Feedback artifacts, cars and semantics - Page 11

AKSA · 26th February 2004, 05:26 AM

Because there is no open loop voltage gain, and therefore no feedback factor?

Hugh

Steve Eddy · 26th February 2004, 05:52 AM

Quote:

Originally posted by AKSA
Because there is no open loop voltage gain, and therefore no feedback factor?

Is there open loop voltage gain in an opamp configured as a unity gain buffer?

se

AKSA · 26th February 2004, 07:11 AM

OLG is fundamental to calculating feedback factor.

The ratio of OLG to CLG is, as I'm sure you realize, the feedback factor.

If you wish to prove me wrong, go right ahead. It's not important. In truth, the question is, as someone put it, roaringly funny.

Hugh

Steve Eddy · 26th February 2004, 08:18 AM

Quote:

Originally posted by AKSA
OLG is fundamental to calculating feedback factor.

Yes, I'm aware of that.

Quote:

The ratio of OLG to CLG is, as I'm sure you realize, the feedback factor.

Yes, I'm aware of that.

Quote:

If you wish to prove me wrong, go right ahead.

Nothing to prove wrong.

Quote:

It's not important. In truth, the question is, as someone put it, roaringly funny.

Then let me rephrase it so you perhaps can understand what I was asking.

If one directly feeds the opamp's output into its inverting input, creating a unity gain buffer, does that loop not contain the open loop gain of the opamp?

In other words, if you follow the loop around, the opamp's open loop gain is part of that loop. You have the input, the summing node, then the opamp's open loop gain, the output which goes into the feedback network which is then fed to the summing node.

Point I was getting at was that the opamp's open loop gain doesn't just disappear. If it did, then feedback wouldn't be able to do the things that it does.

So now let me get more to the point.

Let's say you've got an unknown opamp with an unknown open loop gain configured as a unity gain buffer.

You can determine that it's gain is 1. But you're not going to know what the open loop gain is and subsequently the feedback factor. So what do you do? You configure it as an open loop amplifier and measure its open loop gain.

If it's open loop gain is 100,000, you know that the feedback factor is also 100,000 and you've got 100dB of feedback.

Why is a transistor fundamentally any different?

If a transistor configured as a common-emitter amplifier with say 100 ohms for R_C gives you a gain of say 100, is not the same transistor configured as a common-collector amplifier with 100 ohms for R_E an amplifier which has a feedback factor of 100 and 40dB of feedback?

I don't see any fundamental difference here.

se

johnferrier · 26th February 2004, 11:05 AM

If an amplifier circuit has three stages: 1st stage gain = 15, 2nd stage gain of 75, and 3rd stage gain of 1, it has a open loop gain of 1125 (~67db). Feedback does not change the gain of each of the stages. The signal from the inputs is smaller with more feedback. The more feedback, the smaller the difference signal.

This is kind of interesting. So, with an op-amp with an OLG of 10,000. Configure it as a buffer. Apply 2VAC. Output is 2VAC. However, the difference signal at input of the first stage is 200uVAC. If the CLG is 10, then, of course, the signal is 2mVAC.

I imagine that something similar happens with single/compound active device circuits (meaning the gain of the devices remains the same OL v CL).

JF

johnferrier · 26th February 2004, 02:43 PM

I found out that my input stage is idling at 2mA (which I'm matching JFETs to) and has a maximum variation of 40uA with feedback. Output of the first stage is 3Vdc and maximum of 50mV ac. Open loop, of course, the variation is much higher.

JF

Ultima Thule · 26th February 2004, 04:24 PM

SE,

were you stunned ower my little "trick" in Post #95, or perhaps you didn't discover it...?

Do you think there are "feedback" in diodes too as well as with CC's!??

Cheers!

Steve Eddy · 26th February 2004, 05:28 PM

Quote:

Originally posted by Ultima Thule
SE,

were you stunned ower my little "trick" in Post #95, or perhaps you didn't discover it...?

Do you think there are "feedback" in diodes too as well as with CC's!??

No, I don't.

se

Nelson Pass · 27th February 2004, 03:08 AM

You guys are very entertaining (in a perverse way) but you
really need to look at the question from another angle.

An example would be: Ask the question, "What is different
about each transistor's experience in the two configurations?"

Steve Eddy · 27th February 2004, 03:15 AM

Quote:

Originally posted by Nelson Pass
You guys are very entertaining (in a perverse way) but you
really need to look at the question from another angle.

An example would be: Ask the question, "What is different
about each transistor's experience in the two configurations?"

Could you elaborate just a bit on what you mean by "experience"?

se

26th February 2004, 05:26 AM	#101
AKSA diyAudio Member Join Date: Sep 2001 Location: Melbourne, Australia	Because there is no open loop voltage gain, and therefore no feedback factor? Hugh __________________ Aspen Amplifiers P/L (Australia) www.aksaonline.com

26th February 2004, 07:11 AM	#103
AKSA diyAudio Member Join Date: Sep 2001 Location: Melbourne, Australia	OLG is fundamental to calculating feedback factor. The ratio of OLG to CLG is, as I'm sure you realize, the feedback factor. If you wish to prove me wrong, go right ahead. It's not important. In truth, the question is, as someone put it, roaringly funny. Hugh __________________ Aspen Amplifiers P/L (Australia) www.aksaonline.com

26th February 2004, 04:24 PM	#107
Ultima Thule diyAudio Member Join Date: Jan 2004 Location: Koskenkorva Land	SE, were you stunned ower my little "trick" in Post #95, or perhaps you didn't discover it...? Do you think there are "feedback" in diodes too as well as with CC's!?? Cheers! __________________ "If transistors are blueberries and FETs are strawberries, then tubes must be.. pears" Michael 29th January 2010

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26th February 2004, 11:05 AM	#105
johnferrier diyAudio Member Join Date: Sep 2003 Location: WA	If an amplifier circuit has three stages: 1st stage gain = 15, 2nd stage gain of 75, and 3rd stage gain of 1, it has a open loop gain of 1125 (~67db). Feedback does not change the gain of each of the stages. The signal from the inputs is smaller with more feedback. The more feedback, the smaller the difference signal. This is kind of interesting. So, with an op-amp with an OLG of 10,000. Configure it as a buffer. Apply 2VAC. Output is 2VAC. However, the difference signal at input of the first stage is 200uVAC. If the CLG is 10, then, of course, the signal is 2mVAC. I imagine that something similar happens with single/compound active device circuits (meaning the gain of the devices remains the same OL v CL). JF

26th February 2004, 02:43 PM	#106
johnferrier diyAudio Member Join Date: Sep 2003 Location: WA	I found out that my input stage is idling at 2mA (which I'm matching JFETs to) and has a maximum variation of 40uA with feedback. Output of the first stage is 3Vdc and maximum of 50mV ac. Open loop, of course, the variation is much higher. JF

27th February 2004, 03:08 AM	#109
Nelson Pass The one and only Join Date: Mar 2001	You guys are very entertaining (in a perverse way) but you really need to look at the question from another angle. An example would be: Ask the question, "What is different about each transistor's experience in the two configurations?"