For the VCF Parker/Steiner i need 2 matched transistor BC547
i find this schematic in an thread, but which Diode must i’ve to use in this circuit please
1N5817 can do the trick ?
thx
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A 1N5817 is a Schottky diode which is really different from a junction diode. The symbol is for a junction diode like a 1N914, In4148, 1N4001, etc. Any silicon junction diode should be fine at these low currents. Not super sure what the rest of the circuit is though and that may affect the choice. You could use the Vbe of another transistor as a diode too.
Oh, you are using this to match beta! . I think the diode is to keep the CB junction biased pretty low. Use a junction diode to keep Vcb reverse biased.
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Ray Wilson’s transistor matching circuits are considerably more elaborate (and thoroughly described), whether they work better or not I have no idea.
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thanks @feralbeagle & @analogoutput 
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I still match mine with just a breadboard and my meter. Given the range of tolerance and build build quality in transistors today you’ll never get that perfect match without testing hundreds so keep it simple and go for the best match out of 10. 20 if you’re a tad perfectionist.
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Thank you, I’m already forcing myself to do it, and at the beginning I even thought not to do it at all, just take 2 in the bag et voilà , i will see that tomorrow but indeed I will not test hundreds of them
what can be the consequences in the module with 2 transistors not matched ?
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Constantly questioning yourself if you should have matched the transistors is probably the biggest risk. If you match them fairly close, then that is one less thing to be concerned about later.
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Consequences? Basic one is too much current flows into one and it over heats. The next is issues with forward voltage. In modules? Meh, your module may not function properly or worse will work fine and then pftttt (imagine blue smoke)
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For the circuits that use matched transistors, it is not likely that the transistors will pass enough current cause significant heating. Diff amps, current mirrors, log converters, etc. are low current where the current is limited by a driving current source or a resistor. If you are matching output transistors on a power amp, a mismatch could cause extra distortion but will probably still work ok in a good (safe) design without causing damage. Matching improves the performance of a design but a mismatch should not do damage.
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This is for a VCF so presumably an exponential converter. Matched transistors are used to stabilize against temperature drift. Main consequence of poor match is that the cutoff frequency would wander as the circuit heats up or room temperature changes.
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Having matched my fair share of transistors for power amps, combos, sonar and power monitors was I wrong about the difference in current?
I accept that in a filter we match to maintain temperature and that the current is fairly low. But we also sandwich the transistors together for a temp match so that the current is even. The imbalance will, as I said cause a module to misbehave. It will also damage the transistor over time. I’m not predicting blue smoke every time but it would cause more than just a drift and the resulting failure could seriously affect other components. The matched transistors are part of a circuit that relies on a balance in current (however small) any significant offset risks the circuit.
Meh. Sorry. Lost an amp head as a child, old yeller we called him. Pop took him out to the woods and he never came back. As you were.
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thx @Farabide @analogoutput @feralbeagle
i take all that and make a matched circuit this afternoon
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I spent quite a while yesterday and the day before messing with this. I used a circuit posted by Kassutronics which is the same as the above except the top resistor was 4.7k.
All that’s going on there is that resistor plus the diode form a simple voltage reference, ~600 mV, for the collector voltage. Base is at ground and emitters are a little above -12 V. You put a reference transistor on one side and leave it there, and put transistors to check on the other side and measure voltage between the two emitters. Then you pair off transistors that give nearly the same measurement.
At first I built the matching circuit on a solderless breadboard and spent some time trying out an idea for expediting the process and generally trying to get consistent and sensible results. Ended up deciding the breadboard was not the way to go, especially after discovering the reading would change if I moved the jumper that ran from base to ground from one spot on the ground rail to another! So I built the circuit on stripboard, abandoned trying to improve the method, and just got on with it. Resistors were matched to 0.1%, 100.5k each, though if my CircuitLab simulation is correct, a 0.1k difference corresponds to only a 24 µV change in the measurement and I can only measure to precision of 0.1 mV.
With the stripboard, results seemed to be consistent and stable at the ~0.2 mV level. Out of 23 transistors tested all were within 2.5 mV of my reference transistor (and all read higher than the reference, apparently I picked a very atypical transistor for my reference) and I was able to pair off 20 of them with measurements differing by ≤ 0.1 mV.
(What you’re measuring is a voltage difference ∆V corresponding to ∆I/R where ∆I is the difference between the two emitter currents and R is 100k (or 100.5k in my case). Each current is about 11.4 V / 100.5k ~ 114 µA. Measuring ∆V to 0.1 mV corresponds to measuring ∆I to ~1 nA, or a relative difference in emitter currents of about 1 part in 100,000. Not bad for a multimeter and a piece of stripboard. Note, though, that the emitter voltages also differ, so the underlying quantity of interest, I_S, isn’t directly proportional to the emitter current. Per the CircuitLab simulation, it looks as though an emitter current difference of 1 part in 100,000 corresponds to a difference in I_S of something like half a percent.)
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