If I put my notes here, I might be able to find them again later!
Matching JFETs
with only two resistors, a 9 V battery, and a voltmeter...
The current-voltage relationship for a jfet device is approximately a quadratic expression defined by just two parameters, the saturation current, I_dss, and the pinch-off voltage, which I'll call V_gs0.
I = I_dss (1-V/V_gs0)^2
In principle, therefore, to characterize the device all we need is two data points (I1, V1) and (I2, V2) to solve the expression above for I_dss and V_gs0. We don't need to measure I_dss or V_gs0 directly.
All you need to do is connect the jfet device-under-test (DUT) as shown, and measure the voltages across two different source resistances. That's it. The excel worksheet computes the I_dss and V_gs0 values for you (or you can do it by hand, the formulas are provided.)
The math is a bit messy, but if you can solve a quadratic expression it's easy enough.
*****
Note: I found it was important to include a small bypass capacitor (0.1 uF ceramic at least) near the jfet or else the system oscillated, giving unstable and incorrect voltage measurements.
Note: If the jfet transfer curve conforms to the mathematical expression, then the I_dss and V_gs0 values obtained with this method will be the same as the measured values. If it does not, if the current V1/R1 and V2/R2 are close to the source current you plan to run the jfet at, then the interpolation method will actually yeild a more accurate representation of the jfet behavior at the operating point, even though the I_dss and V_gs0 values are not meaningful and do not model the device at other operating points.
The current-voltage relationship for a jfet device is approximately a quadratic expression defined by just two parameters, the saturation current, I_dss, and the pinch-off voltage, which I'll call V_gs0.
I = I_dss (1-V/V_gs0)^2
In principle, therefore, to characterize the device all we need is two data points (I1, V1) and (I2, V2) to solve the expression above for I_dss and V_gs0. We don't need to measure I_dss or V_gs0 directly.
All you need to do is connect the jfet device-under-test (DUT) as shown, and measure the voltages across two different source resistances. That's it. The excel worksheet computes the I_dss and V_gs0 values for you (or you can do it by hand, the formulas are provided.)
The math is a bit messy, but if you can solve a quadratic expression it's easy enough.
*****
Note: I found it was important to include a small bypass capacitor (0.1 uF ceramic at least) near the jfet or else the system oscillated, giving unstable and incorrect voltage measurements.
Note: If the jfet transfer curve conforms to the mathematical expression, then the I_dss and V_gs0 values obtained with this method will be the same as the measured values. If it does not, if the current V1/R1 and V2/R2 are close to the source current you plan to run the jfet at, then the interpolation method will actually yeild a more accurate representation of the jfet behavior at the operating point, even though the I_dss and V_gs0 values are not meaningful and do not model the device at other operating points.
Total Comments 6
Comments
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im not sure what you mean by percision is more important than accuracy?Posted 24th March 2016 at 06:00 AM by Dadbeh
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Precision means how much deviation you find in a series of measurements on the same device. Accuracy is how close the average of those value is to the true value of the parameter being measured.
So while this technique may not give the most accurate value of the saturation current (compared to direct measurement), if the result for device A gives the same result as for device B then you can nevertheless be confident that the two devices are well-matched.Posted 24th March 2016 at 08:50 AM by rjm
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Hi, I am a bit confused by your table, on the left, under constants, you have R1,R2,R_Source, all of different values. In your previous blog entry, you provide a spreadsheet where 2 resistor are used to calculate vgs_0 and idss.
first question:
How does R_source fit in the picture???
second question:
can we modify the provided spreadsheet to include the calculation of g_m?Posted 4th April 2016 at 12:33 PM by BaffleChallenged
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Thanks for your input. I've uploaded a revised spreadsheet that hopefully clarifies some things. I've added an extra line where can now calculate the transconductance at a given current.
The table in the supermatched jfets post uses the same calculation as the worksheet posted here, but then goes an extra step and says "ok, now for the R_source used in the actual circuit where you are going to use this jfet, what is the transconductance?" That's a different value (usually) from the test values R_source1 and R_source2 used here.Posted 4th April 2016 at 11:58 PM by rjm
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Thank you very much!
I am sure it will be useful in my next project. I will try to build an amplifier without feedback. I am still waiting for parts, but I will try to measure the component I have on hand using your methodology.
Again, thank you for sharing with the DIY community.Posted 5th April 2016 at 12:22 AM by BaffleChallenged
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Please use ver. 1.2. I was an idiot and forgot to divide by V_gs0 for the transconductance in 1.1.Posted 5th April 2016 at 01:57 AM by rjm
