Whoops said:
Whoops said:
ln76d said:
joaquins said:
I bought 400 BC550C (I know, you win) and they have between 400 and 500, some have little more but non over 600, maybe I'm in the wrong country and they send the worst for here... Too bad for me.
JS
I found that some multimeters shows different values of HFE. I did comparision of three mutimeters all shows different value of HFE. Two show HFE with difference of +/-20 but the third mm show difference more than 100...
I also have the same problem.
All my DMM measure different values and the difference is quite high, can be from 20% to 50% difference in measuring. And this was done in 5 different DMM.
One DMM measures my BC184C in the 550 - 620 range, while other measure the same transistors in the 320 to 370 range.
Makes it hard to know who has the correct HFE values
Any advices of a device I could use to measure HFE that is precise and trustworthy?
Anyone knows whats the base current that HFE should be measured to select BC184C for a Neve 1290/1272 type circuit?
It's advisable to select at least the first one of each darlington config to higher than 600 HFE, but at what base current should the HFE be 600?
Ok, let's slow down. The exact value of the HFE does not matter, it shouldn't. It doesn't care if one DMM measures twice as much as another. The HFE changes hugely with conditions, and DMM are really bad at get any estimate about that value, it's a baaad thing to use that HFE value to do any math. In fact, a circuit shouldn't rely at all in that value.
DMM provide a easy and dirty way of measuring such value at one condition, you could compare 2 transistors with it, and find two alike each other. In a diferential input pair, at an opamp, HFE has little to do of how it works, but it does play a role in the bias current at the base. Having a larger HFE makes lower bias current at the base and higher hFE makes higher signal gain in the first stage, both good for noise performance. Having similar HFE makes lower current offset, not a big deal either in audio applications.
VBE matching determines the offset input of an opamp, also not the best thing to measure as a diode with a DMM but it gives an idea. My goal was to measure the few hundreeds of transistors to sort them out and not end having tens of mV offset, or hundreeds in HFE differences. I wouldn't use that matching for a precision log converter or a low offset discrete opamp for metrology application.
I also measure at room temp, no time to settle, connect the transistor, count to 5 to get a steady reading and take note. A proper measurement of those two values would be with the transistor biased as in the actual circuit and in thermal balance after a while of heating with that bias. In there measure the base current and the collector current and that would give the HFE, the VBE is a direct measurement, much greater matching of VBE could be archived using a transistor as reference and measuring the difference between that and the DUT. That way you can use a lower range (if your DMM supports a lower range) and get better results.
To measure hFE you should introduce a signal to the circuit and mesure the AC current at base and collector, that ratio will give hFE. I don't think vBE has any impact in audio applications, but the AC voltage between base and emitter could be measured with signal and from there and AC current at the base the rπ or hie could be estimated. While you are at it measure the AC voltage between collector and emitter and you can build the model of your particular device and play it in spice. You are still missing the capacitances that Miller will ask you for, but for our <20khz world that hardly matters.
JS
