ElectronicsI_L24 - Lecture 24 Stabilizing the Collector...

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Lecture 24
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Stabilizing the Collector Current If we can prove that the emitter current will be stable, then we also can show that the collector current will be too The two currents differ from each other in active mode by very little. The first requirement essentially makes variations in the emitter base voltage have little effect (this is for Temp. variations) The second reduces the dependency of the emitter current on Beta. Note that both these requirements can and do conflict with other requirements for an amplifier, so as always compromise! BE BB V V >> 1 + >> β B E R R
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Example 1 • We want to choose the proper resistors such that we will have an emitter current of 1mA from a power supply of +12V • The nominal beta for the transistor is 100 • We have a few rules of thumb we can use: –V BB =1/3 V CC –I C R C =1/3 V CC • These rules will give us a possible voltage swing at the collector of about 1/3 of the supply voltage
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• Using these rules of thumb, we have a base voltage of 4V • If the BJT is in active mode, then the emitter voltage is: • We can get the emitter resistor now: • Another design rule is that we want the voltage divider network to have a current of I E to 0.1I E flowing through it. • Neglecting the base current, we have
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This note was uploaded on 02/13/2012 for the course PHYSICS 16.365 taught by Professor Staff during the Summer '10 term at UMass Lowell.

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ElectronicsI_L24 - Lecture 24 Stabilizing the Collector...

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