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Old 20th December 2011, 09:34 PM   #21
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Hi Mir,

I'm not sure if I understand you correctly. But I'm afraid you are missing one important point: The whole thing (regarding AB-II as well as Class-I) is based on nonlinear FB as far as the bias loop is concerned. This scheme approaches the so called geometric mean relationship between the currents through the top and bottom OP devices, i.e Idp * Idn = constant. To keep the product of these current exactly constant is not an aim in itself (any relationship will do), rather the aim is to prevent that one of the two OP devices gets completely turned off. If this might happen the, relationship between the two currents is undefined by definition.

Going back to your suggestion (if I understand you correctly), if you linearize the bias FB loop, then, above a certain output current, one of the OP devices will be completely turned off. That's exactly what we don't want.

BTW, I've simulated such approaches, but to no avail; much more distortion.

Cheers,
E.

PS: As for schematic capture, why don't download the student version of Micr0Cap V10. It's for free and the easiest and fastest SW you can get.
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Old 21st December 2011, 11:19 AM   #22
dadod is offline dadod  Croatia
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Hi Edmond,
I case if used more then one pair of output transistors what is better to connect, only one pair to error feedback and auto bias transistors, or do it from each pair? In my schematic resistors are R58, R60, R59 and R61.
Damir
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Old 21st December 2011, 11:35 AM   #23
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Hi Damir,

From each pair (of course).
BTW, if R47=R55=R54=R56=2.4 Ohms
(or R47=R55=2.2 Ohms and R54=R56=2.7Ohms)
(or R47=R55=2.7 Ohms and R54=R56=2.2Ohms)
the bias tempco becomes almost zero.

Cheers,
E.
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Old 21st December 2011, 12:51 PM   #24
mt490 is offline mt490  Australia
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Ahh, so the doubled transistors in the ABII input area (Q1 and complement in post 19 ABII schema) is for tempco compensation. Is that right?
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Old 21st December 2011, 01:40 PM   #25
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Default 'doubled transistors'

Hi MT

No, it are the resistors R47, R55, R54 & R56 that reduce the tempco.
Regarding the 'doubled transistors', see: HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles

Cheers,
E
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Old 21st December 2011, 02:57 PM   #26
dadod is offline dadod  Croatia
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Hi Edmond,
With your settings for AB2 OPS bias current is 100mA(33mV on 0.33R) and this is not optimal according to D. Self. How to decrease bias current(cca 75mA) and keep thermal and non switching condition optimal?
Damir
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Old 21st December 2011, 03:36 PM   #27
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Hi Damir,

You are referring to the so called Barny Oliver optimum point, i.e. 26mV across both emitter resistors. In case of AB2 your are simply bound to this 33mV (across a single resistor). I am not sure whether this is a real disadvantage or not. I have to figure that out.

Cheers,
E.
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Old 21st December 2011, 05:21 PM   #28
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Default Barny Oliver

Hi Damir,

Indeed, some gm doubling takes place. So I simmed the OPS with 26mV across both emitter resistors, being the optimum bias (by means of two multipliers, easy to simulate, harder to build). Guess what? Distortion gets worse! Apparently, different rules apply to output stages with a sliding bias. So forget Barny Oliver with all his wisdom.

Cheers,
E.
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Old 21st December 2011, 05:47 PM   #29
dadod is offline dadod  Croatia
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Default AB2 square wave simulation

Hi Edmond,
I did some simulation with square wave, and it is not very encouraging. If I test with a bit higher input signal it vent berserk.
First test is solo OPS. Slew Rate cca 80V/usec
Second is full amp with no input filter connected.
Third test with input filter connected, Slew Rate cca 30V/usec.
What do you think about it? Not very promising.
Damir
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File Type: jpg EdmondOPS-square-sim1.jpg (166.8 KB, 321 views)
File Type: jpg Non switching EdmondOPS amp-square1.jpg (185.7 KB, 301 views)
File Type: jpg Non switching EdmondOPS amp-square2.jpg (187.3 KB, 290 views)
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Old 21st December 2011, 07:40 PM   #30
miralin is offline miralin  Russian Federation
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Hi Edmond
Once again my poor English leads to possible misunderstanding. It is because the posts to this thread are the first attempts of English language conversation on technical issues for all my life. I’ll make another try some days later.
Cheers Mir
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