I have a simple circuit setup to match transistors. The original circuit has I_{c}=0.1 mA and at that level the transistors stayed at ambient temp and the V_{be} value settled down pretty quickly after handling.
I modified the circuit to pass about 10 mA of current through the transistors since this is more typical of what I will see in built circuit and I noticed the extra heat dissipation resulted in the V_{be} taking about two minutes to settle down, which is fine, but I also noticed that the value is more sensitive to ambient conditions. I think a fly nearby would create enough air flow to change V_{be}.
So, the question is, if two transistors of the same type exhibit a particular delta V_{be} at one I_{c} will it exhibit a similar delta V_{be} at another vastly different I_{c}?
Thanks
--
MIke
I modified the circuit to pass about 10 mA of current through the transistors since this is more typical of what I will see in built circuit and I noticed the extra heat dissipation resulted in the V_{be} taking about two minutes to settle down, which is fine, but I also noticed that the value is more sensitive to ambient conditions. I think a fly nearby would create enough air flow to change V_{be}.
So, the question is, if two transistors of the same type exhibit a particular delta V_{be} at one I_{c} will it exhibit a similar delta V_{be} at another vastly different I_{c}?
Thanks
--
MIke
Since Ic changes exponentially with Vbe, matching transistors using Vbe may not be as accurate as matching transistors using there betas (Ic/Ib). Make Ib (and Vce) constant and measure Ic.
absolute measurements
I contend that we under resourced amateurs cannot hold all the conditions fixed that are necessary to compare absolute measurements to the manufacturer's data.
I recommend comparing Device Under Test (DUT) to reference (REF).
rough sort your devices by whatever is your easiest measurement method.
Select a random device (REF) from a group and compare all the others (DUTs) in that group to REF. Note the difference {REF - DUT } (dif).
if two or more devices compare with dif=-x for each device, then place REF within it's group labeled 0 or REF.
Take two of the comparable devices and fit them as DUT & REF'
You may find that some of the DUTs that have very similar dif are actually well matched.
If temperature affects the device parameter then you must arrange for both DUT & REF to be thermally coupled and that they both have the same voltages and currents passing across/through them.
I contend that we under resourced amateurs cannot hold all the conditions fixed that are necessary to compare absolute measurements to the manufacturer's data.
I recommend comparing Device Under Test (DUT) to reference (REF).
rough sort your devices by whatever is your easiest measurement method.
Select a random device (REF) from a group and compare all the others (DUTs) in that group to REF. Note the difference {REF - DUT } (dif).
if two or more devices compare with dif=-x for each device, then place REF within it's group labeled 0 or REF.
Take two of the comparable devices and fit them as DUT & REF'
You may find that some of the DUTs that have very similar dif are actually well matched.
If temperature affects the device parameter then you must arrange for both DUT & REF to be thermally coupled and that they both have the same voltages and currents passing across/through them.
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