Transistor - Amplifier

Description:

Amplifier is an electronic device, which increases strength of weak signal i.e. when input of low amplitude is provided amplifier converts it to high amplitude signals.

Amplifier is N-P-N transistor with common emitter configuration. For input microphone is used that generates A.C signals and for output load speaker is connected to the circuit.

Transistor is current sensitive device and it has large current gain (β), when operated in active region. Figure shows circuit diagram for transistor C-E amplifier using n-p-n transistor.

For transistor to work as an amplifier E-B junction should be forward biased and C-B junction should be reversed biased.

The voltage sources V_BB and V_CC provides the necessary bias. The signal voltage (V_i) which is to be amplified, is applied between base and emitter. The amplified voltage (V_o) gets developed across the load resistor R_o. When input signal varies slightly, the base current varies which produces large change in collector current.

Input voltage in given by –

V_BB = V_BE + I_BR_i

Changes in the input voltage is given by –

ΔV_i = ΔI_B . R_i

Output voltage is given by –

V₀ = V_CC − I_CR₀

Changes in the output voltage is given by –

ΔV_o = 0 − ΔI_C.R₀

Relationship between the change in input voltage and change in output voltage i.e. voltage amplification is given by –

ΔV_o/ΔV_i = − ΔI_C.R₀/ΔI_B.R_i

We have current gain (β) -

β_ac = ΔI_C/ΔI_B

β_dc = ΔI_C/ΔI_B

Therefore, Amplification of voltage is given by –

A_v = − β_ac R_o/R_i

The negative sign in the gain signifies that when the input voltage increases in positive direction, the output voltage increases in negative direction i.e. there is phase reversal.

Amplification of power or power gain is given by –

A_p = A_v A_i

A_p = β_ac²R₀/R_i

In an amplifier the output is in the form of current gain, amplification of voltage/voltage gain and power gain.