Re: 6DJ8 substitute idea - reverse engineering
A FET is in saturation (the happy region for amplification) if VDS exceeds (VGS - VT).toggle quoted messageShow quoted text
The standard fetron connection is a pseudocascode, where the upper (cascoding) device has its gate connected to the source terminal of the bottom transistor. That connection makes the bottom transistor have a VDS equal to VGS. That automatically guarantees operation of the bottom transistor in saturation if the transistors are reasonably matched depletion-mode devices, without the need for a separate positive bias. The fetron really is designed to drop into the same socket as the vacuum tube it replaces, without requiring rewiring.
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford, CA 94305-4070
On 6/23/2021 13:48, Dave Wise wrote:
You need positive bias anyway, unless the bottom device in the cascode can operate with just a volt or two across it.