Where, the voltage gain of a voltage amplifier is simply the ratio of output voltage to input voltage, i.e.

$$A_v\:=\:\frac{V_{out}}{V_{in}}$$

A voltage amplifier draws a very little amount of power from the load. For these amplifiers, the input signal has very small magnitude, for this reason a voltage amplifier is also called a small signal amplifier.

The voltage amplifiers are used in various applications such as in wireless communication, broadcasting of signals, audio equipment like speakers, etc.

What is a Power Amplifier?

As its name implies, a power amplifier is an electronic device which boosts the power level of an input signal. The circuit diagram of a typical power amplifier is shown in Figure-2. It consists of a power transistor, as a normal transistor cannot be used in the power amplification. Because, in a power amplifier, the transistor must be able to handle huge large amount of power.

Since the power of the signal cannot be directly amplified. In actual practice, a voltage amplifier is placed before the power amplifier which increases the voltage level of the signal. This high voltage signal at the input of the power amplifier generates a high current signal, and the product of which provides a signal of high power at the output of power amplifier.

Therefore, the gain of the power amplifier is given by,

$$A_P\:=\:A_V\:\times\:A_I$$

Where,

$$A_V\:=\:\frac{V_{out}}{V_{in}}\: and \:\:A_I\:=\:\frac{I_{out}}{I_{in}}$$

The power amplifiers are extensively used in several applications like in headphone drivers, TVs, mobiles, music systems, microwave ovens, etc.

Difference between Voltage Amplifier and Power Amplifier

The following table highlights all the significant differences between a voltage amplifier and a power amplifier −

Basis of Difference	Voltage Amplifier	Power Amplifier
Definition	An amplifier which is designed to increase the level of input signal is called a voltage amplifier.	A type of amplifier which is designed to boost the power level of the input signal is called a power amplifier.
Alternate name	Voltage amplifier is also called small signal amplifier.	A power amplifier is also called a large signal amplifier.
Voltage of input signal	The input voltage of a voltage amplifier is very low, of the order of approximately few mV.	The input voltage of a power amplifier is relatively high of the order of few volts.
Nature of output signal	Voltage amplifier has high voltage and low power output.	Power amplifier has high power and low voltage output.
Size of transistor used	The transistor used in the voltage amplifier circuit is smaller in size.	The transistor used in the circuit of the power amplifier has relatively larger physical size.
β value of transistor	The transistor used in the voltage amplifier has relatively high β value, around greater than 100.	The β value of the transistor used in the power amplifier is low, around 5 to 20.
Type of coupling	The R-C coupling is usually used in the voltage amplifier.	Power amplifier always uses transformer coupling.
Collector current (IC)	The collector current of the voltage amplifier is low, approximately equal to 1 mA.	The collector current of a power amplifier is relatively high, greater than 100 mA.
Base region of transistor used	The transistor used in the voltage amplifier has thin base region as it handles low current.	The base region of transistor used in the power amplifier is comparatively thicker as it required to handle large current.
Load resistance (RC)	In the voltage amplifier, the load has high resistance value, around 4 kΩ to 10 kΩ.	In the power amplifier, the value of load resistance is low, around 5 Ω to 20 Ω.
AC power output	The AC power output of a voltage amplifier is low.	The AC power output of a power amplifier is comparatively high.
Output impedance	Voltage amplifier has high output impedance, approximately equal to 12 kΩ.	The output impedance of a power amplifier is low, around 200 Ω.
Need of heat sink	In case of voltage amplifier, there is no need of heat sink.	The heat sink is necessarily required in a power amplifier.
Use	The voltage amplifier is usually used at the first stage of amplification.	The power amplifier is usually used at the last stage of amplification.

Conclusion

The most significant difference between voltage and power amplifiers is that a voltage amplifier increases the magnitude of voltage of the input signal, while a power amplifier raises the power level of the input signal.