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Old 28th April 2010, 11:46 AM   #641
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Quote:
Originally Posted by AndrewT View Post
and the difficulty as Iko has pointed out is how to determine the final gain that has been created.
Using +-0.1% resistors in two gain stages will give 60+-0.04dB accuracy. That's good enough for me. But how do we measure/confirm it?
Using 1% resistors gives a tolerance of ~+-0.4dB.
Ok, it is clear. I still haven't figured out what all the fuss is all about. +-0.04dB?? thats fantastic. Do you worry if your power amp has 28.6dB instead of the design 26.28??
IMHO building a calibrator to verify the gain of a 60+/-0.04dB measurement amp is, shall we say, unusual?
But YMMV.

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Old 28th April 2010, 11:54 AM   #642
AndrewT is offline AndrewT  Scotland
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no fuss. He was asking and did not get an answer.

I built a precision attenuator using T resistors for attenuation of -0.05dB to -20.0dB. By cascading these T attenuators I can set up any attenuation from 0dB to -61dB in steps of 0.05dB.
But, I have a problem. The -10dB attenuator is within my accuracy tolerance.
I have no way to check that the -20dB attenuators are within my selected tolerance limits. I just have to hope that my DMM techniques have got it good enough.
When I use >19.95dB attenuation I rely on the accuracy of the -20dB ( or 2 of them). But I cannot measure it. Iko is trying to solve the same problem it seems.
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Old 28th April 2010, 12:02 PM   #643
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Quote:
Originally Posted by AndrewT View Post
no fuss. He was asking and did not get an answer.

I built a precision attenuator using T resistors for attenuation of -0.05dB to -20.0dB. By cascading these T attenuators I can set up any attenuation from 0dB to -61dB in steps of 0.05dB.
But, I have a problem. The -10dB attenuator is within my accuracy tolerance.
I have no way to check that the -20dB attenuators are within my selected tolerance limits. I just have to hope that my DMM techniques have got it good enough.
When I use >19.95dB attenuation I rely on the accuracy of the -20dB ( or 2 of them). But I cannot measure it. Iko is trying to solve the same problem it seems.
Use the -10dB attenuator. Measure the output voltage.
Then use the -20dB atten. Measure the voltage. Calculate the error.
The absolute accuracy of your meter is irrelevant, it's the ratio that's important. Only hope that the meter is linear between the 2 indications.
If the 20dB att is switchable, set to -10 and measure that way.
Depending on the att Zin, make sure you drive it from a low impedance.

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Old 28th April 2010, 12:14 PM   #644
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Originally Posted by janneman View Post
Use the -10dB attenuator. Measure the output voltage.
Then use the -20dB atten. Measure the voltage.
that cannot confirm the accuracy of the -20dB attenuator.
I have thought about this a lot over the last 12years since I built the attenuator, but so far I have not found a method available to me that can confirm the accuracy of the higher attenuation settings. Using a DMM cannot do that.
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Old 28th April 2010, 12:26 PM   #645
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Quote:
Originally Posted by AndrewT View Post
that cannot confirm the accuracy of the -20dB attenuator.
I have thought about this a lot over the last 12years since I built the attenuator, but so far I have not found a method available to me that can confirm the accuracy of the higher attenuation settings. Using a DMM cannot do that.
Yes you can get the -20dB att accuracy related to the -10dB, which as you said is within your requirement.

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Old 28th April 2010, 02:00 PM   #646
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Sorry about the fuss Jan

The reason I would still want the gain calibrator is this. After I built the LT1028 amp, out of curiosity, I also built a discrete amp with four 2SK170V paralleled in the first stage cascoded by a 2SC1571, and one 2sk170BL in the second stage. With the opamp it is indeed a matter of faith and using precision resistors for setting the gain. For the discrete amp it's a bit of a different matter. I would like to keep the discrete amp for low noise measurements because it shows lower self-noise at the output than the LT1028 based amp.

Normally I wouldn't care about exact gain, but if this amp is used as an instrument, it would be nice to have somewhat clear specs for it.

Andrew, I'll start a new thread soon about the calibrator, and perhaps we can talk about your attenuator there, I'm interested in what you've explored.
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Old 28th April 2010, 02:13 PM   #647
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Sorry about the fuss Jan

The reason I would still want the gain calibrator is this. After I built the LT1028 amp, out of curiosity, I also built a discrete amp with four 2SK170V paralleled in the first stage cascoded by a 2SC1571, and one 2sk170BL in the second stage. With the opamp it is indeed a matter of faith and using precision resistors for setting the gain. For the discrete amp it's a bit of a different matter. I would like to keep the discrete amp for low noise measurements because it shows lower self-noise at the output than the LT1028 based amp.

Normally I wouldn't care about exact gain, but if this amp is used as an instrument, it would be nice to have somewhat clear specs for it.

Andrew, I'll start a new thread soon about the calibrator, and perhaps we can talk about your attenuator there, I'm interested in what you've explored.
The issue is the same for opamp or discrete, the secret is to measure or calculate the open loop gain. Even an opamp has a gain that is lower than the calculated value from the resistors; it can be precisely set if you know the open loop gain with the wellknown formula Gcl=Gol/(1+beta*Gol) where Gcl is closed loop gain, Gol is open loop gain and beta is the attenuation of the feedback network. You can go down as deep as you like by calculation the impact of, say, the Zout on the feedback resistor value, or the Zin on the attenuation value.

The same procedure is followed with a discrete of course; the formula's have no way to know what type of amp they are being used for

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Old 28th April 2010, 03:12 PM   #648
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Doesn't variation between device samples throw off the calculations? Maybe I don't understand the issue correctly, but if I take the data sheet specs for a 2sk170 and then use the formulas with precision resistors then I'll trust the gain is within some set tolerance, irrespective of device variation? I'm asking because I don't have experience with this.
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Old 28th April 2010, 03:30 PM   #649
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Originally Posted by ikoflexer View Post
Doesn't variation between device samples throw off the calculations? Maybe I don't understand the issue correctly, but if I take the data sheet specs for a 2sk170 and then use the formulas with precision resistors then I'll trust the gain is within some set tolerance, irrespective of device variation? I'm asking because I don't have experience with this.
Yes, that's the reason (one of them) that we use feedback, to eliminate device variations, aging, what have you.
Of course all bets are off if you use the amp open loop. Then you need to individually adjust for device variations and you are subject to aging, drift...
I don't know which type your discrete is, is it open loop?

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Old 28th April 2010, 03:43 PM   #650
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In fact, Art Kay demonstrates the thermal effects in one of the papers -- the thermal effect can seem as if it's noise. This is why Williams mounts the JFETs in (I guess) some kind of potting compound.
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