(Solved): 5 An npn junction transistor is used to switch the current through a \( 12 \mathrm{~V} 6 \mathrm{~ ...

5 An npn junction transistor is used to switch the current through a \( 12 \mathrm{~V} 6 \mathrm{~W} \) lamp. The input signal to the transistor is from a logic circuit that gives an output of \( 5 \mathrm{~mA} \) at \( 5 \mathrm{~V} \). The circuit is shown below. (a) The base-emitter voltage of the transistor when switched on is \( 0.7 \mathrm{~V} \). (i) Calculate the voltage across the resistor, \( \mathrm{R} \), when the transistor is swifched on. (ii) Calculate the resistance of \( \mathrm{R} \) required to limit the base current to \( 5 \mathrm{~mA} \). (iii) Resistons of this value are not available. Which two identical resistors from the E24 series would combine to give the required resistance? (iv) Write the colour code for one of these resistors if it has a \( 5 \% \) tolerance. (b) (i) Calculate the collector current of the transistor shen the lamp is fully switched on. (ii) Calculate the required current gain (ratio of collector current to base current) of the transistor in this circuit.