Well, that sounds like very good advice. In the headphone amp, the base current (simulated, sorry) is about 180 ua. Using this value gives me a value for R4 & R8 of 77k. That gives a collector current of -4.2 ma.
So. I'm a bit confused now. 3.9 meg only gives 3.69 ua of base current and a collector current of -148 ua. Surely the device is conducting more with the lower base resistor value?
I think the best thing for me to do is breadboard a simple test jig, set to give 180 ua of base current and calculate hFE by measuring Ic and Ib, indirectly by measuring the voltage across the base and collector resistors. Knowing the hFE of each device will make selecting them easier as well.
Thanks though, everyone, for the very helpful comments and discussion.
Hi Clockworktent,
Yes. Set up the jig so the collector (~= emitter current except with low beta parts) so the parts are passing the expected current in the application. If your current is too low, the parts may not be matched (probably not) where they are used. Size the collector resistors in the jig to give approx 10 VDC across the part maybe to make sure it remains in the active range, below 5 VDC will affect the readings so you want more than that.
If you look at spec sheets for semi manufacturers, the e-c test voltage is often near 10 VDC. I set my beta balancer project so the transistors see approximately that, and made the CCS tail current adjustable in steps so you could test the pair at the current they would be used at. You're trying to avoid too much DUT temperature rise during the test. Put the parts in tight physical contact to equalize their temperatures as that affects beta quite a lot. This was a jig to match transistors for differential pairs and has an NPN and PNP section. I've used it for many decades, before I joined DIYAudio in fact. Many people have built it, and it is very effective. I think three different boards were designed for it by members here.
No, lower base current means lower collector current. It is not an inverse relationship. You could probably use a 250K pot between the two bases (no common connection) and dial it to the point where you see the expected collector current. This should be in the ballpark for the rest of the parts, but feel free to adjust as required. Don't get hung up on an exact current as it will vary with many factors. For low beta, a lower value pot would make adjustment easier. Don't use a 10 turn.
Yes. Set up the jig so the collector (~= emitter current except with low beta parts) so the parts are passing the expected current in the application. If your current is too low, the parts may not be matched (probably not) where they are used. Size the collector resistors in the jig to give approx 10 VDC across the part maybe to make sure it remains in the active range, below 5 VDC will affect the readings so you want more than that.
If you look at spec sheets for semi manufacturers, the e-c test voltage is often near 10 VDC. I set my beta balancer project so the transistors see approximately that, and made the CCS tail current adjustable in steps so you could test the pair at the current they would be used at. You're trying to avoid too much DUT temperature rise during the test. Put the parts in tight physical contact to equalize their temperatures as that affects beta quite a lot. This was a jig to match transistors for differential pairs and has an NPN and PNP section. I've used it for many decades, before I joined DIYAudio in fact. Many people have built it, and it is very effective. I think three different boards were designed for it by members here.
No, lower base current means lower collector current. It is not an inverse relationship. You could probably use a 250K pot between the two bases (no common connection) and dial it to the point where you see the expected collector current. This should be in the ballpark for the rest of the parts, but feel free to adjust as required. Don't get hung up on an exact current as it will vary with many factors. For low beta, a lower value pot would make adjustment easier. Don't use a 10 turn.
For those interested, I have started the description of an alternative matcher here:
Sorry, I hadn't explained my confusion properly here. I was surprised that if the device wasn't in "forward active mode" Mark wanted to increase the value of the base resistor, lowering the base current. I thought the opposite was required (lower the resistor and increase the current).
Also, after some (LTSpice) testing, I've been very surprised how much hFE seems to change with the base current: Here are the results from my simple test rig:
Measurement: hfe
step RBase i(rc)/i(rb)
1 10k 205.187
2 47k 225.695
3 100k 178.446
4 200k 124.967
5 500k 68.843
6 1meg 41.7192
7 2meg 24.7628
8 3meg 18.1654
So, yes, setting the current to match the application is important!. Unless, of course, I've done something wrong 🙄
Hi Clockworktent,
Okay, makes sense why you might ask about base current.
Transistor models are "best fit". It's nice they showed there is a variation with base current (collector current). If you look on good device data sheets, you will see a beta vs collector current graph that might help you get a feel for it.
Okay, makes sense why you might ask about base current.
Transistor models are "best fit". It's nice they showed there is a variation with base current (collector current). If you look on good device data sheets, you will see a beta vs collector current graph that might help you get a feel for it.