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Lecture18-Understanding+BJT+Circuits

# Lecture18-Understanding+BJT+Circuits - ECE 3040...

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ECE 3040 - Dr. Ying Zhang Georgia Tech ECE 3040: Microelectronic Circuits Lecture 18 Reading: Notes

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ECE 3040 - Dr. Ying Zhang Georgia Tech Recapture: Single Transistor Amplifier Analysis 1.) Determine DC operating point and calculate small signal parameters (see next page) 2.) Convert to the AC only model. DC Voltage sources are shorts to ground DC Current sources are open circuits Large capacitors are short circuits Large inductors are open circuits 3.) Use a Thevenin circuit (sometimes a Norton) where necessary. Ideally the base should be a single resistor + a single source. Do not confuse this with the DC Thevenin you did in step 1. 4.) Replace transistor with small signal model 5.) Simplify the circuit as much as necessary. Steps to Analyze a Transistor Amplifier Step 2 Step 3 Step 4 Step 5 Step 1 Important! 6.) Calculate the small signal parameters (r π , g m , r o etc…) and then gains etc…
ECE 3040 - Dr. Ying Zhang Georgia Tech ± BJT Base Charging Capacitance ± BJT Depletion Capacitance ± Complete BJT Small-Signal Model ± Understanding BJT Circuit Agenda

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ECE 3040 - Dr. Ying Zhang Georgia Tech BJT Base Charging Capacitance (Diffusion Capacitance) + = = 0 0 ' 1 1 ' ' dx e e n qA dx e e p qA Q dv dQ C n T D p T D L x V v po L x V v no D D D Diffusion Recall for a diode: Neglect charge injected from the base into the emitter due to p+ emitter in pnp Excess charge stored is due almost entirely to the charge injected from the emitter. In active mode when the emitter-base is forward biased, the capacitance of the emitter-base junction is dominated by the diffusion capacitance (not depletion capacitance). Sum up all the minority carrier charges on either side of the junction
ECE 3040 - Dr. Ying Zhang Georgia Tech The BJT acts like a very efficient “siphon”: As majority carriers from the emitter are injected into the base and become “excess minority carriers”, the Collector “siphons them” out of the base. We can view the collector current as the amount of excess charge in the base collected by the collector per unit time. Thus, we can express the charge due to the excess hole concentration in the base as: F C B i Q τ = or the excess charge in the base depends on the magnitude of current flowing and the “forward” base transport time, τ F , the average time the carriers spend in the base.

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Lecture18-Understanding+BJT+Circuits - ECE 3040...

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