I am designing a transistor matcher and curve tracer.
Got it running sort of ok.
Measures hfe of npn and pnp ok and compares well with my DMM for readings.
I then tried a darlington npn transistor and the hfe is 31 !
I assume base voltage doesnt change when checking hfe.
My base current comes from base driver voltage (through 1k resistor) - base driver voltage at transistor turn on divided by a 1k resistor.
It seems with a darlington the base voltage goes up as its driven harder.
This messes up the calculated base current.
So I am having to add a differential amp across base resistor to calculate accurately the base current.
Seems odd it works fine with none darlington or low hfe transistors.
There is a problem with high hfe none darlingtons too.
Waiting for a new pcb now to see how well it works.
Got it running sort of ok.
Measures hfe of npn and pnp ok and compares well with my DMM for readings.
I then tried a darlington npn transistor and the hfe is 31 !
I assume base voltage doesnt change when checking hfe.
My base current comes from base driver voltage (through 1k resistor) - base driver voltage at transistor turn on divided by a 1k resistor.
It seems with a darlington the base voltage goes up as its driven harder.
This messes up the calculated base current.
So I am having to add a differential amp across base resistor to calculate accurately the base current.
Seems odd it works fine with none darlington or low hfe transistors.
There is a problem with high hfe none darlingtons too.
Waiting for a new pcb now to see how well it works.
Turn on voltage is about 1.2 volts.
Its seems to go up to 1.4 when its turned fully on.
The 0.2 volts change messes up the calculations for hfe.
I have to be careful not to bodge it or the normal npn/pnp wont read right.
The answer is obviously to measure V across 1k resistor for an accurate measure.
I have added a differential amplifier to measure it.
Its seems to go up to 1.4 when its turned fully on.
The 0.2 volts change messes up the calculations for hfe.
I have to be careful not to bodge it or the normal npn/pnp wont read right.
The answer is obviously to measure V across 1k resistor for an accurate measure.
I have added a differential amplifier to measure it.
Normal "turn on " for a Darlington is 2X a single BJT which is =>0.7 V therefore =>0.7V X 2= 1.4V+
If I look at the curves on the pc screen the low hfe npn seems to rise slightly faster than the darlington. So it must be the darlington base voltage varying by 0.2V that accounts for the difference.
Your circuit should be designed to measure base current with a fixed collector current. Base current error should be fine.
In other words, diode wire the DUT with a suitable sized, or selectable resistor value (range control!) in series with the base. V(Rb)=IbRb. Put a much smaller resistor in series with the voltage supply to set the current as (Vsupply-IbRb-Vbe)/Rlimit.
If you're measuring to match Vbe, then measure that at the same time, at the collector current you intend to use for bias.
In other words, diode wire the DUT with a suitable sized, or selectable resistor value (range control!) in series with the base. V(Rb)=IbRb. Put a much smaller resistor in series with the voltage supply to set the current as (Vsupply-IbRb-Vbe)/Rlimit.
If you're measuring to match Vbe, then measure that at the same time, at the collector current you intend to use for bias.
If you don't want the base current error, just use math to readjust the collector current to be correct, since you're measuring Ib.
there may be a small 30 ohm ish resistor across the BE junction on the output transistor for speedup. this would cause a double knee at .7v and again at 1.4v when low currents are applied.
I went with a differential amp in the end across the 1k base resistor.
I know Ic, so Ic/Ib = hfe
I know Ic, so Ic/Ib = hfe