Electronics Basics #36 | Three Input Paths and Finite Transistor Output Resistance

Instructor: Dr. Süleyman Burak ÇELİK

What loads your signal source? Identify the input and output ports before combining resistors. In a grounded-emitter, voltage-divider-biased BJT, R1, R2 and r-pi are three parallel input paths. At the collector, RC and finite transistor output resistance ro are in parallel. Only base current controls the beta-times-current source. Based on original TR lesson 51. A labelled supplemental example uses ICQ=1 mA, beta=100, VT=25 mV, R1=30 kohm, R2=15 kohm, RC=1 kohm and ro=50 kohm. It gives gm=40 mS, r-pi=2.5 kohm, total input resistance 2 kohm, output resistance about 980.392 ohm and stage gain about -39.216 V/V. With Rs=1 kohm, source gain is about -26.144 V/V; a 1 mV source change gives about 0.667 mV at the base and -26.144 mV at the collector. Four model-driven 3D instruments share one current scale, so iin=i1+i2+ib. Numerical calculations hold the matching positive signal peak. These are slowed small-signal instruments, not electron motion or a physical wiring diagram. Assumptions: established forward-active DC bias, grounded emitter, midband, finite ro, unilateral hybrid-pi model, negligible internal capacitances and no external output load. Practical input coupling must preserve the DC bias. Small-signal changes are not the full node voltages. Chapters: 0:00 What loads your signal? 0:41 Separate DC bias from the AC model 2:02 One input node, three paths 3:08 Follow the collector current 4:35 Build a numerical example 7:55 Reconnect the signal source