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Old 13th February 2014, 12:04 PM   #1
Elvee is online now Elvee  Belgium
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Default Opto-controlled bias

Here is an idea for an automatic bias control of a PP output stage.

It has probably been already explored, but here is my take anyway.

I think the schematic is self-explanatory enough; there are two options: bypassed or not.

When the base spreader is bypassed, the behavior exactly mimics that of a conventional circuit, except it is automatic.

If the bypass cap is removed, the circuit operates in real time, and becomes a non-switching class B.

The effect is noticeable in the reduced THD figure. Note that there is some ringing, because I didn't take the care to add any compensation, but these are not difficult issues
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Old 13th February 2014, 12:43 PM   #2
tvrgeek is offline tvrgeek  United States
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Nelson Pass has a patent on it. I am sure it can be updated by now. The big Adcom used it. I was thinking about using a IR detector to look at the output transistor case. Maybe a little less lag.

He also has a circuit to compensate for the driver stage so the bias servo using a thermister only had to figure out the output stage.
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Old 13th February 2014, 12:49 PM   #3
ATAUDIO is offline ATAUDIO  Austria
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Basically it is very interesting and I commend your long time efforts on this topic.

However I am non proficent on the simulator and from the schematic I cannot see how it works.
The left side opto couplers have an open output (goes to a-b?) and the right side are even short. They could be replaced by two LEDs for that.
I am sure I miss something, however.

On the general idea I am more concern about the linearity of the OPTO response. Those stuff normally have an output that passes quite sharply from interdition to saturation. Maybe you might have to work with the base pin of the opto transistor, that in this opto model seems exposed.
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Old 13th February 2014, 01:03 PM   #4
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by tvrgeek View Post
Nelson Pass has a patent on it. I am sure it can be updated by now.
Interesting, it would be nice to see how he implemented it

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Originally Posted by ATAUDIO View Post
However I am non proficent on the simulator and from the schematic I cannot see how it works.
The left side opto couplers have an open output (goes to a-b?)
Yes they're in parallel with R5

Quote:
and the right side are even short. They could be replaced by two LEDs for that.
I am sure I miss something, however.
Problem with any random LED is that they won't match the characteristics of the active opto.
Opto's come in 4-pack, which means good matching and tracking with temperature

Quote:
On the general idea I am more concern about the linearity of the OPTO response. Those stuff normally have an output that passes quite sharply from interdition to saturation. Maybe you might have to work with the base pin of the opto transistor, that in this opto model seems exposed.
The linearity is completely irrelevant here, because they arent in the noble path: they're in in an orthogonal control loop having no direct effect on the output
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Last edited by Elvee; 13th February 2014 at 01:06 PM.
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Old 13th February 2014, 01:33 PM   #5
ATAUDIO is offline ATAUDIO  Austria
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OK, starting to understand now.
The circuit does not thermally compensate for bias, it justs does auto-bias when needed in the crossover region.
U2 and U4 are OK, especially if the 4N25 is a quad pack, good practical idea.
But when is it that the U1 and U3 internal LEDs are not lit?
It seems to me that they are always forward biased with a good current something like (50-2Vled-Vdiode)/10K i.e. about 4mA.
What did I get wrong?
Are the Schottky diodes necessary for the basic functioning of the circuit?
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Old 13th February 2014, 01:47 PM   #6
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Originally Posted by ATAUDIO View Post
OK, starting to understand now.
The circuit does not thermally compensate for bias, it justs does auto-bias when needed in the crossover region.?
Yes, but of course the automatic bias works against any type of perturbation, thermal included
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U2 and U4 are OK, especially if the 4N25 is a quad pack, good practical idea.
The 4N25 isn't a quad, that is just what I had available in the simulator, but in reality quads can be found easily
Quote:
But when is it that the U1 and U3 internal LEDs are not lit?
When the corresponding transistor passes a sufficient current, they aren't lit, but that's not where interesting things happen: it is when the current becomes too low in any of the transistor: the opto then raises the current to a minimum value, about 180mA as you can see on the real time version
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Are the Schottky diodes necessary for the basic functioning of the circuit
No, their role is simply to avoid wasting power whilst keeping a high enough sensitivity not be annoyed by matching or stability problems, but the details are unimportant and, if you build a real amplifier based on this concept, you have to understand how it works and design it from scratch: I used convenient values in my example, but they are certainly far from optimum in many ways
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Old 13th February 2014, 01:50 PM   #7
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I have found the Pass patent:
Patent US4752745 - Opto-isolated bias circuit for operating push-pull amplifiers in class A and ... - Google Patents

It works very differently from my idea, and is suited to class A. In class AB, it will bring more problems than it solves
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Old 13th February 2014, 03:15 PM   #8
ATAUDIO is offline ATAUDIO  Austria
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I have used the same Schottky "trick" to keep the efficiency high of a multiple pair CFP output, putting them in parallel to each OP Re.

But using them as in your circuit, I fail to see how the Opto "senses" when the OP is drawing current. With the Schottky in place the maximun voltage variation that it can see is about Vd, so a few hudred mV. It seems to me too less to turn off the internal LED.
I mean U1 LED anode is always at +V1 (or , at least, at +V1 -Vd1) and the cathode of U4 LED is always at -V2, so it looks that U1 always "ON" (output saturated). Withour D1 /D2 and adjusting R3, it should work.
Where am I wrong?
Great concept, BTW.
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Old 13th February 2014, 05:20 PM   #9
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Originally Posted by ATAUDIO View Post
I have used the same Schottky "trick" to keep the efficiency high of a multiple pair CFP output, putting them in parallel to each OP Re.
When they are across the Re's, they bring serious non-linearities, unless you use some tricks like Broskie's class C.

Here, in the collectors their influence is minimal, through Early effect or similar.
With the opto bias, no emitter resistance is required, which is best for linearity

Quote:
But using them as in your circuit, I fail to see how the Opto "senses" when the OP is drawing current. With the Schottky in place the maximun voltage variation that it can see is about Vd, so a few hudred mV. It seems to me too less to turn off the internal LED.
I mean U1 LED anode is always at +V1 (or , at least, at +V1 -Vd1) and the cathode of U4 LED is always at -V2, so it looks that U1 always "ON" (output saturated). Withour D1 /D2 and adjusting R3, it should work.
Where am I wrong?
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but that's not where interesting things happen: it is when the current becomes too low in any of the transistor: the opto then raises the current to a minimum value, about 180mA as you can see on the real time version
Remove the schottkys if you prefer, they change nothing to the operation of the circuit.
Consider the pair of LEDs as a virtual diff amp, without explicit transistors
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Old 13th February 2014, 06:15 PM   #10
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I have found the Pass patent:
Patent US4752745 - Opto-isolated bias circuit for operating push-pull amplifiers in class A and ... - Google Patents

It works very differently from my idea, and is suited to class A. In class AB, it will bring more problems than it solves
I bet that is why they quit using it. The motivation was probably to not have to factory set bias. Any hand-operation is expensive. I think the 585 used it.
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