No-global-loop amplification
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matze
diyAudio Member

Join Date: Sep 2012
Quote:
 Originally Posted by traderbam Comparison You can set 1/B = TRX so the closed loop gain is independent of A but the feedback loop gain is ABX, the same as the conventional system.
The third last line on the sheet seems to agree with results from post #135 and #128.

diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by vzaichenko OK, in my system X = 1, making the formulas even simpler. Now, here's the point - overall gain in my system is equal to T. It's independent of A - that's exactly the condition for ODNF to work properly.
Ok. ODNF => 1/B=TRX (X=1 and T= your G and R=5.1k)
Quote:
 The error channel's loop characteristics, shown in the post #145, practically do not influence the amplifier's overall characteristics - they are strongly canceled-out after all. If I ground the output of A - nothing happens except the distortion increase, as there's no (-error) injection anymore. Neither the amplitude nor the phase responses change.
No.
By your own measurement, the op-amps apply 18dB of NFB at low f. This will cause the op-amps to try to control the output voltage and this certainly will affect the overall characteristics of the amp. Eg: try measuring the output resistance. I also notice in post #142 that the overall unity gain f has doubled or more with ODNF and the phase response slightly straightened.

It is tempting to think that the closer the output voltage tracks the input voltage the less feedback exists. But this isn't so! Feedback affects the sensitivity of the circuit to input/output difference regardless of what that difference is. The feedback is always "fully on".

The absence of change of the overall closed loop gain does not imply there is no feedback. The op-amps are simply using their local excess voltage gain to control the global closed loop gain to be the same as before. Because that's what you have told them to do.
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Last edited by traderbam; 2nd February 2018 at 01:01 PM.

diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by matze The third last line on the sheet seems to agree with results from post #135 and #128.
Barring mistakes, the diagrams and the equations should agree.
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vzaichenko
diyAudio Member

Join Date: Jan 2010
Location: Moscow, Russia
That's the way it works

In my particular case E = 18db, however, as 18db / (1+18db) is rather close to 1, it does not influence the overall gain / phase characteristics of the amplifier, as I mentioned earlier.

Once again, the dominant component in the formula is A, so the amplifier's gain, phase and other characteristics are defined by A.
Distortion D is a sum of Da and Dx - so all distortion components are corrected.
E only influences the accuracy of the error correction - the higher E means the lower Error at the output.

ODNF does not really change the "character" of the amplifier evenly decreasing the harmonics amplitude. I see it in my prototype measurements. That's exactly what I like it to do
Attached Images
 ODNF formula.JPG (77.3 KB, 1285 views)
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matze
diyAudio Member

Join Date: Sep 2012
Quote:
 Originally Posted by vzaichenko Once again, the dominant component in the formula is A, so the amplifier's gain, phase and other characteristics are defined by A. Distortion D is a sum of Da and Dx - so all distortion components are corrected. E only influences the accuracy of the error correction - the higher E means the lower Error at the output. ODNF does not really change the "character" of the amplifier evenly decreasing the harmonics amplitude. I see it in my prototype measurements. That's exactly what I like it to do
OK.

Cheers,
Matthias

vzaichenko
diyAudio Member

Join Date: Jan 2010
Location: Moscow, Russia
Quote:
 Originally Posted by traderbam By your own measurement, the op-amps apply 18dB of NFB at low f.
No, because of the reason I mentioned - the "+" input of the opamp was grounded for that measurement . Connecting it to the input makes a big difference, making the real influence looking as 8 / (1 + 8) = 1db (18db = 8 times). This is what I also see in simulation if I connect the "+" input to the signal source.
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If you can't explain it simply, you don't understand it well enough (c) Albert Einstein
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diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by matze The third last line on the sheet seems to agree with results from post #135 and #128.
Yes #135 does except that I have treated the IPS as a transconductance amp and my equation includes R. I haven't checked #128.
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diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by vzaichenko No, because of the reason I mentioned - the "+" input of the opamp was grounded for that measurement . Connecting it to the input makes a big difference, making the real influence looking as 8 / (1 + 8) = 1db (18db = 8 times). This is what I also see in simulation if I connect the "+" input to the signal source.
Re post #154 diagram
The transfer function is

Vo/Vi = [ (AR + K)X ] / ( 1 + KBX )

The feedback loop gain is KBX.
Can you estimate KBX for your circuit?

Re measuring feedback
You need to connect everything up in the sim as in the proper operation of the circuit. Then insert an ac voltage source (ac=1 dc=0) into the feedback loop. Then measure the ratio of the voltages either side of the voltage source. Do not apply any input signal.
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 2nd February 2018, 01:48 PM #159 traderbam   diyAudio Member     Join Date: Jan 2002 Location: Earth Looking at the schematic in post #49, a quick calculation (always a mistake!) gives me KB = 8.4 or 18dB. Do you get the same figure? __________________ Everything matters.
 2nd February 2018, 01:50 PM #160 vzaichenko   diyAudio Member     Join Date: Jan 2010 Location: Moscow, Russia You're trying to consider K as a gain stage. From the overall amplifier's point of view, it's not. It's a small helper, producing the difference between the input signal and attenuated output signal. The transfer function: Vo/Vi = AX And then, we have additional component from the error channel, looking like: KBX / (1 + KBX) This additional component sets the rate of distortion cancellation. Nothing else. Assuming, the amplitudes at the "+" and "-" inputs of the opamp are equal. __________________ If you can't explain it simply, you don't understand it well enough (c) Albert Einstein http://vzaudio.com/

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