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 Transistor curve tracer 
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1047
Location: Lacey, WA USA
I built one long ago with a 555 timer, 4017 decade counter, Op Amps, and a few other parts.

Should be simpler with a Teensy 3.x. Let's do it!

Image

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Steve Greenfield AE7HD


Mon Oct 24, 2016 4:17 pm
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1047
Location: Lacey, WA USA
So... assuming a maximum output of 100mA with 10 levels, and 3 scales (full scale 1mA, 10mA, and 100mA), and a beta that can be anything from 10 to 300.

The maximum base current required is 100mA/10 = 10mA.
The minimum base current required is 100uA/300 = 330nA.

Quite a range. The Teensy 3.1 and up have a 12 bit DAC built-in. That is only 4096 levels. So I'm thinking MOSFET switching of resistors.

Calibrate the voltage from the DAC to base current by putting 1V on the base resistor, measuring the base voltage, subtract, calculate.

Or modify a current transmitter IC to the purpose.

See "span adjust" on pin 8, 9, and 10:
http://www.ti.com/lit/ds/symlink/xtr110.pdf Rather expensive at $16 each

See Rset on pin 7:
http://www.ti.com/lit/ds/symlink/xtr111.pdf Rather cheap at $2 each

This circuit is bidirectional, which would help in making this work for PNP also, and for JFETs. The voltage source will also need to be bidirectional, then.
Image Iout =Vin/R6
http://www.next.gr/circuits/VOLTAGE-TO-CURRENT-CONVERTER-l51228.html

A much simpler voltage to current source/sink:
Image
http://www.seekic.com/circuit_diagram/Power_Supply_Circuit/VOLTAGE_PROGRAMMABLE_CURRENT_SOURCE.html

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Steve Greenfield AE7HD


Tue Oct 25, 2016 9:36 am
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1047
Location: Lacey, WA USA
To handle the other quadrants requiring negative voltages, this circuit works as a non-inverting amp when the switch is open, and an inverting amplifier when the switch is closed:
Attachment:
File comment: S1 Open, Av = 1
S1 Closed, Av = -1

20161025_112837crop.jpg
20161025_112837crop.jpg [ 134.38 KiB | Viewed 281 times ]


If this were just displayed on an oscilloscope screen, this would not be necessary.

Of course, another possibility is to level shift the analog inputs of the Teensy so that 1/2 of Vcc is 0V on the component. The Teensy 3.x has ADCs that are 16 bit, 13bit ENOB. So I might go that way.

It might take more than 12 bits of DAC to get an output fine enough to adjust base current over a wide enough range.

Then it either sets the base current at the desired amounts for each line of the curve set, or measures the collector current and adjusts the base current to get collector current at the desired amounts.

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Steve Greenfield AE7HD


Tue Oct 25, 2016 2:08 pm
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1047
Location: Lacey, WA USA
The Teensy Audio Library could be of great help with this, simple to created a sawtooth/ramp waveform.

https://www.pjrc.com/teensy/td_libs_Audio.html

The 12 bit ADC and DAC will be of great use for this, too.

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Steve Greenfield AE7HD


Wed Jun 07, 2017 9:25 am
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1047
Location: Lacey, WA USA
For a very simple method to drive a constant current, a relatively high voltage and a resistor will act as a constant current source into the BE junction. Then the current can be varied by changing the voltage.

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Steve Greenfield AE7HD


Thu Jul 20, 2017 2:06 pm
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