Question

Describe the function of a transistor as an amplifier with the neat circuit diagram. Sketch the input and output wave forms.

Answer 1

i. Amplification is the process of increasing the signal strength (increase in the amplitude)

ii. NPN transistor is connected in C_E configuration

iii. To start with, the Q point is fixed to get a maximum signal swing at the output.

iv. R_C – to measure the output voltage

C₁ allows only AC signals to pass,

Bypass capacitor C_E provides a low resistance path

Coupling capacitor C_C is used to couples the next stage amplifier.

V_s input source signal

I_C = BI_B `(∴ "B" = "I"_"C"/"I"_"B")`

V_CE = V_CC – I_CR_C

             Transistor as an amplifier

Input and output waveforms showing 180° phase reversal.

Working of the amplifier:

a. Working of the amplifier:

1. Input signal V_s increases the forward voltage across the emitter-base. I_B increases. also increases I_C also increases β times.
2. This increases the voltage drop across R_c which decreases V_CE.
3. Therefore input signal in the positive direction produces an amplified signal in the negative direction at the output. Hence output signal is reversed by 180°.

b. During negative half cycle:

1. Input signal V_s decreases the forwarded voltage across the emitter-base. As a result, I_B decreases I_C increases.
2. Increase in I_C decreases potential drop across R_c and increases V_CE.
3. Input signal in negative direction produces an amplified signal in the positive direction at the output.
4. Therefore 180° phase reversal is observed during the negative half signal.