Question

+15V (10) The circuit at right detects a person walking through a dark room at night and turns on a light bulb so that the person can see. The circuit uses an infrared (ir) detector that outputs just over 5V when it detects the heat from a person walking past. TheR1 infrared detector outputs 2V when there is no person +15V +15V bulb present. R3 15V ir detector The diode in the circuit above is there so that no negative voltage is fed into te transistor· THe diode has a 0.7V voltage drop across it when it is forward biased. The transistor in this circuit must drive the bulb as the Op-amp can only supply a maximum of 20 mA at 15 V and the bulb needs 400 mA to function. The transistor has a β of 100 and a re of l volt when it son and passing 400 mA. ( What base current must be injected into the transistor in order for 400 mA to pass through the bulb? When 400 mA is passing through the bulb, the voltage across the bulb is 14.7 volts. (vi What region is the transistor working in and how do you know this? (vii) What power is being dissipated in the transistor when 400 mA is passing through the bulb? (iv No protection diode across the bulb is needed to protect the transistor in this circuit. Expain why not

Answer 1

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