Electronics Basics #42 | Zero Gate Current, Half the Signal? MOSFET Input Divider

Instructor: Dr. Süleyman Burak ÇELİK

The MOSFET gate takes practically no current. Why can half the signal disappear before it reaches that gate? Start with the source, input network and output, then follow the bias and small-signal calculations. This lesson covers original TR lesson 58. A supplemental enhancement-MOSFET example uses VDD=12 V, R1=900 kohm, R2=300 kohm, RS=1 kohm, RD=4.7 kohm, VT=1 V and k=1 mA/V², including the one-half coefficient. The valid operating point has VG=3 V, ID=1 mA, VS=1 V, VGS=2 V, VDS=6.3 V and gm=2 mS. The other quadratic root is outside the assumed conducting branch. In the midband ideal-bypass model, Rin=R1 parallel R2=225 kohm, Rout=RD=4.7 kohm and gate-to-output gain is -9.4. An ideal signal source gives 1 mV at the gate and -9.4 mV at the drain. Add 225 kohm signal-source resistance: the gate receives 0.5 mV and the output becomes -4.7 mV. The bias resistors draw input-network signal current even when gate current is zero. Signal-source resistance is not the transistor source resistor. The two language-independent 3D experiments use one calculated small-signal model, a common input-voltage scale and a separately declared output scale. The ideal AC-grounded source still has its 1 V DC bias. Assumptions: saturation, small signals, midband, ideal bypass and coupling, infinite ro, no body effect or external output load, negligible gate leakage and internal capacitance. The closing comparison answers the opening question; the source-to-output gain and stage gain are different quantities. Chapters: 0:00 Follow the signal before the transistor 1:18 Establish a valid DC operating point 3:20 Find the input and output resistances 5:26 Separate two different gains 6:54 Compare the same amplifier with two sources 8:19 Answer the question and state the limits