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Question
With the help of a neat circuit diagram, explain the transistor as an amplifier?
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Solution
Working of an amplifier:
- The circuit of an amplifier using an n-p-n transistor in CE configuration is shown in the figure.
- When the input voltage Vin is not applied, applying the Kirchhoff’s law to the output loop, we can write, VCC = VCE + ICRL
- Similarly, for input loop,
VBB = VBE + IBRB - When the input AC signal is applied, Vin is not zero. Thus, the voltage drop across the input loop will now be,
VBB + Vin = VBE + IBRB + ΔIB (RB + ri) ....(1) - The AC signal applied adds the current of ΔIB to the original current flowing through the circuit. Therefore, the additional voltage drop in the input loop will be across resistor RB (= ΔIBRB) and across the input dynamic resistance of the transistor (= ΔIBri).
- From equation (1),
Vin = ΔIB (RB + ri)
As, RB is very small, we can consider,
Vin = ΔIBri - The changes in the base current IB cause changes in the collector current IC. This changes the voltage drop across the load resistance because VCC is constant. We can write,
ΔVCC = ΔVCE + RLIC = 0
∴ ΔVCE = -RLIC - The change in the output voltage ΔVCE is the output voltage Vo hence we can write,
Vo = ΔVCE = `beta`ACRLΔIB
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