Question

With a neat circuit diagram, explain the working of a single stage $CE$ amplifier. Draw the frequency response curve and discuss the results.

Answer 1

Functions of Circuit Elements :

Biasing Circuit : The resistances $R_1$, $R_2$, $R_E$ forms the biasing and stabilization circuit.

Input Capacitance $C_{in}$ : This is used to couple the signal to the base of the transistor. If this is not used; the signal source resistance will come across $R_2$ an thus change the bias. $C_{in}$ allows only a.c. to flow.

Emitter bypass Capacitor $C_E$ : This is connected in parallel with $R_E$ to provide a low reactance path to the amplified ac signal. If it is not used, then amplified ac signal flowing through $R_E$ will cause a voltage drop across it, thereby shifting the output voltage.

Coupling Capacitor $C_O$ : This is used to couple the amplified signal to the output device. This capacitor allows only a.c. to flow.

Working : When a weak input ac signal is applied to the base of the transistor, a small base current flows. Due to transistor action, a large ac current flows through $R_C$ and a large voltage appears across $R_C$ and hence at the output. Therefore, a weak signal applied to the base appears in the amplified form in the collector circuit. Voltage gain of the amplifier is the ratio of the amplified output voltage to the input voltage.

Frequency response and bandwidth : The voltage gain for different input frequencies can be determined. A graph is drawn by taking frequency along $X$-axis and voltage gain $\left(A_V\right)$ along $Y$-axis. It can be seen that the gain decrease for very low and very high frequencies but remains constant over mid frequency region.

$f_L$: Lower cut off frequency is defined as the frequency in the low frequency range at which the gain of the amplifier is $\frac{1}{\sqrt{2}}$ times the mid-frequency gain $\left(A_M\right)$.

$f_U$: Upper cut off frequency is defined as the frequency in the high frequency range at which the gain is $\frac{1}{\sqrt{2}}$ times the mid-frequency gain $\left(A_M\right)$.

Bandwidth : is defined as the frequency interval between lower cut off and upper cut off frequencies. $BW=f_U-f_L$