semiconductor curve tracer - Page 1
Forum section: electronics / projects

author: slow_rider
author_rank: Regular Contributor
timestamp: December 15, 2013, 08:32:33 pm
content: I'm starting to work on a MCU based device to test transistor, FETs and diodes and plot their I/V curves. My intention is to use integrated solution as much as possible and retain high accuracy as much as possible.The 1st thing I want to design (or find a adequate circuit) to drive a transistor's base and FET's gate. It's tempting to use a DAC into an op-amp stage and through a resistor to the transistor's base and sample the voltage drop across the resistor though a very high impedance op-amp. The problem here might be a 10K - 100K resistor I will be using. That might drift over temp and which will hurt the accuracy of the reading. A better solution might be using a current source / sink and a very small resistor for sampling but I could not find a dual polarity current source that can output as little as 1uA in a manner I control accurately using "sane" digital means.Suggestions are welcomed!

author: Paul Price
author_rank: Super Contributor
timestamp: December 16, 2013, 03:57:41 am
content: You need to create several different constant current source for biasing bipolar transistors and and an adjustable constant voltage source for biasing FETs.You can create switchable constant current sources by building a power supply that connects to a resistor on the emitter of a PNP transistor. Connect a LED from the +voltage source to the base of this transistor. Use a high-valued resistor connected to the collector of a NPN transistor to the base of the PNP transistor so that you can turn on/off with your control circuitry. The collector of the PNP transistor is your constant-current output bias. Be sure to use a zerer diode or two LED's in series to limit the base-emiter voltage to <4Volts to prevent damage to the Base-Emitter of the D.U.T. By adjusting the emitter resistor value on the PNP transistor you can create a fairly good constant current source. You can in this way create 1uA, 10ua...100mA current sources. The higher constant-current switchable sources may require heat sinking.You can just use a D2A converter feeding a buffer op-amp to create bias for FET testing.You can use a DAC to

author: calexanian
author_rank: Super Contributor
timestamp: December 16, 2013, 05:36:11 am
content: Are you doing your tests at low frequency steps? Like old style curve tracers? Most of them would have selectable drive ranges of both voltage and impedance in a switchable fashion. Fets being high impedance are adjusted by the voltage range regardless of impedance at low freq and DC. In the RF range the gate capacitance has to be compensated for.

author: slow_rider
author_rank: Regular Contributor
timestamp: December 16, 2013, 06:10:46 pm
content: Paul Price - I'm on the edge of my seat here!!! You did not finish the last sentence. I would very much like to use a DAC + very simple analog circuitry (like a buffer) to eliminate the need for, well, additional analog circuitry.Do you have some info on the rage of impedances and can these be applied to CCS designs? This won't be an RF device, max 20 or 100KHz.