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Electronic circuits and components * Course Index * Introduction * About the Author * About this Course * Feedback * Course Navigation * How to use this Course * TINA * Locktronics Fundamentals * Introduction: Fundamentals * Units and Multiples * Electricity * Electronic Principles * Electrical Circuits * Alternating Current * Assessment: Fundamentals Passive Components * Introduction: Passive Components * Resistors * Capacitors * Inductors * Transformers * Batteries, Fuses, Lamps and Switches * Assessment: Passive Components Semiconductors * Introduction: Semiconductors * Diodes * Transistors ## Transistor Construction ## Transistor Symbols and Connections ## Transistor Operation ## Current flow Model ## Examples: Current Flow Model ## Transistor as a Current Amplifier ## Worksheet: Transistor as a Current Amplifier ## Transistor as a Switch * Logic Gates * Assessment: Semiconductors Passive Circuits * Introduction: Passive Circuits * Series and Parallel Connections * Kirchoff's Laws * Potential and Current Dividers * Passive Time Variant Circuits * Assessment: Passive Circuits Active Circuits * Introduction: Active Circuits * Power Supply Circuits * Operational Amplifier Circuits * Transistor Amplifier Circuits * 555 Timer Circuits * Assessment: Active Circuits Parts Gallery * Introduction: Parts Gallery * Passive Component Images * Semiconductor Images * Other Images * Quizzes |
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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.
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