Electronics Basics #34 | Emitter Degeneration: Why Choose Less Gain?

Instructor: Dr. Süleyman Burak ÇELİK

Why deliberately reduce an amplifier’s gain? Compare two common-emitter stages with the same separately established collector bias. Model-driven 3D instruments show the same input, inverted outputs, and a smaller base-current change with an unbypassed emitter resistor. We derive the input resistance and voltage gain step by step using the hybrid-pi model, then calculate every intermediate result in a worked example: IC = 1 mA, beta = 100, VT = 26 mV, RC = 2 kohm and RE = 200 ohm. Input resistance becomes 22.8 kohm and unloaded voltage gain is about −8.772. A separate example adds source resistance and output loading. Assumptions: forward-active bias, small signal, midband, infinite transistor output resistance, and omitted bias-network loading. The 3D instruments display calculated signal changes, not electron motion or a complete wiring diagram. Corresponding instruments share a scale across stages; different physical quantities use different scales. Check actual bias, headroom and frequency limits before applying this model to hardware. Chapters: 0:00 Start with the gain-versus-input tradeoff 1:44 The emitter is no longer signal ground 3:17 Follow both currents through the emitter resistor 4:33 Keep the minus sign and the assumptions 6:43 Compare two stages at the same bias current 14:03 Include the source, load and model limits