Question

What do you understand by amplification? Make the diagram of a p-n-p transistor common amplifier and explain the process of amplification and calculate the formula for voltage gain.

Answer 1

Transistor Amplifier:

Amplifier is an active electronic device of which the amplitude of output signal obtained is more than the amplitude of the input signal applied on it. Input signal is generally alternating current or voltage (a.c.). the process of increase in the amplitude of the signal is called amplification. In this there is no change in the shape and frequency of the signal. The necessary energy required for the increase in the amplitude of the signal is obtained from the dc power supply in the amplifier circuit. Figure shows the block diagram of an amplifier.

One type of amplifier, which is used in our daily life and which is known to us is the sound frequency amplifier. With the help of a microphone, the sound signals are converted into electrical signals of same frequency and hence injected in the amplifier and output of which high amplitude electrical signals are obtained which are again changed into sound signals by the loudspeaker. Hence, we hear a high intensity sound.

The ratio between output and input signals for an amplifier is known as amplification factor or gain. If the input signal voltage is V_i and output signal voltage is V₀ then;
Voltage amplification factor or voltage gain

Hence, for amplifier all the three amplification factors are corelated.

Transistor, when used in the form of an amplifier its one terminal of input and output circuits, is common. Input signal whose amplification is to be done is used in the input circuit and output circuit has load resistor R_L. The amplification process by a transistor is possible for both the configuration of common base and common emitter. But the voltage current and power gain for common emitter amplifier are more than the common-base amplifier.
Hence, here we will only study about common-emitter amplifier.

Common-Emitter Amplifier:

Figure shows a PNP transistor common-emitter circuit. Here, emitter is common for both the input and output circuits. With the help of battery V_BB the emitter-base junction is provided forward biasing. Whereas with the help of battery V_CC (> V_BB ) reverse biasing is provided to the collector – emitter junction. V; is signal voltage whose amplification is to be done.

A load resistor R_L is connected to output circuit on which amplified voltage V₀ is obtained.

If alternative signal V_i is absent then potential difference V_BE present on base-emitter junction is constant. Due to this the input current I_B and the output current I_C related to it is of constant and direct nature. In the presence of V_i the potential difference of base-emitter junction changes. If there is very less change in I_B and change is I_B and there is change in related collected current I_C and change is i_C. According to definition;
Current Gain:

Since, β >> 1, therefore output signal current is more than input current. Hence, in common emitter configuration the current is amplified. [In sign A_ie the element e represents common emitter configuration.]

Voltage Amplification:

Voltage amplification or voltage gain by definition;

Here, V₀ – Alternating voltage generated on load resistor R_L
= i_CR_L
and V_i = i_bR_ie
where R_ie is the input resistance in common – emitter configuration
\(A_{v e}=\frac{i_{c} R_{L}}{i_{b} R_{i e}}=\beta \times \frac{R_{L}}{R_{i e}}\)
\(\frac{R_{L}}{R_{i e}}\) is called the resistance gain for common-emitter configuration.
Therefore, A_ve = A_ie × resistance gain.
Because β >> 1, therefore R_L and R_ie are same hence;

If A_ve ≈ β >> 1 then voltage amplification is possible. If R_L > R_ie, then A_ve > β >> 1) more voltage resistance is obtained.