 Hello everyone. In today's session, we are going to learn about power amplifiers. Let us get basic introduction of power amplifiers. At the end of this session, students can classify and define power amplifiers with performance parameters. These are the contents of my session. Let us see the general power amplifier system. The general power amplifier system consists of input signal pickup transducer, a preamplifier, a voltage amplifier, a power amplifier and output transducer. The typical example of this general power amplifier system application is a public address system or a sound amplifier system. The input signal pickup transducer, for example microphone, converts one form of energy into electrical energy. It converts sound energy into electrical energy whose output is very low level a voltage signal is applied to a preamplifier. The output of microphone, that is the input signal pickup transducer, cannot be used to drive output transducer because its signal level is very low. So it is given to a preamplifier. It is a small signal amplifier whose input resistance is very high and it is a small signal amplifier. So it raises signal voltage level and current level of the signal from a microphone. It is applied to a voltage amplifier. Output of preamplifier, that is a voltage amplifier signal is applied to a voltage amplifier. Voltage amplifier is a small signal amplifier with high input resistance and high output resistance. It amplifies a voltage signal and output of this voltage amplifier is applied to a power amplifier. So voltage amplifier raises voltage signal, voltage level of the input signal. It is given to the input of power amplifier. A power amplifier is a large signal amplifier whose output is high power. So its input should be sufficiently large a voltage signal. So that is why the output of a small signal voltage amplifier is given to the input of power amplifier. A power amplifier increases voltage level as well as current level of the input signal and with high output driving capability the output high power signal is given to output transducer. For example, the output transducer is a speaker. It converts electrical energy into a sound energy. Power amplifier is basically a power converter. It converts DC power from DC power supply into output AC signal power that is delivered to output load under the control of input AC signal. Let us compare a voltage amplifier and power amplifiers. In considering the parameters current gain for voltage amplifier high current gain a transistor is used. Current gain with greater than 100 are used in a voltage amplifier whereas the current gain of transistor in power amplifier is low it is in the range of 20 to 50. Collector load for a voltage amplifier is high about above 10 kilo ohm and for power amplifier it is low it is in the range of 5 to 20 ohm. Input voltage for voltage amplifier is a low level signal it is in the order of milliholt. For power amplifier the input signal voltage level is about 2 to 4 volts. Current collector current IC for voltage amplifier collector current is low about 1 milliampere for power amplifier the collector current it is high it is above 100 milliampere. Power output for voltage amplifier the power output is low and for power amplifier power output is high the power output is about hundreds of watts. Power dissipation capability for voltage amplifier power dissipation capability is less than 0.5 watt and for power amplifier it is more than 0.5 watt. Output impedance output impedance of voltage amplifier is high about 10 kilo ohm for power amplifier it is low about 200 ohm coupling input and output coupling for voltage amplifier generally resistor capacitor that is RC coupling is used because of its small size request less space and low cost RC coupling is used for voltage amplifier. Average for power amplifier always transformer coupling is used for power amplifier to couple the amplified output signal of amplifier to output low. Now let us perform performance parameters of power amplifier let us measure performance or effectiveness of power amplifier by considering different parameters of power amplifier like collector efficiency collector efficiency is a ability of power transistor used in the power amplifier to convert DC power from DC power supply or battery to AC signal power at the output it can be defined as the ratio of AC output signal power to DC input power or zero input signal condition or DC power from DC power supply. So efficiency eta is equal to AC power delivered to output load upon DC power input supplied in collector circuit of transistor power transistor from DC power supply so that is PAC upon a PDC power dissipation capacity power amplifier power transistors are used power transistors are designed to handle large amount of current so that it is a ability of a power transistor to dissipate heat developed in it during its operation. So power dissipation capacity of a power transistor used in a power amplifier can be increased by mounting with the metal case like aluminum sheet metal case is attached with the power transistor it is known as a heat sink a transverse heat developed in power transistor in surrounding atmosphere distortion distortion that the change of output from the input is known as distortion in power amplifier the output amplified output signal is distorted its shape is not exactly similar to input signal AC signal shape so power amplifier handles large signals at the input and output side for large input signal transistor operation becomes a non-linear causes a harmonic distortion. So that is why the output signal amplified output signal is distorted and its waveform is not exactly same as input signal waveform. So distortion should be as less as possible for power amplifier as well as efficiency of power amplifier should be as large as possible. Now let us classify the power amplifiers generally power amplifiers are classified according to their mode of operation or amount of variation in output signal or one complete cycle of input signal that is conduction angle of transistor. So power amplifiers are classified as and named as class C power amplifier class B power amplifier class AB power amplifier class C power amplifier that is the first type of power amplifier that is class A power amplifier. The transistor power transistor by C conditions in class A amplifier are registered to locate it is a percent point a Q point a DC operating point at the exactly center of load line in active region in active region so that output current signal varies or current output current flows for 360 degrees of input signal cycle. A transistor conduction angle is about 360 degrees it has less efficiency and produces a minimum distortion. As a transistor conducts all the time or for complete cycle of input AC signal the efficiency of class A amplifier is very less. So power dissipation in output signal collector circuit is more and as output is signal waveform is nearly same as in input signal waveform so distortion in class A amplifier is very less. So efficiency of class A amplifier it is in the range 25 to 50 percent. Class B second type of power amplifier is known as class B power amplifier. The power transistor by C conditions are adjusted to locate it is a DC operating point or percent point a Q point on load line in the cutoff region. So that output current signal varies only for either 180 degrees of input signal cycle. The output current flows only during positive or negative half cycles of input signal. It has high efficiency and produces more distortion as the transistor conducts only for 180 degree of input signal cycle. So efficiency is relatively higher than the class A power amplifier and since output current is in terms of pulses and output current is discontinuous. So it produces relatively more distortion as compared to class A amplifier. Class A B amplifier the power transistor by C conditions are adjusted to locate Q point on load line just above the cutoff region. So that output current signal varies in the range greater than 180 degree to less than 360 degrees of input signal cycle. The output current flows for more than half cycle but less than full cycle of input signal. It requires push pull arrangement to get output power for both cycles of the input signal. Now fourth type of power amplifier is known as class A power amplifier. The power transistor by C conditions are adjusted to locate it is a Q point on load line below the cutoff region. So that output current signal varies for less than 180 degree of input signal cycle the output current flows in the form of pulses. It has high efficiency and produces more distortion. It is used in two line amplifier. Since output current flows in terms of pulses the distortion is more and since power transistor conducts for less than 180 degree of input signal cycle. So efficiency of class A amplifier is relatively higher. For short time student can pause video here and think over why the efficiency is very less for class A amplifier. For class A amplifier the power dissipation in the output collector circuit is more. Since power transistor remains conducting for complete cycle of input signal for 360 degrees. So power dissipated in zero input signal condition in collector circuit output circuit is higher. So efficiency of class A amplifier is very less. These are the references. Thank you.