 In this session, we are going to learn about Class C power amplifiers. At the end of this session, students will be able to explain the operation of Class C amplifier with its design equations. These are the contents of today's presentation. Let us get a basic introduction of Class C power amplifier. In Class C power amplifier, the DC biasing conditions for power transistor are adjusted to locate its DC operating point or crescent point on a load line below the cutoff line so that output collector current signal varies for less than 180 degree of input AC signal cycle. The convergent efficiency of Class C power amplifier is very high. It is about 90%, but distortion in the output signal is also high. It produces more load power than Class B power amplifier, since the output current of Class C power amplifier flows in the form of pulses. If resistor load is connected in the collector circuit of power transistor, it contains more number of harmonics. This amplifier is used in a tuned circuit and Class C power amplifier is tuned to fundamental frequency of output current signal. Because of this, Class C amplifier are also called as narrow band amplifiers. Class C power amplifier is used to amplify only small band of frequencies around the resonant frequency of the circuit. The need of large inductor and capacitor is avoided in the resonant circuit. The resonance frequency of resonant circuit depends upon the values of inductor and capacitor. As frequency increases, the size and value of inductor capacitor decreases. Therefore, a Class C power amplifier that amplifies small band of frequency around the resonant frequency is always operated at a radio frequency greater than 20 kHz. This figure shows the circuit diagram for basic Class C power amplifier. The Class C power amplifier consists of a parallel amplifier, which consists of a parallel connection of inductor and capacitor called as a tank circuit. This tank circuit is connected in the collector circuit of a power transistor. The input AC signal to be amplified is coupled to the input of amplifier using capacitor coupling with the use of capacitor C1. Output amplifier signal is provided to load RL using capacitor coupling. Output load resistance RL is coupled at the output of Class C amplifier using capacitor coupling with the use of capacitor C2. The base emitter junction when output collector current is zero in the absence of input AC signal. When input AC signal is applied, the base emitter junction of power transistor, base emitter diode along with capacitor C1 and resistor RB connected between base and emitter acts as a diode clamper. So this diode clamper clamps some negative portion of input AC signal with respect to zero level. When emitter base diode conducts, the operating point of Class C power amplifier moves towards saturation region. Due to resonant action of a parallel tank circuit, the output signal is supposed to be a sine wave signal. If output tank circuit consists of an inductor and capacitor is tuned to a fundamental frequency of signal under amplification, the parallel reactance of a tank circuit becomes infinite and this leaves only dynamic load resistance in the collector circuit of a power transistor. The efficiency of this Class C power amplifier is approximately 90 percent. So due to these reasons, Class C power amplifier is used as power output stage. Now this figure shows DC and AC conditions for Class C power amplifier. The DC operating point of Class C amplifier is selected below cutoff line. For variation of input base signal, the output current flows in the form of a pulses as the working of a Class C amplifier moves along the load line. Now let us discuss the design equations for a Class C power amplifier. When tank circuit is tuned to frequency of input signal with high quality factor, the parallel resonance occurs at frequency FR is equal to 1 upon 2 pi root LC. So, the resonant frequency of parallel tank circuit depends upon the values of inductor and capacitor. So at this frequency, peak to peak voltage across load resistor RL reaches to its maximum value and amplified output signal is the sine wave signal. Under zero bias condition, the operating point of Class C amplifier is below the cutoff line. So output collector current at Q point ICQ is equal to zero and collector to emitter voltage that is quiescent collector voltage VCQ is equal to its maximum value VCC. The saturation current flowing through transistor power transistor ICSAT is equal to VCC upon RC where RC stands for AC load resistance, AC load resistance seen by the collector. When quality factor of tank circuit greater than 10, the bandwidth of resonant circuit will be BW equal to FR upon Q that is resonant frequency divided by quality factor of tank circuit. The quality factor of inductor QL is equal to XL upon RS where XL is reactance of inductor coil and RS stands for series DC resistance of that is internal resistance of inductor coil and overall Q of the circuit is RC upon XL where RC stands for AC resistance seen by the collector of power transistor. In practical Class C amplifier, the quality factor of inductor coil is more than 50 and overall quality factor of the circuit is greater than 10. The maximum load power depends upon the input signal swing and power dissipation in transistor. Due to less conduction angle of a transistor that is less than 180 degree, the average power dissipation of power transistor is very low. The maximum output voltage swing across a tank circuit on either side of collector to emitter voltage at Q point BCQ is Vcc minus Vcsat. Since Vcsat that is collector to emitter voltage under saturation condition is very small that is negligible, the peak to peak value of output voltage reaches to 2 Vcc. The maximum power delivered to load RL is PL equal to VPP square upon 8 RL. The maximum load power occurs when entire AC load is used. The transistor power dissipation because of resonant tank circuit all harmonics are filtered to get sinusoidal voltage with fundamental frequency FR. The maximum voltage is approximately 2 Vcc hence transistor should be selected with collector to emitter voltage VcQ Vco rating greater than 2 Vcc. So total DC power in Class C amplifier is equal to PL that is load power plus power dissipated in transistor and power dissipated in inductor coil. Now students can pause video here and think over this question why the Class C amplifier is called as narrow band amplifier. You know Class C amplifier is used in tuned circuit when Class C amplifier is tuned to fundamental frequency of signal under amplification. It amplifies a small band of frequency around a resonant frequency. So that is why Class C amplifier is also known as narrow band amplifier. These are the references. Thank you.