E Gadgets 7: JFETs in Real World
Last Updated on January 7, 2022
First Published on January 6, 2022
I've introduced JFETs, BF256B, 2SK209-BL, and J112, for my impedance transformer. And I've also introduced a circuit which put a diode in parallel with Gate to Source of JFET for bass guitars. In this post, I'd like to describe more detail about each JFET and the circuit.
This JFET is manufactured by Fairchild (Now "onsemi"). To know its characteristics, you should also refer the datasheet for BF256A. In my examination, I found characteristics of BF256B is unique because its drain current is cut by positive voltage, i.e., it's like P-channel (but it's called as N-channel). "Figure 2. Correlation Between -VGS(off) and IDSS" of the datasheet of BF256A (BF256A/D. September, 2001 - Rev. 3) seems to show variants of BF256 series, therefore the difference between BF256A and BF256B seems to be drain current on 0V at Gate to Source. However, in the datasheet of BF256B, Gate is placed on the opposite side (No. 1) of the side (No. 3) in pinout of ordinal N-channel JFETs with TO-92-3 (TO-92) package. I examined BF256B, and I concluded Gate of BF256B seems to be No. 3 as well as ordinal N-channel JFETs. When I tested with the "correct" pinout, the unique characteristics remained. I used BF256B as a first JFET in my projects, and I started to search other JFETs whose drain current are at least 6mA as much as the first one. BF256B is for VHF and UHF amplifiers, and its useful to make equipment for amateur radio. Besides, I think BF256B would be used as a 180 degrees phase inverter in my impedance transformer.
This JFET is manufactured by TOSHIBA. This has enough transconductance/transadmittance to hold power of signal on the output of an impedance transformer. Although it's available with TO-236 (similar to SOT-23) packages in Japan, the sound is nicely powerful like in my demonstration.
This JFET is manufactured by Fairchild (Now "onsemi"). The drain current is 5mA at minimum, but I think it's the real one in the world. It's useful to be implemented in a plug of a guitar cable because this JFET has not only SOT-23 package, but also TO-92 package.
As I applied in the opto compressor, I put a diode in parallel with Gate to Source of JFET for bass guitars. In Japan, this idea was filed as a utility model for microphones (実全昭50-146533). This idea was also filed as a patent of a semiconductor device (特開昭61-160963). Basically, JFET used to have biasing with negative voltage on Gate to Source. The first stage of transistor guitar amps and boost pedals which are known as "bright" high input impedance is typically JFET with biasing. I think this method comes from preventing a grid leak of a tube. The phenomenon occurs on positive grid voltage, making further grid leaks. Whereas, JFET doesn't have a leak like a tube. Gate to Source of JFET is just a diode. When the gate-source voltage surpasses around 0.6 volts, current flow occurs to clip surpassed voltage, but it's reversal of a leak of a tube. Beside, when the gate-source voltage falls below -0.6 volts, the additional diode in this idea starts current flow to hold voltage. Fig. 6 (第6図) of 特開昭61-160963 describes an application without biasing.
* パイオニア株式会社. マイクロホン装置. 実全昭50-146533. 1975-12-04.
* 三洋電機株式会社. コンデンサマイク用半導体装置. 特開昭61-160963. 1986-07-21.