An n-p-n transistor in a common-emitter mode is used as a simple voltage-amplifier with a collector current of 4 mA. The terminals of a 8 V battery is connected to the collector through a load-resistance `R_L` and to the base through a resistance `R_B`. The collector-emitter voltage `V_(CE)=4V`, the base-emitter voltage `V_(BE) = 0.6 V` and the current amplification factor `beta_(dc) = 100`. Then

Explanation

Potential difference across `R_(L)`

`=8V-V_(CE)=8V-4V=4V`. Now `I_(C)R_(L)=4V`
`R_(L)=(4)/(4xx10^(-3))=10^(3)Omega=1kOmega`
Further for base emitter equation, `V_(C C)=I_(B)R_(B)+V_(BE)`
or `I_(B)R_(B)=` Potential difference across `R_(B)`
`=V_(C C)-V_(BE)=8-0.6=7.4V`
Again, `I_(B)=(I_(C ))/(beta)=(4xx10^(-3))/(100)=4xx10^(-5)A`
`therefore R_(B)=(7.4)/(4xx10^(-5))=1.85xx10^(5)Omega=185kOmega`