Electronics Basics #38 | Common Base: Why Is the Gain Positive?

Instructor: Dr. Süleyman Burak ÇELİK

Why does the collector rise when the emitter rises? Keep the base at AC ground and follow the signs: the base-emitter voltage decreases, collector current decreases, the resistor drop decreases and the collector voltage rises. Derive Rin=rpi/(beta+1), Av=+gm RC and Rout=RC from the common-base hybrid-pi model. Based on original TR lesson 53. Numerical values are a supplemental teaching example, not original source givens: IEQ=1.01 mA, beta=100, VT=25 mV and RC=2 kohm give ICQ=1 mA, gm=40 mS, rpi=2500 ohm, Rin about 24.752 ohm and stage gain +80 V/V. An ideal +0.25 mV emitter signal produces a +20 mV output. With Rs=50 ohm, source loading reduces source-to-output gain to about +26.490 V/V and output amplitude to about 6.623 mV. Model-driven 3D instruments distinguish emitter voltage, base-emitter voltage, incremental collector current and output voltage. Input voltages, current and output voltage have explicitly different fixed scales. They show small AC changes, not electron trajectories or full DC voltages. A negative current increment reduces the positive DC current; it does not reverse the full collector current. Assumptions: a settled forward-active bias point, small signals, AC-grounded base, infinite transistor output resistance, negligible capacitance and no external output load. Signal ground does not mean zero DC bias. Distinguish stage gain from source gain and voltage gain from current gain: alpha=beta/(beta+1) is below one. The closing experiment answers the opening question. Chapters: 0:00 Raise the emitter: what happens? 1:24 Choose the port and keep the signs 2:45 Derive the three stage properties 4:52 A complete numerical check 6:59 Include the source and return to the question