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Question
For a CE-transistor amplifier, the audio signal voltage across the collector resistance of 2 kΩ is 2 V. Suppose the current amplification factor of the transistor is 100, find the input signal voltage and base current if the base resistance is 1 kΩ.
Solution 1
Given:
Output voltage, Vo = 2 V, output resistance, Ro = 2 kΩ, base resistance, Ri = 1 kΩ
Current amplification factor, β = 100
Then input signal voltage is calculated as:
`V_0/V_i=R_0/R_ixxbeta=>2/V_i=2/1xx100`
`=>V_i=10mV`
Now, collector current is
`I_C=V_0/R_0=2/2=1mA`
Therefore, base current is
`I_B=I_C/beta=(1mA)/100=10muA`
Solution 2
Collector resistance, RC = 2 kΩ = 2000 Ω
Audio signal voltage across the collector resistance, V = 2 V
Current amplification factor of the transistor, β = 100
Base resistance, RB = 1 kΩ = 1000 Ω
Input signal voltage = Vi
Base current = IB
We have the amplification relation as:
Voltage amplification = `V/V_' = beta R_C/R_B`
`V_i = V R_B/(beta R_C)`
`= (2 xx 1000)/(100 xx 2000) = 0.01 V`
Therefore, the input signal voltage of the amplifier is 0.01 V.
Base resistance is given by the relation:
`R_B = V_i/I_B`
`= 0.01/1000 = 10 xx 10^(-6)mu^6 A`
Therefore, the base current of the amplifier is 10 μA.
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