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Transistor as a Switch

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Electronic circuits often need to control large currents and voltages from signals that are much smaller. The circuit shows how a signal of only 4V at 10mA, can, in conjunction with a transistor acting as a current amplifier, be used as an electronic switch that controls a current of 250mA at 12V. In order for the circuit to operate correctly, the transistor must have a current gain (h_FE) of greater than 25. Note that, when the transistor is in the 'on' state, its collector-emitter voltage (V_CE) falls to a very low value (approx. 0.2V). In this condition we say that the transistor is saturated and no further increase in base current will reduce the value of V_CE. Conversely, when the transistor is in the 'off' state, the value of V_CE will be very close to that of the supply (i.e. 12V). In this condition we say that the transistor is cut-off . These two states are summarized in the table.

Parameter	TR1 "OFF"	TR1 "ON"
Base current, IB	0 mA	10 mA
Collector current, IC	0 mA	250 mA
Base-emitter voltage, VBE	0 V	0.65 V
Collector-emitter voltage, VCE	12 V	0.2 V (approx.)

<^< Worksheet: Transistor as a Current Amplifier | Course index | Logic Gates >^>