Another Bloak Hack...

Ah yes, a Zobel! ... forgot to put one of those in. I'll try it.

1N4148 tempcos are in fact not that good a match to bipolar Vbe ones. But it's usually in the right direction away from thermal runaway.

One gotcha with Vbe multipliers often overlooked: it's not just Vbe that is involved, but as well beta. And when beta rises with T the voltage divider loading is less, which is a countervailing effect to the Vbe decrease with T.

An integrated darlington is often a good compromise. The loading on the divider of a usefully high impedance is so small that the variation in beta has little effect. However what does go on is that at low current densities the Vbe tempco goes up, so you can get into an overcompensated regime.

As a late neurophysiologist used to intone to an old acquaintance of mine:

Stephen, science is so difficult. But science is more fun than anybody.

[quote author="Samuel Groner"]

The diamond buffer would be another alternative.

Samuel[/quote]

I'm high on diamonds.

[quote author="bcarso"]Stephen, science is so difficult. But science is more fun than anybody.[/quote]
Ah, I like that. Very well put!

I'll give the diamond suggestion a shot as well... never tried one before.

OK ... Zobel cleaned things up nicely.

And, I like the diamond as well... what do you think of my little bootstrap arrangement? Dunno if anyone's done that before - just seemed like an obvious idea to me... should improve the drive into the output transistor.

Here's the schematic for my comparisons:

[deleted]

Comments? I've never done a diamond buffer before... if you spot anything which needs fixing, please let me know.

[quote author="hifizen"]OK ... Zobel cleaned things up nicely.

And, I like the diamond as well... what do you think of my little bootstrap arrangement? Dunno if anyone's done that before - just seemed like an obvious idea to me... should improve the drive into the output transistor.

Here's the schematic for my comparisons:



Comments? I've never done a diamond buffer before... if you spot anything which needs fixing, please let me know.[/quote]

The bootstrap is an a.c. way of getting the equivalent of a current source with the added advantage of allowing beyond-the-rail swing.

Some also bootstrap the collectors of the input devices to remove most of the effects of collector-base capacitance. It's all good; whether it helps in a closed-loop arrangement needs to be evaluated on a case-by-case basis.

OK, now this is really starting to look nice in spice.

This version appears to be stable into 1nF at unity (10K feedback resistor) and +6dB (10K + 10K feedback network) gains. Current limit hits the wall at about +90mA and -110mA, and it can deliver 20Vp-p into 150ohms with the peaks just brushing the edge of current limiting diode conduction. This meets my criteria for headphone driving very nicely. Output stage bias appears rock solid over temperature. My 'mock' LED bias model is a bodge, but appears to simulate accurately.

It will definitely need a servo to keep the output DC in check, but I haven't decided yet whether I want to servo at the base or emitter of Q9.

This is actually a reply and a reflection from analogs amp design
above, which I also make a bow for Tk to as well. :grin:
It really works well in real world. I put a 170GR as J1, Q5 a 550B with a 470 ohms as emitter tail, beta 350. Q1 &Q2 matched with beta 450.
Q3 & Q4 BD 139/140 matched. The output gave a dc offset at 0,4V.
I tested to get it zero but leaved it. A change of R4 from 130 ohm to
115 ohm gave me about -0,4V instead. I believe its gonna be hard to make it stay at 0V even with a trim , its gonna drift up and down some 100mV. The current at the output stage is 8 mA! Perhaps to low?
R5 was made 10kohms//47 pF and made it stable to gain of 1 and upwards. It´s also very quiet, even at gain of 1000. (60 dB) I´m not
gonna strain it more then 50 dB in my applications. R10 was added a cap
at 2700 microF to ground. So thanks for your contribution, this is a fine amp. Cheers Bo

I accidententally lost all my posted schemos, I have to upload them again when I get the time.
Great work, Bo.

analag

So this weekend I finally got around to building the circuit, and did some preliminary testing. Probably time to move this topic over to the Lab section - I'll start a new thread there soon. For now, I'll just report my results...

First testing was on the diamond buffer output stage, open loop (with 2SC4793 / 2SA1837 instead of BD139 / BD140). To set up the correct DC conditions, I constructed the entire circuit (no servo) and trimmed it approximately for DC offset. I then ran it open loop by grounding the +IN node and injecting signal directly at the Q6 base using a function generator.

Square wave response looked good, with a small well-damped overshoot. However on the sine sweep, I ran into trouble at about 7MHz. At that point, the output suddenly collapsed into a distorted sawtooth, and did not recover until the frequency was backed off all the way down to 4MHz. One of the limitations of the bootstrap jobbie is that R7 and R10 appear as AC loads on the output, so there's a tradeoff between driver current and output loading. 2mA seems to be too low (I was optimistically hoping otherwise), as the drivers looked to be going into cutoff while the stage was distorting. The cause of the frequency-related hysteresis isn't entirely clear to me yet, but starved drivers in combo with the bootstrap appear to be the culprit. I'll try and post some waveforms soon. As much as I like the the bootstrap aesthetically, to bias the drivers hotter would put more load on the output than I want.

So for now, I'll replace the bootstrap arrangement with CCS-connected JFETs and see how that does. I have 2N5462 and 2SK246BL on hand. The 5462s are only a 40V part, while the K246s are 50V, but the K246 is not as easily available or as inexpensive as the 5462. I was hoping not to have to go this route, because I wanted to avoid using hand-picked parts in the circuit. I guess one can always pad the FET down to the right current with a source resistor, but this still involves part selection. The other alternative would be a couple more BJTs and a bunch of resistors. *sigh*

One last note - I think I'm going to servo at the CCS transistor emitter, as this provides the lowest possible noise (lower resistance at both the base and emitter). Further, if I move the servo to Q3 instead of Q9, the noise-dominating input stage is removed from the servo loop, and the two-capacitor opamp network can change to a single-capacitor version.

test

[edit] - was having difficulties posting last night... didn't think this message actually got posted.

OK, I've started a new thread over in the Lab to continue the development and real-world testing.

"DCBloak on the bench..."