Electronics Basics #35 | Two Input Paths: What Does the Signal Source Drive?

Instructor: Dr. Süleyman Burak ÇELİK

Your source current is not all transistor base current. In an emitter-biased amplifier, one path goes through the bias resistor and the other enters the transistor. Start with that split, establish the DC operating point, then derive the parallel input resistance and the voltage gain seen by the source. Based on original TR lesson 50. A clearly labelled supplemental example uses VCC=12 V, RB=470 kohm, RC=2 kohm, RE=1 kohm, beta=100, VBE=0.7 V and VT=26 mV. The DC collector current is about 1.979 mA and VCE about 6.043 V. Total input resistance is about 84.023 kohm; unloaded stage gain is about −1.955 V/V. Adding Rs=10 kohm and RL=10 kohm gives source-to-output gain about −1.456 V/V. Model-driven 3D instruments use one shared ampere scale, so their signed displacements satisfy iin=iRB+ib. Numerical calculations hold the matching positive signal peak. These are slowed small-signal instruments, not electron motion or a physical wiring diagram. Assumptions: forward active, midband, infinite ro, unbypassed emitter resistor, and AC coupling that preserves DC bias. Check bias, headroom and frequency limits before using the model in hardware. Chapters: 0:00 What does the signal source actually drive? 1:16 Separate the bias circuit from the signal model 2:32 Two paths, one input voltage 4:26 Define the gain before using a formula 5:46 A complete supplemental example 11:43 Add the source and load, then check the limits