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Old 6th May 2008, 07:26 PM   #1
corrieb is offline corrieb  South Africa
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Default Confusion: NFB, OLG, Unity Gain

Hi,

I don’t know how to phrase this question but maybe someone can help me. I have been doing some reading through Mr. D. Self’s books and it brought me to a point where I am confused when it comes to negative feedback.

Components C4, R10 and R11 from image2.pdf form the negative feedback. If we for a moment forget about C4 and the -3dB low pass roll-off point then the resistor components calculates to 30.4 dB. What do we call this value? Is this the negative feedback factor, ratio ...?

From what I have read it is best to choose an amount of xdB of NFB at 20 kHz to ensure stability. I have also learned that once open loop gain has reached P1 it falls with 6dB/octave when standard milliard compensation is used and the same goes for the NFB available.

For me to get the xdB NFB available at 20 kHz do I subtract the open loop gain at that specific frequency from the calculate component value as set above to get the available NFB at that frequency.

Image3.pdf is my calculations and image1.pdf is the ouput(top green), phase shift(bottom green), open loop gain(top red) and resistor calculation(top blue) for my design so far. When we talk about unity gain, when does this occur based on the simulation? Is this when open loop gain reaches the resistors calculated value?

If the questions are unclear please indicate.

Any assistance will be appreciated or reference to a threat or site.

Thanks CorrieB
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Old 7th May 2008, 05:47 PM   #2
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The 2 resistors set the circuit closed loop gain
everything beyond the 30.4dB is then feedback.
If there is 130.4 dB OLG an 30.4 Closed LG, then 130.4-30.4=100dB feeedback.
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Old 7th May 2008, 06:40 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

The resistors set the closed loop gain.

As Self states there is no fixed level of feedback.

Quote:
The open-loop (O/L) gain has two regimes;
flat below the lowest pole and falling at 6dB/octave above it. (HF)

LF gain is: LF gain= gm*beta*Rc Eqn 1
HF gain is: HF gain= gm/(w*Cdom) Eqn 2
Pole freq is: P1= 1/(Cdom*beta*Rc) Eqn 3

Where:gm is input stage transconductance. Beta is VAS current gain.
Rc is VAS collector impedance w= 2*pi*freq
The unity gain criteria is confusing until you realise the closed loop
gain attenuates the output by that amount at the inverting input.

So the open loop gain has to be lower than the closed loop gain
before the phase hits 180 degrees. The higher the closed loop
gain the less feedback there is and higher open loop bandwidth
can be used.

For a unity gain buffer you really have to slug the amplifier, as the
inverting input is at the output. For fast op-amps to be used at
reasonable gains it is undesirable that they are unity gain stable,
because at higher gains they will be overcompensated.

Selfs level of feedback at 20KHz comes from equation 2 and is a rule
of thumb in essence for typical output stages. In reality varying the
feedback factor at 20Khz for a given closed loop gain is also moving
the "unity gain" point.

/sreten.
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Old 8th May 2008, 12:17 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I'm going to take you a little further than you asked.
Let's interpret your gain and phase plots.
The OLG becomes 0dB @ 10MHz.
The phase is about 170 to 175 degrees @ 10MHz.
the phase margin is 180 - 170 = 10degrees at best.
Adding a tiny bit of capacitance to this amp will make it into an oscillator.
It will probably ring quite badly into a resistive load with a 5 to 10 degree phase margin.
Most recommend at least 45degrees of phase margin for good characteristics. However, even with an adequate phase margin, this will change when a reactive load or cables are added to the amp. You now need to compensate the amplifier to retain the near 45degree phase margin into a wide variety of reactive loads.
Some recommend 60degrees of phase margin but this can cut off transients and make the amp sound too smooth. This often happens when an inductive load is applied to the output of a poorly compensated amplifier.

So firstly. change the amp gain and phase to obtain an adequate phase margin at an open loop 0dB gain.
Then, do the difficult part. Compensate the amp to be tolerant to reactive loading.
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Old 8th May 2008, 01:22 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by AndrewT
Hi,
I'm going to take you a little further than you asked.
Let's interpret your gain and phase plots.
The OLG becomes 0dB @ 10MHz.
The phase is about 170 to 175 degrees @ 10MHz.
the phase margin is 180 - 170 = 10degrees at best.
Adding a tiny bit of capacitance to this amp will make it into an oscillator.
It will probably ring quite badly into a resistive load with a 5 to 10 degree phase margin.

......
Hi,

Completely wrong. It is conservatively overcompensated.
Gain margin is ~ 25dB and phase margin ~ 130 degrees.


/sreten.
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Old 8th May 2008, 01:24 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by sreten
Completely wrong. It is conservatively overcompensated.
Gain margin is ~ 25dB and phase margin ~ 130 degrees.
please explain.
Am I looking at the wrong plot/graph?
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Old 8th May 2008, 01:32 PM   #7
sreten is offline sreten  United Kingdom
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Hi, Gain is set at 30dB, not unity, /sreten.
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Old 8th May 2008, 01:37 PM   #8
AndrewT is offline AndrewT  Scotland
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Ignore your byline for a moment and indulge me.
Why can't I see what you're getting at?
A more detailed explanation may help me understand.
Quote:
So the open loop gain has to be lower than the closed loop gain
before the phase hits 180 degrees. The higher the closed loop
gain the less feedback there is and higher open loop bandwidth
can be used.
is the the crucial criteria?
Does my case only apply when unity gain is required?
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Old 8th May 2008, 02:00 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by AndrewT

Does my case only apply when unity gain is required?
Hi,

Yes. The higher the stage gain the less feedback there is.
e.g. a NE5534 op-amp is stable for gains > 3, but not unity.

/sreten.
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Old 8th May 2008, 02:09 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi Corri,
sorry.
I hope we both learned something.
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