Power Amplifier

Define conduction angle in power amplifier?

The time during which the transistor conducts i.e. (the collector current is non zero) when an input sinusoidal signal is applied in a power amplifier is defined as Conduction angle.

What are the types of distortions encountered in amplifiers?

There are mainly three types of distortions encountered in amplifiers. They are

a) Non linear distortion: It occurs due to non dynamic curve of input output characteristics in amplifiers. New frequencies appear in the output due to non linear distortion. It is often encountered in large signal amplifiers.

b) Frequency distortion: This type of distortion occurs when the signal components of different frequencies are amplified differently.

c) Phase distortion: Phase distortion occurs due to unequal phase shift of signals of different frequencies.

Frequency and phase distortion occurs when the signal frequencies lie outside the band width of amplifier.

What are types of power amplifier based on conduction angle?

There are mainly four types of amplifiers based on conduction angle. They are

a)      Class A amplifiers

b)      Class B amplifiers

c)      Class AB amplifiers

d)     Class C amplifiers

What is class A, Class B, Class AB, Class C amplifiers?

Class A amplifier: In class-A amplifiers the collector is biased at a value greater than the amplitude of AC signal current. Hence the conduction angle is 360 Degrees i.e. the Class A stage conducts for the entire cycle for the input signal.

Class B amplifier: Class B amplifiers are biased at zero DC bias collector current. Hence it conducts only for half of the input signal cycle, so the conduction angle for class B amplifier is 180 Degrees.

Class AB amplifier: In class AB amplifiers the biasing current is non zero but much smaller than the peak current of the sine wave signal. As a result the transistor conducts for interval slightly greater than half a cycle. The conduction angle is slightly greater than 180 Degrees.

Class C amplifier: In class C amplifier the transistor conducts for an interval less than the half cycle. Hence the conduction angle is less than 180 Degrees.

What is push-pull amplifier?

A push pull amplifier is a special type of arrangement used in class B amplifiers where the active device pair push (supply) current and pull (absorb) current from load. Push pull amplifiers are more efficient than the single ended power amplifiers. Because of the symmetric construction of two sides of the amplifier the even harmonics are cancelled and output signal distortion can be minimized. Another advantage of the push pull amplifier is the effect of ripple voltage that may be contained in the power supply is balanced out. The main disadvantages of the push pull amplifier are the need for two identical transistors and the requirement of bulky and costly coupling transformers used for phase splitting.

What is complementary push-pull amplifier?

Complementary Push-pull amplifiers use two “complementary” or matching transistors, one being an NPN-type and the other being a PNP-type with both power transistors receiving the same input signal together that is equal in magnitude, but in opposite phase to each other. This results in one transistor only amplifying one half of the input waveform cycle while the other transistor amplifies the other half of the input waveform cycle with the resulting “two-halves” being put back together again at the output terminal. In complementary push pull amplifier neither an input nor an output transformer is needed. The main difficulty with this circuit is the if there is unbalance in the characteristic f the two transistors considerable distortion will be introduced.

 What is non-linear distortion in power amp?

Non-linear distortion occurs due to non dynamic curve of input output characteristics in amplifiers because of which the output waveform differs from the input. Active devices such as transistors, FET’s have linear input output characteristic for small signal variations around the Q-point. But in case of large signal variations around the Q-point considerable non linearity is introduced in the output.

Consider a power amplifier which is given a input of A*sin (w*t), the output is in the form of A1*sin (w*t) + A2*sin (2*w*t)+A3*sin(3*w*t)+……… The ratio of |A2/A1| is called as second harmonic distortion. Similarly |A3/A1| is called third harmonic distortion and so on. The total harmonic distortion is defined as D = (D1^2+D2^2+D3^2+…….)^(1/2). Therefore the total power is given as Pt= P(1+D^2) where P is the power delivered at fundamental frequency.

If the input consists of two different frequencies w1 and w2 then the inter modulation frequencies (sum or difference frequencies) w1+w2 and w1-w2 are introduced into the output

What is tuned amplifier? Where it is used?

Tuned amplifier is one in which a tuned circuit is introduced in to the output circuit which draws maximum power output only for a specified range of frequencies. Example of a tuned amplifier is shown below

tuned amplifier

tuned amplifier

The tuned amplifier shown in the figure will deliver maximum power only in the bandwidth of LC resonant circuit. Tuned amplifiers are used in video and radio frequency applications. They are used as frequency selective amplifiers in multi communication channels such as radio and TV broadcasting through wireless medium where each station is assigned a frequency band. They are used in RF/Wireless transceivers and intermediate stages of IF amplifiers.

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