A series fed directly coupled class a amp is shown in fig.

Fixed biasing is being used for the circuit and resistance $R_B$ is adjusted in such a way that ‘Q’ pt. lies exactly at the centre of load line.

Transistor ‘T’ is a power transistor and level of voltage that can be applied at i/p is of the order of a few volts.

The circuit shown is capable of handling a large amt. of power however the voltage gain of the circuit is not very high, because this is Power amp.

This amp is called as series fed directly coupled amp because, the load $R_L$ is connected directly (coupled directly) to the collector circuit without using any coupling technique.

Graphical Representation of series fed directly coupled amplifier is as shown, i.e., i/p and o/p voltage waveforms are pure sine waves, and there is no harmonic distortions present in the waveform.

The transistor remain in the ‘active region’ for all the values of i/p sig and never enters into the saturation or cut off regions.

The transistor conducts for the complete cycle of ac i/p i.e. for 360 degrees. Thus the angle of collector current flow is 360 degrees or full cycle.

The i/p sig is amplified, without introducing any distortions. Thus harmonic contents in o/p will be low.

As the transistor continuously operates in its active region, the voltage $V_{CE}$ across it and current $I_C$ through it, both are simultaneously high.

Therefore a large power will be dissipated in the transistor in the form of heat. Therefore the efficiency of class a power amp. Is low. In fact it is the lowest of all the power amp.

Typically the efficiency (n) of a class a power amp lies between 25% to 50%

Advantages:

i) The circuit design is simple

ii) Less number of components are required. Hence it is cheaper and less bulky.

iii) Output transformer is not used as the load is directly coupled.

Disadvantage:

i) Large power dissipated and power wastage takes place in power transistors.

ii) Heat sink is essential which makes the circuit bulky.

iii) Poor efficiency

iv) The o/p imp is high and there is no imp matching techniques used, hence low imp loads such as loud speakers cannot be driven efficiently.

BJT as class C amplier:

Class C amp cannot be used as audio amp.

However we can use it in tuned circuits.

The LC tuned circuit acts as load for transistor Q.

The dc source - $V_{BB}$ biases the transistor below the cut off. RFC is radio frequency choke. It is equivalent to a short circuit for the dc operating condition but its reactance is very high at radio frequency.

The ac i/p voltage $V_i$ is applied to base of the transistor. The voltage is superimposed on - $V_{BB}$.

The transistor will be forward biased and will conduct for an angle which is less than 180 degree.

When these current pulses are applied to LC resonant circuit a full cycle of o/p sig is obtained at resonant frequency of tuned circuit. Thus the o/p voltage produced across the LC circuit is a sine wave and its frequency is equal to the resonant frequency of the tuned circuit.

Property of tuned Circuit: Current pulses

States that if we apply a current pulse to a tuned circuit then it generates damped voltage oscillations at its o/p.

The amplitude of oscillation is proportional to size of current pulse and decay rate is proportional to time constant. -This is called flywheel effect in a tuned circuit. So if we apply train of current pulses at frequency ₣ which is equal to resonant frequency. of tuned circuit. Then we will obtain a sinusoidal sig at frequency ₣ at o/p.

Class C is limited to use at only one frequency, i.e., resonant frequency ₣.