Electronics Basics #37 | Emitter Bypass: Same Bias, Higher Gain?

Instructor: Dr. Süleyman Burak ÇELİK

Why can a capacitor make a BJT amplifier output larger without changing its settled DC bias? Follow the emitter voltage and the two AC current paths. Start with the physical circuit, derive the exact unbypassed hybrid-pi input branch, then compare an explicitly ideal bypass with a finite capacitor. Based on both pages of original TR lesson 52. All numerical examples are supplemental: VCC=12 V, R1=82 kohm, R2=22 kohm, RC=2 kohm, RE=1 kohm, beta=100, VBEQ=0.7 V and VT=26 mV. The consistent bias solution gives ICQ about 1.553 mA and VCEQ about 7.324 V. Without bypass, the transistor input branch is about 102.674 kohm, total input resistance about 14.839 kohm and stage gain about -1.948 V/V. The ideal bypass reduces input resistance to about 1.526 kohm and gives stage gain about -119.497 V/V. Include Rs=1 kohm when comparing the source-to-output response. With finite CE=470 microfarads at 1 kHz, source-gain magnitude is about 72.191 V/V, with a small phase departure from exact inversion. Model-driven 3D instruments show ie=iRE+iCE and opposite capacitor AC charge changes; current and charge have separate labelled scales. These are explanatory instruments, not electron trajectories. Assumptions: established forward-active DC bias, small signals, infinite transistor output resistance, idealized input/output coupling, no external output load and no internal transistor capacitances. The capacitor is open at steady DC, not an AC short at every frequency. This is not a complete amplifier bandwidth prediction. The closing experiment answers the opening question and explains the trade-off between gain and source loading. Chapters: 0:00 One capacitor, a different response 1:22 Let the emitter move 3:25 A path for the changing current 4:45 Compare the same operating point 7:38 Include the source and the frequency