4QD-TEC Electronics Circuits Reference Archive
Test Ramp generator
This is a ramp generator suitable for driving the base of a power transistor, or the gate of a MOSFET, for curve generation purposes. It gives a nice, linear ramp of zero to some voltage. The output then goes to zero and stays at zero for a pre-set time before the next ramp starts.
This is a perfect waveform for power device testing, as the duty cycle ramp:rest can be set to say 1:10, or better. So the Device Under Test (DUT) is working (getting hot) for 10% of the time, then resting the rest of the time. That makes it possible to do high current testing without requiring massive heatsinks.
Also, the voltage output of the ramp can be 20v or more peak, so is very suitable for driving the horizontal deflection of an oscilloscope.
How it works
There are two 'identical' oscillators. Identical that is, apart from the fact that one is the complement of the other. One uses NPN transistors where the other uses PNP and vice versa. All diodes and the power supply are of course reversed, so one is therefore drawn 'upside down' in the above drawing.
This is so that the ramp polarity can be switched from positive going (for testing NPN devices, to negative going, to test PNP devices.
The ramp is fed, via a suitable resistor, to the base of the DUT. The resistor is chosen to suitably define the peak base current fed to the DUT. The output is fed via a complementary darlington emitter follower, so can give several amps of current if suitable assembled. The trwo diodes in the output are there because the emiter followers 'loose' 2 diode drops (emitter-base voltages) so wuithout the diodes, the output voltage waveform would not hit zero volts.
The two oscillators are boxed: the green box generates a negative going ramp, for testing PNP devices and the red box, a positive going ramp, for testing NPN transistors.
The basic operation of the emitter coupled multivibrator has already been described. Here it is again. The transistor numbers are the same as in that article.
Tr4 is an addition: its base is biased at constant voltage from the 6v8 zener and it has a variable resistor in its emitter, so it supplies a constant current to charge the ramp capacitor. That causes the capacitor to charge at a constant dV/dt - just what we require for a linear ramp.
Tr1 defines the rest period. We are not interested in how linear this ramp is, so a constant current is not required.
The same sort of waveform can (of course) be obtained from a few op-amps. It's shown in the TEC Library package 'Waveform Generators' but the circuit is reproduced here.
© 2001-2011 4QD-TEC
First published: 16th March, 2001.