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NEET PHYSICSMedium

A common emitter amplifier has a voltage gain of 50, an input impedance of 100 Ω100 \text{ } \Omega and an output impedance of 200 Ω200 \text{ } \Omega. The power gain of the amplifier is:

A

500

B

1000

C

1250

D

50

Step-by-Step Solution

Given: Voltage gain, Av=50A_v = 50 Input impedance, Ri=100 ΩR_i = 100 \text{ } \Omega Output impedance, Ro=200 ΩR_o = 200 \text{ } \Omega

The power gain (ApA_p) of an amplifier is given by the ratio of output power to input power: Ap=PoutPin=Vout2/RoVin2/Ri=(VoutVin)2×RiRoA_p = \frac{P_{out}}{P_{in}} = \frac{V_{out}^2 / R_o}{V_{in}^2 / R_i} = \left(\frac{V_{out}}{V_{in}}\right)^2 \times \frac{R_i}{R_o} Since Av=VoutVinA_v = \frac{V_{out}}{V_{in}}, we have: Ap=Av2×RiRoA_p = A_v^2 \times \frac{R_i}{R_o}

Substituting the given values: Ap=(50)2×100200A_p = (50)^2 \times \frac{100}{200} Ap=2500×12=1250A_p = 2500 \times \frac{1}{2} = 1250

Alternatively, we can first find the current gain (β\beta): Av=β×RoRiA_v = \beta \times \frac{R_o}{R_i} 50=β×200100    β=2550 = \beta \times \frac{200}{100} \implies \beta = 25

Then, Power Gain = Voltage Gain ×\times Current Gain: Ap=Av×β=50×25=1250A_p = A_v \times \beta = 50 \times 25 = 1250

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