TRANSISTOR CURVE TRACER FOR CRO

A full wave bridge rectifier provides both a +ve and -ve unregulated voltage.
Fed thru a series diode; 3 terminal regulators gives +5v, +15v and -15V rails.
The positive and negative 100Hz pulsating voltages (isolated from filter electrolytics by the diode) are used for the following:
As a varying collector voltage and X axis sweep voltage for CRO display. (+ve going for NPN device, and -ve going for PNP device)
Pulsating +ve voltage is fed to 4096 schmitt trigger .
Its O/P gives us a fast rise time square wave "clock" pulse of 100Hz to feed the 7474 cascaded freq' divider stages.
A resistive divider (47K/10K) at the input was selected on test (SOT) to give the best 1:1 mark/space ratio. A diode clamps 4096 input to Vcc + 0.7v (otherwise either I.C. would fail due to excessive input voltage level).
The 7474 'D' flip-flop is configured as two divide by 2 stages, and with a basic resistive Digital to Analogue convertor forms a staircase voltage generator. The 4,2,1 weighted digital o/p is fed into a DIL resistor package of 10K per leg .
1 leg, and 2 and 4 legs paralleled are used to give the 4:2:1 "weighting" required.
I found that the current input to the "1" leg of the ADC had to be attenuated with an extra external 4.7K resistor.
This gave an even looking 'staircase' waveform on the CRO.
The 8 step staircase voltage ramp is used to source base bias current to the Transistor under Test.
IC2 op-amp is a simple inverting or non-inverting gain amplifier (depending on switch position PNP/NPN ).
The emitter load voltage is fed to the Y plates of the CRO.
With each successive collector voltage pulse the base bias current to the transistor increases by one "step".
As can be seen from the schematic an 8 level sequence occurs and then repeats. By correct adjustment of the X and Y voltage amplifier CRO controls; an on screen display as in the photo can be obtained.
This is similar to the transitor characteristic diagrams seen in manufacturers data books.
Although not much good for obtaining "empirical" data,
(as the device really isn't calibrated)
It would be of great use to determine similarity between an number of identical type number transistors in order to obtain a good matched pair (or "quad") etc.


Further reference:
Transistor curve tracer (Japanese site)