Power amplifier is an output stage of amplifier optimized to provide high output power gain. To drive the transducer whether it is a loud speaker e.t.c. it is required that the AC small signal which will be an audio signal in this case should be amplified enough to drive the transducer (loud speaker). The output stage should have output resistance as low as possible so that the amplifier can deliver the output signal without the loss of gain. The block diagram of an audio amplifier is shown in the figure
Audio amplifier block diagram
The LM386 is a power amplifier designed for use in low voltage consumer applications. DS1868 is a digital potentiometer for gain or volume control.
During this process of power amplification distortion is introduced in the output signal. There are mainly three types of distortions encountered in amplifiers. They are
a) Non linear distortion: It occurs due to non linear 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 i.e. gain of the amplifier is a function of frequency.
c) Phase distortion: Phase distortion occurs due to unequal phase shift of signals of different frequencies.
Frequency and phase distortion normally occurs when the input signal spectrum contains frequencies which lie outside the band width of amplifier.
Non-linear distortion occurs due to non linear dynamic 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. Accordingly in power transistors due to non linear response, new frequencies are introduced in the output and will result in distortion of input AC signal. A parameter called total harmonic distortion which is the RMS value of harmonic components of the output signal excluding the fundamental is used to quantify the conversion efficiency of power amplifier.
Consider a power amplifier which is given a input voltage of Vi = A*sin (ω*t), the response of transistors will be of the form
output = a*Vi+b*Vi2+c*Vi3+d*Vi4+………
Hence the output of the power amplifier will be in the form of A1*sin (ω*t) + A2*sin (2*ω*t)+A3*sin(3*ω*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 = (D12+D22+D32+…….)(1/2). Therefore the total power is given as Pt= P*(1+D2) where P is the power delivered at fundamental frequency. The total harmonic distortion is a measure of fidelity and efficiency of power amplifier and should be as low as possible.
If the input consists of two different frequencies ω1 and ω2 then the inter modulation frequencies (sum or difference frequencies) ω1+ω2 and ω1+ω2 are introduced into the output
The transistors in the power amplifier stage will be dealing with large power signals. Accordingly the power dissipation at the junctions will be high. Care must be taken to keep the power dissipation as low as possible to improve the overall efficiency by delivering maximum generated output power to load. Due to high power dissipation the junction temperature increases and there will be rated temperature beyond which the transistor will get damaged. Hence the power amplifier stage should be equipped with efficient heat sinks to remove the generated at the junctions.
A high power efficient power amplifier prolongs the battery life employed as power supplies to bias the transistors, reduces the cost by eliminating the need for cooling mechanism.
Conduction angle in Power Amplifiers
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. The Output power amplifier stages are classified based on conduction angle.
Classification of Power Amplifier
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
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.
The response of a) Class A power amplifier b) Class B power amplifier c) Class AB power amplifier d) Class C power amplifier for sinusoidal input signal. The red line indicates the output signal the black line indicates the input signal drawn on different scales for input and output signal amplitude.