Hello,
Just finished sweeping through more than 200 BJTs for my MC/MM phono preamp, measuring Hfe and Is indirectly through Vbe (with base/collector tied) using diode test function of a DVM. A bit of a PITA if you ask me, but you gotta do what you gotta do right? Or do we? Throughout this endeavor I couldn't help but wondering, does any audio manufacturer goes through this trouble ? Still have 200 more of the BC549/BC556 to measure...
Thank god for spreadsheets, I can sort the data set by Hfe then Vbe or inversely in order to get the matches. I also produced statistics from the 2SC2547D/2SA1085D data and right off the bat I can tell you I can't get beta matching for the two polarities out of 50 NPN and 100 PNP for the input transistors. I read here that this is normal for the epitaxial transistors.
Descriptive statistics also put forward the following properties:
1. Hfe is generally higher for NPN than PNP, even if they belong to the same beta group ("D" suffix);
2. Hfe varies much more than Vbe regardless of polarity;
3. Hfe variance is much higher for NPN than PNP (300.2 vs 47.8)
4. Vbe variance is also higher for NPN than PNP (1.75 vs 0.81), but less variable than Hfe like mentioned in point 2;
5. In my sample, PNPs have Hfe range lower than specified in the datasheet... Bad batch, bad luck, fakes ????
That being said, I'm wondering what to aim for in different parts of the circuit (see below), keeping in mind that beta match may be impossible between polarities. I know that:
- Joris
Just finished sweeping through more than 200 BJTs for my MC/MM phono preamp, measuring Hfe and Is indirectly through Vbe (with base/collector tied) using diode test function of a DVM. A bit of a PITA if you ask me, but you gotta do what you gotta do right? Or do we? Throughout this endeavor I couldn't help but wondering, does any audio manufacturer goes through this trouble ? Still have 200 more of the BC549/BC556 to measure...
Thank god for spreadsheets, I can sort the data set by Hfe then Vbe or inversely in order to get the matches. I also produced statistics from the 2SC2547D/2SA1085D data and right off the bat I can tell you I can't get beta matching for the two polarities out of 50 NPN and 100 PNP for the input transistors. I read here that this is normal for the epitaxial transistors.
Descriptive statistics also put forward the following properties:
1. Hfe is generally higher for NPN than PNP, even if they belong to the same beta group ("D" suffix);
2. Hfe varies much more than Vbe regardless of polarity;
3. Hfe variance is much higher for NPN than PNP (300.2 vs 47.8)
4. Vbe variance is also higher for NPN than PNP (1.75 vs 0.81), but less variable than Hfe like mentioned in point 2;
5. In my sample, PNPs have Hfe range lower than specified in the datasheet... Bad batch, bad luck, fakes ????
That being said, I'm wondering what to aim for in different parts of the circuit (see below), keeping in mind that beta match may be impossible between polarities. I know that:
- The parallel transistors of the input cascode need to be matched for Is; that is have the same collector current for a given Vbe for equal current sharing between them;
- Each leg of a long-tailed pair should be matched in beta.
- By which parameter to match the cascode's top transistors?
- Hfe influences the device's dynamic emitter resistance (re), but when emitter degeneration resistors (Re) are present, Hfe matching is more relaxed. So do I need to place the closest beta matches where the re/Re ratio is highest, e.g. where the degeneration resistor has the least effect on total output impedance?
- Do the CCS transistors need to be matched from one rail to the other? Matched for which parameter?
- Do the current buffer transistors (discrete Darlington configuration) need to be matched, and if yes by which parameter, total Darlington combined gain from rail to rail? I feel these positions are less critical since gain will be high anyways...
- The long-tailed pairs input devices should be matched in beta, but that is for the lowest input transistor since the LTPs are implemented as a cascode. The top transistors are tied by their bases, should they be matched for Is like the input parallel trios?
- Joris
Last edited:
Well, a good design does not depend on matching transistors.
You mentioned re the dynamic emitter resistance. The way to mitigate the effects of re (which drifts with temperature and of course affects the balance of the circuit) is to swamp it with emitter degenerating resistors. If the emitter resistors are much larger than re then the effect of re on the circuit is greatly mitigated.
There are other benefits to this method too, like increased stability and of course reduction in distortion from the linearizing effect of the degenerative resistor (I assume you know that re changes with a bunch of different parameters including bias current).
Doug Self goes into extreme detail on this topic in his Audio Power Amplifier Design book. He covers it in the chapter on input stage design. But the same principles apply to any differential input circuit.
You mentioned re the dynamic emitter resistance. The way to mitigate the effects of re (which drifts with temperature and of course affects the balance of the circuit) is to swamp it with emitter degenerating resistors. If the emitter resistors are much larger than re then the effect of re on the circuit is greatly mitigated.
There are other benefits to this method too, like increased stability and of course reduction in distortion from the linearizing effect of the degenerative resistor (I assume you know that re changes with a bunch of different parameters including bias current).
Doug Self goes into extreme detail on this topic in his Audio Power Amplifier Design book. He covers it in the chapter on input stage design. But the same principles apply to any differential input circuit.
I can hardly see, but when I looked at your circuit it occurred to me that the parallel input transistors would greatly benefit from closely matched, individual emitter resistors (6 resistors instead of the 2 shown). Each resistor should be 3x the value of the resistor shown in the circuit (which I can't see).
OK now I see they're 39 ohms. Get six-1% 100 ohm resistors (I recommend Dale CMF series) for the emitter resistors. Easy!
OK now I see they're 39 ohms. Get six-1% 100 ohm resistors (I recommend Dale CMF series) for the emitter resistors. Easy!
Thanks Eddie for your reply and suggestion. I take good note of it, but for the current project it is impossible to add parallel emitter resistors to the input as the PCBs are already made for it. It is a clone of the phono section of a vintage integrated amp I own - An exercise to understand it in order to service it if necessary, and also to have an MC phono preamp alternative when that amp is being serviced (30 years old 'lytics need replacing). It is also an interesting project by itself, as I have modified the circuit for operation on lower voltage rails and other small mods.