ElectronicsI_L25 - Lecture 25 Graphical Analysis • Much...

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Unformatted text preview: Lecture 25 Graphical Analysis • Much like the MOSFET amplifiers, the transconductance amounts to the slope of the collector current curve around the DC bias values • The BJT’s have a much higher transconductance than MOSFET’s • This is because we are taking a slope off a diode curve Input Resistance • If we want to determine the resistance seen by v be we need to first evaluate the total base current: • The signal component of this is: • If we substitute for the transconductance: • The small signal input resistance between the base and emitter is denoted r π • Finally, replacing g m and using beta to substitute for the collector current: be T C C C B v V I I i i β β β 1 + = = be T C b v V I i β 1 = be m b v g i β = m b be g i v r β π = ≡ B T I V r = π Input Resistance at the Emitter • Sometimes our input signal will enter the emitter terminal • Thus we need to determine the emitter resistance for a small signal input • The total emitter current is: • The emitter signal current is:...
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ElectronicsI_L25 - Lecture 25 Graphical Analysis • Much...

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