Substitutions for the 2N5485 JFET

A 2N5485 Junction Field Effect Transistor is used in Lab 12 to introduce readers to the JFET and to build an automatic gain circuit (AGC). Unfortunately, this part is becoming difficult to find. It is no longer stocked at any of the major distributors.  They still appear to be readily available by sellers on Amazon and eBay, however, the devices  we purchased on eBay were defective and unusable. (You can test a device by completing the plot in Lab 12L.4.1 and seeing if you get a graph similar to Figure 12L.17.)

The relevant parameters for the Lab 12 JFET are its I_DSS (drain-to-source current with V_GS=0) and its V_GS(off) (the voltage from gate-to-source required to obtain a drain current, I_D, of near zero). The latter is a negative value in an N-Channel JFET.

These specifications for the 2N5485 are:

A very close replacement is the J210 TO-92 3L ROHS JFET:

The J210 TO-92 3L ROHS sold by Digikey was not in stock when we wrote this but they claim one week lead time. It is also available on eBay. We bought 20 pieces on eBay for less than $10 and they worked fine.

The closest currently available part we could find on Digikey (and at other distributors) is the J113:

It is not quite a perfect match — I_DSS is spec’ed at 1uA not 10nA and the max I_DSS is not specified. We found its I_DSS to be significantly higher than that of the 2N5485 and J210:

Nevertheless, it is only $0.46 and readily available.  However, the higher I_DSS means the part heats up at larger values of V_DS when plotting the VITC curve in Lab 12L.4.1. I_DSS changes as the part heats up (this is implied above in Note 2 for the J210 and Note 5 for the J113) which makes getting a clean plot more difficult.  Be sure to start with low values of V_DS (highest resistance substitution box values) and work up to the highest value (lower resistance values). You might also want to turn the +15V supply off between measurements as you get to lower resistance values. In the plot above, you can see we stopped at a V_DS of about 11V with the J113 because we we getting so much error from device self-heating.

This graph is an updated version of Figure 12L.21 that plots the attenuation of the input vs V_GS for the voltage divider in Figure 12L.20 for the 2N5485, the J210 and, the J113.  All three provide similar levels of attenuation over a similar range of V_GS.  The slight difference between the three is unimportant because the difference amplifier in the feedback loop of Figure 12L.24 adjusts the voltage control input to the Variable Resistor (VR) as necessary to maintain the desired output amplitude.

Our preference is to use the 2N5485 or J210 if you can get it, but the AGC works fine with the J113:

This shows the operation of the Automatic Gain control circuit of Lab 12L.6 using a J113 JFET as the variable resistor.  The yellow trace is the input, a 1kHz sine wave that varies from 10V peak-to-peak down to ~0.5V peak-to-peak over four seconds.  The violet trace is the output of the AGC circuit. It remains at a constant amplitude of about 3V peak-to-peak with only a bit of attenuation as the input gets to its lowest amplitude.