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.
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.
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.
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.
| Related Blog |
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| 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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