S4-1.2 JFET Biasing & Amplifiers

Work through these

• Fixed bias, self-bias and voltage-divider bias for JFETs

  The same three biasing ideas from the BJT unit, redone for a device controlled by voltage rather than current. Self-bias is the FET workhorse, and the reason biasing is harder here is that the same part number varies widely from one chip to the next.

• FET biasing for zero current drift

  There is a specific bias point at which a JFET's drain current barely moves as temperature changes, and finding it is a standard piece of design arithmetic. It is a favourite examination question and also the item that shows why temperature stability is a real engineering worry.

• CS, CD, CG amplifiers via small-signal model

  Common source, common drain and common gate are the FET counterparts of the three BJT arrangements, analysed with a small-signal model that is lighter than h-parameters. Lean on the parallel and most of the work is already done.

• Compare the three FET configurations

  Setting the three FET arrangements side by side on gain, input impedance and output impedance. The comparison is what you actually carry forward, since choosing a configuration is a routine decision and re-deriving each one is not.

• Frequency response of FET amplifiers

  As with the BJT, gain falls away at both ends of the frequency range, and the causes are the coupling network at the low end and device capacitance at the high end. Worth doing carefully here because the same reasoning returns in every amplifier you meet.

• Lab: simulate a CS amplifier and verify gain against hand analysis

  Simulate a common-source amplifier and check the measured gain against what your small-signal analysis predicted. The gap between the two is where the assumptions in the model become visible.