The circuit posted here is of high gain preamplifier with low input impedance of about 4 KΩ. It is capable of vary high gain in order of few hundred if Rc is very greater than RL.
Circuit Description of High Gain Preamplifier
The circuit of high gain preamplifier is build around a low power NPN transistor T1 which is here configured in common emitter mode using self bias or emitter bias. This biasing method is used because it produces excellent operating point stability.
Resistor R1 and R2 combination cross the Vcc supply and produce a positive dc voltage at the bias relative to the emitter. As in simple preamplifier circuit, the input signal is given to base of transistor T1 through coupling capacitor C1. The dc component of current through the resistor R4 cause dc voltage drop, which cause reverse bias at the emitter junction. In the circuit high gain preamplifier high value bypass capacitor C2 offers an almost short-circuit even to the lower frequency component and thus prevent loss of amplification due to negative feedback.
PARTS LIST OF HIGH GAIN PREAMPLIFIER CIRCUIT
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 = 180 KΩ
R2 = 33 KΩ
R3 = 3.9 KΩ
R4 = 1 KΩ
|C1, C3 = 10 µF/25V|
C2 = 100 µF/6V
|T1 = BC148B|