Intro: Build a LapTop CurveTracer
Do it with RRIO quad and a binary counter.
Step 1: Making a CurveTracer
What does it take to do a curve tracer function in spice.
First there is a need for a sweeping voltage which is applied to a transistor's collector.
And a staircase current needsto be applied to the base.
In spice this is done with a dc sweep card.
And X/Y plot can bedone using the current of VCE vs VCE voltage.
So what does it take to do it all on a laptop?
First off, one needs a power supply. A 5V supply is available off the USB port.
Second, two scope inputs are needed.
A typical laptop has a stereo audio input port which can be used for this.
Third, a way to connect at least three wires are needed.
In this case, a jumper cable technique previously discussed appears to do the job.
A RRIO Quad Op amp can be used to make a ramp,
but it turns out a triangle wave works better.
A binary counter with some resistors and an op amp can make the staircase.
Rail to Rail opamps can measure signal off either rail.
NPN and NMOS devices need to read currents going to ground.
PNP and PMOS devices need to read off the VCC supply.
In this circuit a clip lead is being used to select for the polarity (PorN) of the DUT.
Step 3: NPN CurveTracer Waveform
The PorN node needs to be connect to GND for an NPN.
Extra leads (bulk,VG) intended for CMOS devices are being unused.
In this case, the TRI(SAW) node is connected to the collector.
The VB Base lead has a 100k resistor in series with it to the voltage staircase.
The emitter goes to the ICS node.
The two "scope probes" are connected to the TRI(SAW) node and the ICH node.
For a MacBook Pro, a program like MacCRO X.app as previously discussed
can do a X/Y plot to display the waveform.
Step 4: PNP CurveTracer Waveform
The Emitter, Base and Collector connections are the same for both an NPN and a PNP.
But there is a shift in the polarity of currents.
Now the emitter current needs to be sensed off the VCC rail.
This is done by connecting the PorN node to VCC.
The waveform will be displayed inverted.
Step 5: NMOS CurveTracer Waveform
CMOS devices need the VG and Bulk leads.
The Bulk node on a NMOS device needs to be connected to GND.
For a CD4007 device, this node happens to be pin 9.
The PorN node is connected to the bulk lead.
So connecting the PorN node to GND serves two purposes.
The ICS node gets connected to the source.
The drain gets connected to the TRI (Saw) node.
The VG node connects directly to the Gate.
Step 6: PMOS CurveTracer Waveform
Like for Bipolar, the Drain, Gate, and Source are the same for both NMOS and PMOS.
But for PMOS devices, their bulk needs to be attached to the highest VCC potential.
This is pin 11 in the case of cd4007.
When the PorN node gets connected to VCC, the bulk lead will be connected to VCC too.
If one happens to use pins 12,11,10, such the pin 11 is at ICS,
then the bulk is already connected inside the IC.
Step 7: Here Are All the Curves
The curves above show both polarities of Bipolar andCMOS transitors can be displayed.
So now how does the circuit work?
Step 8: The SweepWave and StairCase
The full MacSpice simulation netlist is included in this page.
The heart of the circuit is a simple RC oscillator shown above in the upper left.
The voltage across capacitor C1 sets the oscillator speed.
It gets gained up to produce a clipping rail to rail triangle wave at the TRI port.
This same oscillator is also driving a binary counter.
The outputs of the counter are fed to binary weighted resistors,
and then on to an Op Amp to generate a staircase waveform at the VG node.
Step 9: The Current Measurement
With PorN to ground, the ICS node can measure current from a NPN going into it.
When at VCC, ICS can measure the current that a PNP pulls out of it.
The gain in the simulation is set for 1mA to generates 1volt shift at the ICH node.
The shown circuit has it more like 1.2V per 1mA.
In the simulation, using ramps appears to work OK.
But triangle waves appear to work much better in the actual circuit.
The orginal TRI port has been renamed from SAW to TRI.
Step 10: CurveTracer Format
The curve tracer waveform should be plotting emitter current vs collector voltage.
Do a little gain scaling of current signal,
and then an x/y plot to get the proper curve.
Step 11: A Paper Printed Circuit Board
This method uses a paper PC board method together with wirewrap.
The layout for the PC "Board" can be downloaded below.
The schematic has slightly different values,
because different values seem to work better in the circuit than in the simulation.
In particular the value of the capacitor might need to be different for different laptops.
This method has been discussed earlier as a solderless PC board technique.
The intention is less to avoid soldering, and more to make it much easier.
It provides a easy way to cleanly mount several wires to one lead before soldering.
If humans had eight arms, perhaps then this techinque might not be so useful.
Step 12: No Hurry to Solder
The circuit seems to want to use the wire-wrap to mechanically mount the components.
Now all nodes are both mechanically and electrically solid.
All bare wires are neatly wrapped around a lead.
No hurry to solder, the circuit will work if hooked up correctly.
And unwrapping wires is easier before soldering.
Some enhancements have been added since the first discussion.
The "board" has since been modified to become a box.
Now the circuit can be used from the component side.
Scotch tape has been added to the sides of the box to reduce wear and tear .
And some previously developed jumper cables,
appear to do the job in terms of connecting transistors and clip leads.
Step 13: But So Much for Pretty Pictures
So what good is just looking at pretty curve tracer pictures?
The curve-tracer waveforms are much more useful in units like volts and amps.
This can be done by downloading and installing software some free software.
It will be discussed in the handling waveforms section.