 In today's session, we are going to learn about class A power amplifiers. Learning.com, at the end of this session, students can analyze class A power amplifiers. These are the contents of today's presentation. The power transistor, the amplifying device used in power amplifier, that is the power transistor, is biased in such a way that its output collector current flows continuously for the entire cycle of input AC signal waveform for 360 degrees. Since its DC operating point or Q send point Q is located at the center of the load line and in the active region, the amplifier operates in active region at all times of input signal cycle. The amplifier uses an amplifying device that is power transistor with relatively low current gain and the amplifier with low current gain power transistor is capable of handling large output power or output current. The power amplifier is not providing much voltage gain. The output current signal waveform is same as input signal waveform. So the operation of power transistor is mostly linear. So no distortion is caused in class A power amplifier. The output load resistance RL, output load with load resistance RL is connected either directly in the collector circuit or using transformer or to collector circuit of power transistor. Not all DC power provided in collector circuit of transistor is converted to output useful AC signal power. Since DC power is dissipated in collector circuit is wasted in resistive load and across transistor under zero input signal condition or in presence of input signal condition. Now let us see the first type of class A power amplifier. Considering the way of connecting the output load to the collector circuit of power transistor. That is a series speed class A power amplifier. The input low level signal gain is applied at the base of a transistor and transistor is biased using DC power supply VCC and voltage divider R1, R2 with emitter resistance RE. In such a way that transistor operates all the time in active region at the center of a load line. The output load with load resistance RL is directly connected in the collector circuit of a transistor. So load resistance RL is coming in series with collector and emitter of a transistor and output is taken from a collector. So that is why this amplifier is called as series speed class A power amplifier. Now let us consider a DC bias conditions for series speed class A power amplifier. The first figure shows a class A power amplifier with the Thevenin's equivalent circuit for voltage divider bias with cell bias using resistor R1, R2 and resistor RE. RTH is Thevenin's equivalent voltage applied at the base of a transistor to turn on a transistor and RTH is Thevenin's equivalent resistance coming in the base circuit of a transistor. So DC bias conditions for a transistor is set in such a way that the maximum possible output collector current of a transistor is set at one half of maximum possible output collector current swing. So that is in the range 0 to VCC upon RL. So output collector current IC is the forward current gain times of a transistor with input bias current, input base current IB. Using Kirchhoff's voltage law to output collector circuit of a transistor the output collector current IC is given by IC is equal to VCC upon RL plus RE and collector to emitter voltage output voltage under DC condition is given by BC is equal to VCC minus IC into RL plus RE. So these are the equations for DC output collector current and DC output collector to emitter voltage. Now let us consider AC bias conditions for a series speed class A power amplifier. When a low level input AC signal is applied at the base of a transistor the output collector current IC and output collector to emitter voltage will vary above and below DC bias conditions, DC operating point, Q point values. The second figure shows the figure shows for output characteristics of a transistor with a load line and variation of input output signal in response to the variation of input signal. That is a variation of output collector current from its minimum value 0 to its maximum value VCC upon RL and variation of output collector to emitter voltage. In the range minimum value 0 to maximum value VCC. So the output collector current swings in between minimum value 0 to maximum value VCC upon RL and output collector to emitter voltage provided to load swings in between minimum value 0 to maximum value VCC. Let us find out the efficiency for series speed class A power amplifier. So that is the ability of power amplifier to convert DC power provided in the collector circuit to output AC power delivered to output load. So efficiency is given by eta is equal to PAC upon PDC where PAC is AC power delivered to load divided by DC input power provided to transistor in collector circuit. So DC power PDC is equal to VCC into IC DC voltage into DC current that is VCC into IC that is equal to VCC square upon 2 RL and PAC that is AC power delivered to load is equal to RMS value of output voltage and output current it is given by VCC square upon 8 RL. So efficiency of series speed class A amplifier eta is equal to 1 by 4 that is 25%. This means that only 25% of DC power is converted to AC power delivered to load and remaining 75% of DC power is wasted in resistive load and across transistor. For maximum power transfer it provides poor impedance matching. Now let us think about second type of class A power amplifier that is transformer coupled class A power amplifier. The figure shows the circuit diagram for transformer coupled class A power amplifier. So in this amplifier output load with load resistance RL is not directly connected to collector circuit of transistor it is connected by means of a transformer. The transformer primary winding is connected in the collector circuit of transistor and output load with load resistance RL is connected to secondary winding of a transformer. So due to transformer action the current and voltage swing in the collector circuit are transformed to secondary winding and delivered to load resistance RL. Now let us consider DC conditions for a transformer coupled power amplifier. Under DC condition the load resistance seen by transistor RL DC is equal to RP plus RE that is approximately equal to RE where RP is the primary winding resistance of transformer and RE is the emitter resistance. Since the resistance of primary winding of transformer is very low ideally it is zero so it is in terms of ohm. So load resistance seen by transistor is approximately equal to RE. Considering DC conditions using KBL to output circuit of transistor IC is given by VCC minus VCE upon RE where VCE is collector to emitter voltage. As RE tends to zero IC tends to infinity. So collector to emitter voltage VCE is equal to VCC minus IC into RE. So that is equal to as RE tends to zero and IC tends to infinity so collector to emitter voltage is equal to VCC. So VCE Q that is collector to emitter voltage at Q point is equal to VCC. Under AC conditions whenever input signal is applied at the input of transformer coupled power amplifier a maximum swing in voltage and current is from minimum value to maximum value. So the figure shows output transistor with load line and the range of variation of output collector current and output collector to emitter voltage. So collector current varies from its minimum value zero to maximum value IC max and collector to emitter voltage varies from zero to maximum voltage VCC. So collector to emitter voltage peak to peak value is equal to VCE max minus VCE minimum. That is equal to VCC. Due to inductive effect of primary winding of transformer the minimum value of collector to emitter voltage is equal to VCC. So total collector to emitter voltage peak to peak value is equal to VCC. So peak to peak value of output collector current is equal to IC max minus IC minimum. So IC peak to peak value is equal to 2IC. Let us consider efficiency of a transformer coupled power amplifier, class A amplifier. Efficiency is equal to eta is equal to PAC upon PDC. So PDC is equal to VCC into IDC. So DC voltage into DC current. So PAC is equal to VCC into IDC divided by 2. So for a transformer coupled class A power amplifier eta is equal to 1 half. So that is equal to 50 percent. Thus the efficiency for a transformer coupled power amplifier eta is increased from 25 percent to 50 percent. So power dissipation is minimized due to very low resistance of primary winding of a transformer. And with the use of a transformer the impedance matching for maximum transformer is very good. The student can pause video here for some time and think over this question. Why the efficiency of class A amplifier is more for transformer coupling than direct coupling? In case of direct coupling the power dissipation in the collector circuit is more and it is the best stage of DC power. The DC output load gets AC power as well as DC power. As far as a transformer coupling is concerned due to low resistance of primary winding of a transformer the DC voltage drop across collector load resistance is very less. So power dissipated in the collector circuit is not provided to output load. So power dissipation is less the voltage drop across collector resistance is less. And output AC power is provided to output load due to transformer action. So output load gets only AC power not DC power. So the stage of DC power in collector circuit is very less for transformer coupling. So that is why for transformer coupled class A amplifier efficiency is higher than direct coupling. These are the references. Thank you.