Thursday, 20 October 2016

All about JFETS - matching for synthesizers

This is a very good introductory video on JFETs - junction field effect transistors.

JFETs can be either N or P channel.
The channel conducts current moving from the source to the drain.
A voltage at the gate increases the channel resistance and reduces the drain source current.
Therefore, the FET can be used as an amplifier or a switch.

This is an excellent device.....Peak Atlas DCA Pro.
The bits about JFETS are around 7mins & the PC applications are at 14:20.


We often want to use a JFET as a variable resistor, particularly in phasers.
The problem with JFETs is that it is much harder to make consistent JFETs than to make consistent bipolar devices. Therefore matching them is important.

 These are screen shots taken of the readouts from my Atlas Pro of one of the JFETS.

 Try to match the curves for each JFET along with as many other specs as possible.

I tested twelve J112s.
measuring VGSoff, Idss, VGSon in that order.
 These were the results:

1. Vsg (off) = -2.97V
    Idss = 0.51v
    Vsg (on) = -2.04V


2. Vsg (off) = -3.14V
    Idss = 0.50V
    Vsg (on) = -2.04V

3. Vsg (off) = -2.98V
    Idss = 0.51v
    Vsg (on) = -2.05V

4. Vsg (off) = -2.71V
    Idss = 0.54v
    Vsg (on) = -1.80V

5. Vsg (off) = -2.99V
    Idss = 0.51v
    Vsg (on) = -2.06V

6. Vsg (off) = -2.68V
    Idss = 0.55v
    Vsg (on) = -1.77

7. Vsg (off) = -3.60
    Idss = 0.46v
    Vsg (on) = -2.63

8. Vsg (off) = -2.90
    Idss = 0.53v
    Vsg (on) = -1.98

9. Vsg (off) = -2.66
    Idss = 0.55v
    Vsg (on) = -1.76

10. Vsg (off) = -2.89
     Idss = 0.53v
     Vsg (on) = -1.97V

11. Vsg (off) = -3.17
     Idss = 0.50v
     Vsg (on) = -2.24

12. Vsg (off) = -3.15
     Idss = 0.50v
     Vsg (on) = -2.22V

A huge variation in results from just twelve J112s
I'll go with 1 & 3

1. Vsg (off) = -2.97V
    Idss = 0.51v
    Vsg (on) = -2.04V

3. Vsg (off) = -2.98V
    Idss = 0.51v
    Vsg (on) = -2.05V

Links:
1. Stompville
2. Learning about electronics
3. Geofex
4. Edn.com

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