The Kuartlotron - keantoken's simple error-correction superbuffer

[BigE]

Originally Posted by Joachim Gerhard

Well, i have to ask Dough but i know he had issues.
You know, bashing TKD right in the open without proof ?
Bad me.

I'm not concerned about that. I'm keenly interested in knowing what type of distortion and where it may come from so that I may try to avoid it on future projects.

Those 128 position relay switched resistor networks look very interesting....

[Joachim Gerhard]

I will send him an e-mail.

[keantoken]

Originally Posted by Calvin

Hi,

@Keantoken
certainly a design I like, as one could use two matched Duals or one matched Quad (That340, though they might generate considerable output offset).

I've played with this idea, but the BC5xx/BC3x7 transistors are somewhat special as you can see from my Kmultiplier results. Furthermore, thermal matching only reduces the error that the circuit already has, which is mainly Early and Hfe distortions. For instance if you make a current mirror with a 26mV (whether intentionally or by temperature difference) difference in Vbe, the output currents will be 100% mismatched but they will still track linearly due to the intrinsic nature of the BJTs - it will be a current-doubler-mirror. So I don't think there is much to gain from using matched transistors. I tried using BC8xx duals but they actually performed worse IIRC.

I have difficulties though to follow Your explanations of how its functioning.
For the basic structure (You named it Tringlotron) You claim identical Vbes and Ics.

It is a simplified explanation. If Hfe is above 400, then there's not a lot of error from Ib. Furthermore, Ib errors are relatively benign, because Ib is proportional to Ic, just like Rm. The proportionality of Rm to Ic is the reason a current mirror is still linear even with a Vbe mismatch.

The only identical Vbes are in fact those for the current mirror.
The Vbes of Q1 and Q2 certainly differ.

Vbe in-circuit or by multimeter? Vbes don't actually need to match for this to work.

Also all 4 Ics differ if You account for the base currents.
Introducing emitter resistors in the CM and their Vbes won´t be identical too.
What difference could be expected when using a PNP-CM instead?
I´d expect even slighty lower THD for that one and probabely lower thermal problems!

Yes they differ, but it doesn't matter much because the circuit accounts for this. The emitter resistors are effectively part of the emitters in this schematic, and they cancel each other out. They are only needed to reduce the current in the input transistor.

Using a PNP current mirror won't have matching between Q1 and Q3 or Q2 and Q4 and this will be a significant source of distortion. Without the cancellation of Q1/R2 and Q3/R1, the nonlinearity of a forced (by degeneration) x10 current mirror ratio would cause failed cancellation.

With the Kuartlotron I basically don´t understand what You call error correction here?
Is it just another term used for positive feedback or for the partial cancellation of the Vbes?

It's in-phase cancellation of Vbe curves, log-antilog cancellation, as opposed to out-of-phase cancellation for instance in an EF or an LTP. This way ALL harmonics are cancelled, as well as the circuit can cancel them. For this reason the leftover residual has a benign monotonic profile, rather than a mix of canceled and uncanceled harmonics.

As the Base of Q4 is modulated signal dependent, and hence the base of Q3 there certainly is (positive) feedback, which might have been the source of Joachim´s circuit instability.

As I recall Joachim's circuit was never unstable, but his output cable was resonating in its series mode, which would happen with any high-speed buffer. This part of the Kuartlotron could be considered positive feedback, but positive feedback alone does not cause instability.

[Joachim Gerhard]

Untamed this buffer has enormous speed so earlier versions i build suffered from RF mismatch of the transmission line to my scope.
The version on Keantokens new website is stable.

[keantoken]

Hey Joachim, I added your very stylish image to my Kuartlotron page.

[baswamin]

Keantoken,

Couple of more questions.
For the input snubber, can the capacitance be 33pF? Will this cause any adverse effect on performance? I have 33pF cap lying around and hence I ask.

Lastly, can I use BC546/556 instead of 550/560?

Thanks

[keantoken]

You can increase the capacitor, but don't change the resistor.

BC546C/556C should work just as well. Can you quickly measure the Vbes with a multimeter and report the results? Also, do you know which manufacturer? This info will help with R&D.

[Joachim Gerhard]

Looks good on your page.

[baswamin]

Keantoken,

I measured the Vbe's on the transistors. Both BC546C and BC556C read 0.763V. I dont know if they are suppose to match so well but that is what it read.

I think the manufacturer is Fairchild semiconductor. The 'C' grade is difficult to get and I got these from PMI. hFE is around 520 for npn and 420 for pnp.

Thanks

Originally Posted by keantoken

You can increase the capacitor, but don't change the resistor.

BC546C/556C should work just as well. Can you quickly measure the Vbes with a multimeter and report the results? Also, do you know which manufacturer? This info will help with R&D.

[Zero D]

@ keantoken

Thanx for the additional info Yes i expect it "might" be suitable for HF oscilliscopes etc, as well as RF stuff. Maybe Hewlet Packard etc would be interested in it ?

Anyway, all the best with this & your other designs etc