VBE Multiplier current compensation resistor

[mikebeth]

Regarding increasing the bias setting. How did you measure the distortion as you adjusted the bias level?

I have seen many advocate increasing the bias level for "improved sound quality" without reference to actually measuring THD levels during the process on the grounds that the amplifier would be more "class A-like". Ouch

[Rallyfinnen]

I was watching FFT in ARTA. Higher order harmonics decreased with the Iq slightly increased, and most importantly, the Iq (and dist) was more stable with time/temp, for example after high power bursts it would settle quickly and nicely. A lot better at cold start too. Also added extra heat sink to be on the safe side. Can't remember the Iq, but it was nothing extreme. I just wrote the set Iq on the heat sink for future reference
A bit tedious to measure Iq without emitter resistors though. I first ran it off a bench supply to get current limiting if something went wrong, and later removed a supply jumper on the board and inserted an A-meter.

I too have found over-biasing to be beneficial isn several amps, and this is based on actual measurements like above.

[mikebeth]

Sounds reasonable. I would hesitate to call it over-biasing, perhaps correct biasing.


I have found that the bias level for lowest THD varies with the power level being used for the measurement. You definitely have to measure at a high enough signal level to move the signal swing out from the crossover region which can be quite wide with an EF output stage. I have not found, in my limited experience, that the "optimal" bias setting is the same at all power levels. You have to pick what output level to optimize at.


Also, as you seem to be aware, increased bias level = increased heat dissipation. Manufacturers are sensitive to this given the need to avoid overheating at high ambient temps, poor airflow around the amp. Also, larger heatsinks cost more money, etc.

[tiefbassuebertr]

Originally Posted by mikebeth

Hello all,


Wondering if anyone has done tests to optimize the value of current compensation resistor (R39 on the attached schematic) to minimize the shift in amplifier bias setting with changes in the +B supply rail voltage. If so, what results were obtained.



I just ran a real world test on various values of resistor ( 0 to 91ohms) and found the optimal value to be 36ohms, which is much higher than the conventionally suggested value of about 13ohms for the 10mA VAS current being used. Interestingly this agrees with the optimal value which my LTSpice simulations predicted while designing the amplifier. At that time I suspected my SPICE simulation was erroneous since it did not agree with what I read in certain books on amplifiers.

I had exact to this subject a very old application note (long report) - either from RCA or from Motorola (maybe as part of the introduction of RCA's first generation 2N3055). Unfortunately I don't find it. Maybe one of the members know the exact title resp. headline - thank you very much.

This I have found in the moment:
bjt - VBE Multiplier with Emitter Resistance Cancellation - Electrical Engineering Stack Exchange
Difficulties with Class B Amplifier Biasing - Electrical Engineering Stack Exchange
https://www.diyaudio.com/forums/atta...amplifiers-pdf
Feedforward Class-B Amplifier.
Optimizing the VBE Multiplier
Vbe bias generator insights
THERMAL COMPENSATION OF Vbe MULTIPLIER
Vbe multiplier sensitivity

Check out also this thread:
Dynamic Bias - Active Bias - Automatic/Self biased - Overview wanted

[MarsBravo]

That's a lot of readingstuff tiefbassuebertr, and I'm pretty sure most cancel it out.
What's the steepest diagonal integral conclusion?

[Rallyfinnen]

mikebeth: Yes, sometimes it's a compromise. Sometimes I don't see any ill effects from the higher bias at higher output levels either.

Back to topic:
I once experimented with an emitter resistor on the bias transistor to tune an over-compensating bias circuit (high bias class AB amp). I was able to get it really stable by 'tuning' this resistor. It also correlated with spice simulation. Thoughts on this? For example, I did not check for modulation of the bias voltage at high output.

[mikebeth]

Yes, that makes sense. At higher signal levels the effects of gm doubling or crossover distortion would be less noticeable as they become a smaller proportion of the overall larger signal level. They don't go away, they just get harder to see as they are now a smaller percentage of the output signal.


An emitter feedback resistor is common on common-emitter amplifier circuits, so I guess it could work if tuned properly. I'd have to simulate it in Spice and see what effect it had as opposed to the common method of applying the resistor to the bias transistor collector.

[Rallyfinnen]

I only 'optimized' it for current vs temperature, I never looked at rail voltage variation like you.
One thing I have found interesting lately is to look at the modulation of the bias voltage when the amp is being driven hard. With caps across the bias circuit you will probably see the cap charging and discharging with high level burst input signals.
You can also plot the Iq to see the effect it has, however effects of the quick temperature transients on the transistor chip on Iq will not be seen.
You can however feed bursts to the amp an observe what happens IRL.

[hpavictor]

https://www.edn.com/making-a-class-a...WM5ToeuBdhzTTs

[fotios]

Maybe not related to the thread, but my experiments have proven that BJTs with low Vceo are much faster in temp compensation. For example, the MJE340 with Vceo=300V is very slow compared to e.g. BD135 with Vceo=45V. In a class B amplifier, yet with big supply rails, the Vce of the multiplier is adjusted close to +/-1.75Vdc and never exceeds this level as the signal is in phase at the C and E of it. Simply has an offset of +/-1.75V. I've used BD135 in several amplifiers based on D. Self book (second edition - bought at 2000), with +/-60V, +/-74V, +/-80V supply rails without problem. Excuse me if it is out of topic