Question

With the help of a neat circuit diagram, explain the transistor as an amplifier? 

Answer 1

Working of an amplifier:

1. The circuit of an amplifier using an n-p-n transistor in CE configuration is shown in the figure.
   
   
2. When the input voltage V_in is not applied, applying the Kirchhoff’s law to the output loop, we can write, V_CC = V_CE + I_CR_L 
3. Similarly, for input loop,  
   V_BB = V_BE + I_BR_B 
4. When the input AC signal is applied, Vin is not zero. Thus, the voltage drop across the input loop will now be,
   V_BB + V_in = V_BE + I_BR_B + ΔI_B (R_B + r_i) ....(1)
5. The AC signal applied adds the current of ΔI_B to the original current flowing through the circuit. Therefore, the additional voltage drop in the input loop will be across resistor R_B (= ΔI_BR_B) and across the input dynamic resistance of the transistor (= ΔI_Br_i).    
6. From equation (1),
   V_in = ΔI_B (R_B + r_i)
   As, R_B is very small, we can consider,
   V_in = ΔI_Br_i
7. The changes in the base current I_B cause changes in the collector current I_C. This changes the voltage drop across the load resistance because V_CC is constant. We can write,
   ΔV_CC = ΔV_CE + R_LI_C = 0
   ∴ ΔV_CE = -R_LI_C 
8. The change in the output voltage ΔV_CE is the output voltage V_o hence we can write,
   V_o = ΔV_CE = `beta`_ACR_LΔI_B