06-08-2013, 12:35 PM
CLASS - A AMPLIFIER
Aim
To design and construct a Class – A power amplifier. To observe the output waveform
and to measure the maximum power output and to determine the efficiency
Bias design:
Since voltage amplification is done in the transistor amplifier circuit, We as equal drops
across VCE & load resistance RE. The quiescent current of 1mA is assumed, we assume a
standard supply of 12V.
Drop across RE is assumed to be 1V,the drop across VCE with a supply of 12V is given by 12V-
1V=11V
It is equal to 11/2=5.5V
Now the voltage across the resistance RE is 5.5V
VCE = 5.5V
VC = 5.5V
IC = 1mA
RC = 5.5V/1mA = 5.5KΩ
Instead of using 5.5KΩ , We can use a standard value of 4.7KΩ.
Theory:
The Power amplifier is said to be class-A amplifier if the Q-point & the input signal are
selected such that the output signal is obtained for a full input cycle. For this, position of the Q-
point is approximately at the midpoint of the load line.
For all the values of input signal, the transistor remains in the active region &never enters into
cut-off or saturation region. When an a.c input signal is applied, the collector voltage varies
sinusoidally hence the collector current also varies sinusoidally. The collector current flows for
360°(full cycle)of the input signal. In other words, the angle of the collector current flow is 360°
i.e. one full cycle.
Procedure:
1. Connect the circuit as per the circuit diagram.
2. Set VS=10mV using AFO.
3. Keeping the input voltage constant, vary the frequency from few Hz to 1MHz in regular steps
& note down the correspondingly output voltage.
4. Plot the graph: gain Vs frequency.
5. Calculate bandwidth from the graph.
Result:
The class-A amplifier is designed, constructed and the output waveform is observed. The
maximum power output and the efficiency are determined.