Amplifier | Classification of Amplifier

Amplifier

A power amplifier may be defined as a device that converts DC power into AC power and whose action is controlled by the input signal. Amplifiers can be classified into various categories based on their function, type of device used, no. of stages, type of coupling, frequency to be amplified, bandwidth and based on the position of the quiescent operating point.

Classification of Amplifiers

Amplifiers can be classified into various categories based on their function, type of device used, number of stages, type of coupling, frequency to be amplified, bandwidth, and the position of the quiescent operating point.

(1) Based on the function: Amplifiers can be classified as:

• Voltage amplifier
• Power amplifier

(2) Based on the device used:

• Transistor amplifier
• Tube amplifier
• FET (Field Effect Transistor) amplifier
• I.C. (Integrated Circuit) amplifier

(3) Based on the number of stages:

• Single-stage amplifiers
• Multi-stage amplifiers.

(4) Based on the type of coupling used:

• RC-coupled amplifiers.
• Transformer coupled amplifiers.
• Direct coupled amplifiers.

(5) Based on the frequency to be amplified:

• Audio amplifier.
• Radio-frequency amplifiers.
• Video frequency amplifier.
• Intermediate frequency amplifier.

(6) Based on the bandwidth:

• Wide-band amplifiers.
• Narrow-band amplifier – (Single tuned & Double tuned).

(7) Based on the quiescent (Q) point:

• Class A amplifiers
• Class B amplifiers
• Class C amplifiers
• Class AB amplifiers.

Here we discuss different types of amplifiers based on quiescent operating point.

On the basis of quiescent or Q- point the amplifiers are of the following types.

1. Class A amplifier
2. Class B amplifier
3. Class C amplifier

Class A amplifier

In a class A amplifier, the operating point is fixed such that the output current flows during the complete cycle of the input signal. The transistor operates in the linear region of transistor characteristics. Class A amplifier can amplify low amplitude signals. This output waveform is exactly similar to the input waveform and also has minimum distortion. However, the efficiency of such amplifiers is low. In this amplifier, the maximum collector current can be equal to the saturation current. Maximum value VCE can be equal to Vcc. The output waveform of a class A amplifier is shown as.

Transistors in class A amplifiers always remain forward biased, hence some current will continue to flow through the transistor, even if there is no input signal. This causes a waste of power and results in lower efficiency.

Class B amplifier

In B-class amplifiers, the collector current flows only for half a cycle of signal wave, for the remaining half cycle the transistor is driven into a cut-off region.

The Q-point is located at V_C = V_CC and I_C = 0 on the load line as you can see in Fig. It is at the lower extreme end of the load line. During the positive cycle of collector current, the operating point moves up on the load line and there is conduction through the transistor, but during the negative part of the cycle the transistor is driven into cut-off, and no collector current flows through it.

class-B operation of the transistor has higher efficiency than A-class amplifier operation because here the quiescent current is zero. No power is dissipated when the signal is not flowing during the negative half cycle. Current flows through each transistor in half cycle only. Hence efficiency of class B amplifiers is higher than that of class A amplifiers.

Advantages of Class B Amplifier

• Low-standing bias current.
• Negligible power consumption at a zero input signal.
• Power output is higher than in a class A amplifier.
• Higher efficiency than a class A amplifier.

Disadvantages of Class B Amplifier

• Creates cross-over distortion.
• It requires a stabilised power supply current that requires changes with a change in the input signal.
• Distortion is greater than in a class A amplifier.

Class C amplifier

In class C amplifiers, collector current flows for less than half the input signal’s cycle. Quiescent The operating point is located in the cutoff region. Collector current flows only when the operational point enters the active zone during the positive half cycle. The transistor remains in a cut-off state for more than half a cycle. There is no collector current flowing during this time period.

Such amplifiers produce a high level of distortion. They only produce small pulses. The load in these amplifiers is a resonant or tuned circuit. It is applied in oscillator circuits. Efficiency is higher than in class A amplifiers. The image shows the output of class C amplifiers.

Advantages of Class C Amplifier

• Efficiency is higher than in class A, B, and AB amplifiers.
• More output power can be obtained.
• Less heat is produced in the transistor as it conducts for only less than the input 180-degree signal.

Disadvantages of Class C Amplifier

• Distortion is higher as the transistor conducts less than 180 degrees of the input signal.
• Application is limited to R.F. tuned amplifiers.

Frequently Asked Questions (FAQs)

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