I’ve been on a bit of a feedback kick lately, researching both for projects and to add a page to the website about the topic. Local feedback, in particular, has piqued my curiosity (which is usually fixated on triodes and open loop circuits).
A tube’s grid is an inverting input (with output taken at the anode). This makes it a natural point to loop feedback from the anode. TubeCAD covers exactly this topic in-depth in Partial Feedback. But, my favorite variation on this type of feedback uses a P-channel FET in the configuration shown above. The FET does several nifty things here:
- Provides low impedance fixed bias to the tube grid via the source, allowing for A2
- Provides high impedance input for the preceding stage
- Defines the feedback/gain in the circuit with source resistors R2 and R3
DiyAudio alias SpreadSpectrum provides details on a push-pull build using this circuit on his blog here.