Electronics Basics #43 | Gain Below One: Why Use a MOSFET Source Follower?

Instructor: Dr. Süleyman Burak ÇELİK

Why add a transistor stage whose voltage gain is below one? Compare the same signal source and load with and without a source follower, then work backward through its bias, feedback and port resistances. Covers original TR lesson 59. The supplemental depletion-MOSFET example uses VDD=12 V, VT=-2 V, k=1 mA/V² including one half, RS=1 kohm, RG=1 Mohm and ro=50 kohm. The valid operating point is ID=1 mA, VS=1 V, VGS=-1 V and gm=2 mS. The drain is at AC ground; the source is the output and is not bypassed. Keeping finite ro, unloaded gain is gm/(gm+1/RS+1/ro), about +0.662. Input resistance is RG and output resistance is about 331.1 ohms. A +1 mV gate signal gives +0.662 mV at the source, leaving about +0.338 mV between gate and source. With external signal-source resistance 100 kohm and AC-coupled load 1 kohm, the input divider and output loading both matter. A 1 mV generator delivers about 0.452 mV through the follower, compared with about 0.0099 mV through the direct connection. The supply provides energy: a useful buffer does not need voltage gain above one. The 3D instruments share a ±1 mV scale and one calculated model. Spoken-result cues control the formulas and numerical readouts. Assumptions: midband, small signals, saturation bias, ideal coupling, finite ro, negligible gate current, no body effect or internal capacitance. The closing load comparison answers the opening question. Chapters: 0:00 A useful amplifier with gain below one 1:19 The output follows through feedback 3:36 High input, low output resistance 5:04 Compare the same source and load 7:17 What the follower can and cannot do