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Old 19th December 2011, 03:22 PM   #11
dadod is offline dadod  Croatia
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Sorry, THAT not WHAT of course. Yes expesive, that bather me.
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Old 19th December 2011, 03:43 PM   #12
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Default Class-I NFB-loop

Now, let's have a look at the NFB loop inside Kendall's Class-I output stage.
In fact here three FB paths: one directly from the output, which is a bit positive (-6dB) and two from the sense resistors, which provide the main and negative FB. At larger output currents, say above 250mA, only one of them is active (either from the top RE or the bottom RE). As a result, the phase and amplitude response of the FB loop depends on the output current. See the graph below.

At idle or low output power conditions:
PM = 60 deg @ 1.5MHz
GM = 6.8dB @ 6.5MHz

And at high output power conditions:
PM = 69 deg @ 2.4MHz
GM = 8.3dB @ 13MHz

I'm not really happy with this variability, as you can't optimize the FB loop for any condition (for small and large signals). In practice this means that you have to tailor the frequency compensation for worst case conditions, that is, at high output currents, as in that case the ULGF is highest. But then the small signal compensation is suboptimal.

BTW, the AB2-OPS doesn't exhibit these quirks.

Cheers,
E.
Attached Images
File Type: jpg Kendall2-FB.jpg (120.7 KB, 473 views)
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Old 19th December 2011, 07:10 PM   #13
miralin is offline miralin  Russian Federation
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Hi Edmond
Basic Auto Bias circuit (Fig.1 in http://home.tiscali.nl/data.odyssey/AutoBias_II.html) has an in-out offset equal to difference between Vbe of pnp and npn transistors. Let me suggest some changes which cannot affect overall work of the circuit.
1.Disconnect collectors of Q5 and Q6 from Out.
2.Connect collectors of Q5 and Q6 to their bases.
We have four transistors connected as diodes. Now negligible I5 and I6 flow thru R1 and R2.
3.Interchange Q3 with Q4 and Q5 with Q6. Now we have diodes on the same places but no offset.
Cheers
Mir
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Old 19th December 2011, 08:53 PM   #14
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Default Alternative Bias

Hi Mir,

Do you mean this:

Cheers,
E
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File Type: png AltBias.png (13.0 KB, 451 views)
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Old 20th December 2011, 05:33 AM   #15
miralin is offline miralin  Russian Federation
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Hi Edmond
No. I mean emitters of Q6 and Q5 connected to S1 and S2, not collectors.
I'm sorry that being a beginner I don't know how to draw and so have to use stiff words instead.

Cheers
Mir
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Old 20th December 2011, 10:07 AM   #16
dadod is offline dadod  Croatia
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Default AB2 OPS

The FFT at 1kHz shows perfect harmonics distribution up to almost full power.
First one is 1W//8ohm, second one 35W//8ohm and last one 81W//8ohm.
Even at 80W distortion is 1.34ppm.
Damir
Attached Images
File Type: jpg Non switching EdmondOPS amp-FFT-1W.jpg (172.5 KB, 433 views)
File Type: jpg Non switching EdmondOPS amp-FFT-35W.jpg (171.6 KB, 396 views)
File Type: jpg Non switching EdmondOPS amp-FFT-80W.jpg (179.4 KB, 374 views)
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Old 20th December 2011, 11:32 AM   #17
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Hi Damir,

I know AB2 is pretty good.
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Old 20th December 2011, 02:05 PM   #18
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Default AB-II and siblings

Quote:
Originally Posted by miralin View Post
Hi Edmond
No. I mean emitters of Q6 and Q5 connected to S1 and S2, not collectors.
I'm sorry that being a beginner I don't know how to draw and so have to use stiff words instead.

Cheers
Mir
Hi Mir,

Thanks, now I got it. I'm glad you brought this up as I was already waiting for a good opportunity the discuss possible variations on AB-II.
Below you see a number of alternative circuits together with Kendall's Class-I OPS.

The first circuit is the same as in my website. This one is non-optimal and only meant to illustrate the basic principle. The distortion, even at 20Hz, is a bit high: 144 ppm, and the offset is -22mV.

In the second circuit, where the polarity of Q3 and Q4 has been interchanged, the offset dropped to -7.7mV, but the distortion is even higher: 184 ppm.

In the third circuit, that what you meant, right?, also the polarity of Q5 and Q6 has been interchanged. The offset dropped even further: to -2.6mV. (It would be zero in absence of a input current offset.) THD is the same as in Fig.1.

The first three circuits suffer from the same thing: switching currents from Q5 & Q6, which are not canceled out. The effect is clearly illustrated in fig.7: the black curve, which shows the distortion residual.

This effect can be eliminated by crosswise connecting the collectors of Q5 & Q6 to the node A respectively B. Now the THD dropped to 4.3 ppm. That's nice, but the offset voltage is back. BTW, this is a non-issue if the OPS is embedded in an amp with global NFB as well.
Fig.8, black curve, shows the distortion residual (almost gone).

In Fig.5 I've tried to reduce the offset voltage by interchanging the polarity of Q 3 & Q4. Although the offset dropped to -3.9mV, this is not a good idea, as not only the THD rises to 100pm, but also the distribution of drain currents get highly asymmetrical (that is, unequal minimum currents). The reason for above issues is the (inevitable!) mismatch between the PNP and NPN transistors.

And finally Fig.6 shows Kendall's Class-I OPS, that is, with added cascode transistors Q3 & Q6. THD is even lower: 3.3ppm and the offset voltage only 1.5mV. Very impressive, isn't it? Regrettably, also this circuit has some issues: If R1 plus Zo of the preceding stage isn't equal to R2, the THD rises considerably. Also notice that without cascodes, as in the original implementation by Kendall, the performance will be affected. Therefore, (without cascodes) Kendall recommend high Early voltage trannies for Q1, Q2, Q4 & Q5.

Cheers,
E.

PS: I simmed the distortion at very a low frequency on purpose (effectively at the DC level): In order to make the distortion from the bias circuit visible without the contribution from the OP devices.
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Last edited by Edmond Stuart; 20th December 2011 at 02:13 PM.
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Old 20th December 2011, 02:09 PM   #19
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Hey, WTF, the pictures are gone!
Let's try again:
Attached Images
File Type: png AB2-0.png (13.5 KB, 204 views)
File Type: png AB2-1.png (13.8 KB, 190 views)
File Type: png AB2-2.png (13.3 KB, 154 views)
File Type: png AB2-3.png (14.4 KB, 151 views)
File Type: png AB2-4.png (14.0 KB, 165 views)
File Type: png ABK-5.png (18.0 KB, 198 views)
File Type: png Res-0.png (13.7 KB, 181 views)
File Type: png Res-3.png (9.6 KB, 100 views)
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Old 20th December 2011, 07:57 PM   #20
miralin is offline miralin  Russian Federation
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Default AB-II

Hi Edmond
Your post #18-19 is highly informative (I think not only for me), especially about the huge reduction of error due to the crossconnection on Fig.4.
Now let’s return to the “offsetless” Fig.3, which also is the simplest cirquit. Assuming Out grounded, consider e.g. the upper half of the cirquit an amplifier with emitter of Q1 as noninverting input, base of Q1 as inverting input, and Q5Q3 as a nonlinear divider in a local NFB loop. Resulting nonlinearity of this amplifier is presented by a red curve in Fig.7 and is responsible for all errors (black curve).
So almost the only thing determining errors is the nonlinearity of the divider between nodes S1 and A, and the simplest way to change (correct) the pattern of this nonlinearity is to place resistors in parallel or in series with Q5 and/or Q3 (with necessary variations of I3, of course). May be, this can give even better results than the crossconnection, or improve effect of the crossconnection itself.
You have already simmed similar changes, but mostly around Q1 (e.g. in #429 of HEEEELLLPPP and earlier). Why not to do some more experiments?
Cheers
Mir
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