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Member Blogs Measure JFET drain current with a current mirror - part 2: build and measure
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  • Author Author: Jan Cumps
  • Date Created: 28 Jun 2020 9:24 PM Date Created
  • Views 489 views
  • Likes 7 likes
  • Comments 2 comments
  • jfet
  • analog
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Measure JFET drain current with a current mirror - part 2: build and measure

Jan Cumps
Jan Cumps
28 Jun 2020

I learned a trick from w2aew: how to measure current if you don't have a current probe.

He's using a N-channel JFET as example. I adapted it to a P-channel type - the J176.

image

This second post is the practical part. No talk and simulation. A real circuit real measurements.

Read Measure JFET drain current with a current mirror for schematics and approach.

 

Building the Circuit

 

This is extremely easy. Only four components are needed.

image

The  NPN transistors in the current mirror are BC547B. Any common NPN will do.

The JFET is a P176. and the resistor in my case are actually 2. I didn't have a 20R, so I put 2 11Rs in series.

 

image

 

The power supply is a negative triangle waveform generated by my function generator (approx 4 V p/p).

The difficulty here is to get a negative 0.6 V DC offset from the top of the wave. That requires a bit of fiddling with my generator, with the signal amplitude and offset dials.

The offset is required to jump the diode drop gap created by the current mirror used to measure the JFET's drain current.

The gate-source voltage is easier. I just used a DC power supply and dialed the knob to the required levels.

 

The oscilloscope needs to be set into x - y mode. Because with a P-channel device the world is upside-down, I inverted both oscilloscope channels to get a graph similar to what you find in databooks.

The results can be seen in the video below. The animation at the top of this post is also made with real captures of the circuit.

 

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Related Blog
Measure JFET drain current with a current mirror
Measure JFET drain current with a current mirror - part 2: build and measure
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Top Comments

  • michaelkellett
    michaelkellett over 3 years ago +2
    Hello Jan, Nice to see some basic analogue stuff. NXP do some nice dual transistors that work well in this kind of application (not very breadboard friendly). PMP5201V.115 - Farnell 1524154 - no one seems…
  • jc2048
    jc2048 over 3 years ago in reply to michaelkellett +2
    The gain matching looks good on those NXP parts. On Semi have this NST65010 NST65010 , which is similar on Vbe matching (within 2mV), but the gain matching is nowhere near as good at only 10%. Fiddly little…
  • jc2048
    jc2048 over 3 years ago in reply to michaelkellett

    The gain matching looks good on those NXP parts.

     

    On Semi have this    NST65010NST65010 , which is similar on Vbe matching (within 2mV), but the gain matching is nowhere near as good at only 10%.

     

    Fiddly little package, too. From the picture it looked quite manageable until I realised that the longest dimension was only 2mm.

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  • michaelkellett
    michaelkellett over 3 years ago

    Hello Jan,

    Nice to see some basic analogue stuff.

     

    NXP do some nice dual transistors that work well in this kind of application (not very breadboard friendly).

    PMP5201V.115 - Farnell 1524154 - no one seems to make dual transistors with proper leads any more.

     

    MK

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