JFET amp with current-source load
6 parts
A JFET is depletion-mode — it conducts hard at zero gate bias — so it can bias itself: the input source DC-ties the gate to 0 V while the 470 Ω source resistor lifts the source to +1.73 V, settling at 3.69 mA (measured). Upstairs, the P-JFET plays the same trick off the 12 V rail — source at a mirror-image 10.27 V, gate tied to the far end of its own source resistor — which turns it into a current source that loads the amplifier with pure output resistance instead of a drain resistor. Identical square-law device models with identical self-bias carry identical currents, so the output balances at a measured 5.997 V, almost exactly mid-rail, and the gain is −392 — a hundred times the resistor-loaded common-emitter example. That is why active loads exist.
- V1 2mV·1kHz
- J1 Vp 2V
- R1 470Ω
- JP1 Vp 2V
- R2 470Ω
- V2 12V
