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Home Transistors and Their Operation PointContact Transistors Current, Resistance, Voltage, and Power Gains  
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Current, Resistance, Voltage, and Power GainsAuthor: Leonard Krugman In the average pointcontact transistor, an increase in emitter current of one milliampere will cause an increase in collector current of 2.5 milliamperes. In physical terms, this indicates that one million holes injected by the emitter causes 2.5 million electrons to be injected by the collector. One million of the collector electrons neutralize the holes. The remaining million and a half electrons flow to the base. The ratio of change in collector current to change in emitter current is called the current gain α (Alpha). Thus α = I_{c}/I_{e} where α = current amplification, i_{e} = change in emitter current, and i_{c} = resulting change in collector current. In the typical case described above, α = 2.5 mA/1.0 mA = 2.5
Since the input voltage is the product of the emitter current and the input resistance, and the output voltage is the product of the collector current and the output resistance, the transistor voltage gain equals the current gain times the resistance gain. where:
For the typical case under consideration, the voltage gain equals 2.5 × 67 = 167.5. Furthermore, since the input power is the product of the input voltage and the emitter current, and the output power is the product of the output voltage and collector current, the transistor power gain equals the current gain squared times the resistance gain. For the typical transistor, the power gain equals (2.5)^{2} (67) = 419.


Home Transistors and Their Operation PointContact Transistors Current, Resistance, Voltage, and Power Gains 