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Old 5th April 2008, 12:02 PM   #1
fotios is offline fotios  Greece
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Default Open Loop Gain Measurement? Fotios Need Help!

I request from any of you has an advice for a PRACTICAL MEANS to measure the open loop gain of an amplification arrangement to give me it.
I have the book (not only the pdf version) of D. Self "Audio Power Amplifiers Design Handbook 2nd edition, for that i paid in the year 2000, 17.400 Drachmas=51Euros of today to by it. I have read it and the method proposed by D. Self it is known to me.
So, i ask for a different method.
Thanks in advance for your polite help in me.

Fotios
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Old 5th April 2008, 04:41 PM   #2
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No one response yet? Why? It is a so difficult issue?
To measure the open loop gain i thing, we must remove the main feedback node? maybe also the miller cap which is a nested feedback loop into the main feedback loop? And i don't know how many other nested and well hidden feedback loops into the main feedback of the amplifier?
By removing only the main feedback node, be it so if we close in a shielded box the amplifier as to be insensitive in external interference, only the touch of the scope probe in the output rail can cause great instabilitty and oscillations.
With which means the great companies make the measurement of the open loop gain?
Have any of the experts in this forum the polite kindness to explain me the instrumentation method used in their laboratories?

Fotios
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Old 5th April 2008, 05:12 PM   #3
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I've made open loop measurements before. I believe it is usually done with nested feedback in place, but global feedback removed, so as to find the best compensation network for global feedback. Even after doing it though, I still found the best solution was empirically determining the global feedback network. Doing it mathematically will definitely give you a good place to start, though. Note that I was doing this for a tube amplifier, a solid state amp may be a little more behaved. I used MATLAB with the simulink toolbox for this, but it can be done otherwise with any linear algebra system (MAPLE I believe is free). To actually measure the open loop gain/phase response of the amp, I just used an oscilloscope, but a vector network analyzer or gain/phase meter would definitely be the best way to go, if you can find a good one for the audio band. A computer sound card with some specialty software may be able to do this function with some attenuation and protection circuitry on the input.
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Old 5th April 2008, 06:00 PM   #4
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Quote:
Originally posted by Boris_The_Blade
I've made open loop measurements before. I believe it is usually done with nested feedback in place, but global feedback removed, so as to find the best compensation network for global feedback. Even after doing it though, I still found the best solution was empirically determining the global feedback network. Doing it mathematically will definitely give you a good place to start, though. Note that I was doing this for a tube amplifier, a solid state amp may be a little more behaved. I used MATLAB with the simulink toolbox for this, but it can be done otherwise with any linear algebra system (MAPLE I believe is free). To actually measure the open loop gain/phase response of the amp, I just used an oscilloscope, but a vector network analyzer or gain/phase meter would definitely be the best way to go, if you can find a good one for the audio band. A computer sound card with some specialty software may be able to do this function with some attenuation and protection circuitry on the input.
I have the new and powerfull software "Multi Instrument Pro-3" of Virtins Technology (compared only with SpectraPLUS and above) installed in my PC (Intel C2D E6850 3GHz, Intel M.B. DQ35JO - 1333 MHz FSB, 8Gb Sync. DDR2 800MHz, Win x64 O.S.) and an external sound card E-MU 0404 (192Ks/s locked internally - 24bit); as you can see there is not any most powerfull system from this (for domestic use at least) at this moment.
Tell me please, the circuit arrangement which is appropriate to make the measurement of the open loop gain.

Thanks a lot
Fotios
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Old 5th April 2008, 07:06 PM   #5
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I have also to express a reasonable query:

What for it is necessary IN PRACTICE the measurement of the open loop gain of an amplifier?

The measurement of the closed loop gain in a definite frequency bandwidth with definite also limits of amplitude variations (i.e. +/-3dB or +/-1dB around an average value) it is not enough?

The measurement of O.L.G. it is most important for academic discussions?

Or it is of big importance for the final behaviour of a given amplifying circuit?

Fotios
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Old 5th April 2008, 08:01 PM   #6
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To measure OLG in practice, you need a sinusoidal swept source of often low voltage to keep the amplifier under test away from clipping. It is important for any amplifier but is sometimes or often overlooked because it can sometimes be an unnecessary step in design. If there are feedback problems such as instability or oscillation, they can often be traced back to open loop response. Knowing the open loop response and the feedback factor (complex) can pinpoint troublesome frequencies where feedback becomes positive, and knowing that one can modify the circuit to (hopefully) correct it. I recommend reading up on phase margin and stability.
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Old 5th April 2008, 09:58 PM   #7
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Quote:
Originally posted by Boris_The_Blade
To measure OLG in practice, you need a sinusoidal swept source of often low voltage to keep the amplifier under test away from clipping. It is important for any amplifier but is sometimes or often overlooked because it can sometimes be an unnecessary step in design. If there are feedback problems such as instability or oscillation, they can often be traced back to open loop response. Knowing the open loop response and the feedback factor (complex) can pinpoint troublesome frequencies where feedback becomes positive, and knowing that one can modify the circuit to (hopefully) correct it. I recommend reading up on phase margin and stability.
Firstly thanks a lot for your interest.
It is easy to obtain a fine sinusoidal sweep from 20 to 20000Hz by my very good Hameg function generator of 10MHz and to inject the signal in the input. What must to do further? To disconect the feedback node of amplifier for making the measurement?

Fotios
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Old 5th April 2008, 10:43 PM   #8
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Default Goggle?

I used the words "measure open loop gain" and Google
I got lot's of hits.

http://www.ece.ucsb.edu/Faculty/rodw...pamp_hints.pdf
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Old 5th April 2008, 10:57 PM   #9
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Default Re: Goggle?

Quote:
Originally posted by peranders
I used the words "measure open loop gain" and Google
I got lot's of hits.

http://www.ece.ucsb.edu/Faculty/rodw...pamp_hints.pdf
Thanks P-A for the link. It is a base from which i can start to modify a most powerfull version to do the measurements directly in my big power projects.

Fotios
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Old 5th April 2008, 11:23 PM   #10
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Hi Fotios

It is difficult to measure the open loop gain of a high gain amplifier!

The current view is that the OLG should be measured without disturbing the feedback path.

To measure OLG using feedback components in place, ground the input. If the feedback resistor is Rf and the voltage tap resistor Rg, it is often the case that Rg is connected through a capacitor to keep DC gain low while increasing AC gain. Disconnect this decoupling point (capacitor or resistor) and apply a signal generator to this point instead. The open loop gain is obtained from the output voltage divided by the voltage measured at the feedback tap point.

You will need a sensitive low noise high impedance meter to measure the feedback tap point voltage, which can be around 1 mV for 10V output or so. Normally the gain would be measured with the normal load in place.

Maybe an oscilloscope would work if it did not cause oscillation. Some people have built a differential transistor pair to clip across the feedback point using a jumper wire to make this measurement, but of course this may change the base resistance. But a couple of FETS may work, and give a measureable gain (2 to 5x) which may be enough, even if they have a high distortion (the signal level is low so that even bipolars will be fairly linear- and can be degenerated to make sure).

cheers
John
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