Gain Measurement test

[stanleystan]

Gain Measurement Test:

Step#1.) Set the input voltage either AC or DC at the nominal input voltage and measure the output voltage
Step#2.) Now set the input voltage to Half of the nominal input voltage and measure the output voltage
Step#3.) Now set the input voltage to double of the nominal input voltage and measure the output voltage

The Ratio for each step1 thur step3 would all have the same ratio gain? true or false?

The Gains Ratio should be the same if the input voltage is half or double the nominal input voltage?

What kind of gain measurement test is this called?

Why would a tech do this type of test for what application?

Does this test actually tell you if the gains of each stage is working and all the op amps are working? the resistor and cap values are in tolerance?

If the circuit under test has multiple parallel paths and feedback paths would this gain measurement test still work?

Having parallel paths on the input or output of gain stages would add or subtract to the gain ratio of the gain stage? this would change the gain input and output ratio depending on the inputs voltage?

 

[Audioguru]

You can have an opamp in a normal circuit work normally or have the same opamp in the same circuit with its output overloaded. An oscilloscope will show an overloaded output that is clipping and a meter will show lower than normal output level. You might think that the gain is reduced instead of seeing than the output is overloaded.

You need to understand which resistors and capacitors in a circuit set its gain.

Gain for an opamp is determined by negative feedback in a 2-resistors voltage divider. If one of those resistors has another resistor switched to be in parallel with it then the gain increases or decreases depending on which resistor in the voltage divider.

 

[stanleystan]

The Ratio for each step1 thur step3 would all have the same ratio gain? true or false?

The Gains Ratio should be the same if the input voltage is half or double the nominal input voltage?

What kind of gain measurement test is this called?

Why would a tech do this type of test for what application?

 

[Audioguru]

We do not know if the amplifier is linear (output level is gain x input level) or if it has too much input level which causes the output to clip, or if the amplifier is logarithmic. It might be a compressor or expander amplifier.
If its output level is only a little below clipping with a nominal input level then obviously the output will be clipping when the input level is doubled and it is a linear amplifier.

Usually a sinewave is the input for a gain test and an oscilloscope shows the output which must not be clipping, must not have hum and must not have noise. A tech would test the gain if the output level is wrong when the input level is correct.

Aren't you called Walters, Billy Mayo, Danny Davis, Castillovanv, Vancastillo, ParkerMike, PrescottDan and many other names that have been banned from here and from other electronics forums??

 

[stanleystan]

A tech would test the gain if the output level is wrong when the input level is correct.

Yes i know

But what is this gain test called when you half the input and measure the output and then double the input and measure the output?

What type of gain test is this? it is used for what?

When you half or double the input signal the output should have the same gain ratio? or will the gain ratio be different?

"This is for linear amps"

We do not know if the amplifier is linear

It's linear, all the gain stages are linear

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When you half or double the input signal and measure the output voltage

Does this test for sure tell you that all the Op amps or transistors in the stages from first input stage to the last output stage are working?

Or does this test not tell if the Op amps or transistors in all the stages are working

 

[Audioguru]

If an amplifier is linear then its gain does not change when its input level is changed. Why bother testing gain at different levels?

If you feed a sinewave to the input then all the stages are working properly if the output is at the correct level with no distortion, no hum and no noise.
You should connect its rated load and you should see that the loaded output level goes as high as normal.
What about heating and supply current? Measure them to see if they are normal.

 

[PrescottDan]

If an amplifier is linear then its gain does not change when its input level is changed. Why bother testing gain at different levels?

a tech told me this is a trick to do

you test the gain at different input levels, this will verify that all the op amps , transistors are working? is this true?

Also if the resistors and caps are out of tolerance and the caps are leaky in the gain stages would this test hold true?

Why would a tech want to test the gain at different input levels? what is this really testing?

Each gain stage is linear, so the output is linear right? yes

Why bother testing gain at different levels?

The tech told me this is a way of check the gain ratio at different input levels

the output voltage ratios should be the same for
half the input voltage
Nominal input voltage
double the input voltage

If the ratio is different for all 3 then the transistors beta , IC has gain issues , the resistors are out of tolerance, the capacitors are out of tolerance, the gain capacitor are leaky

 

[Audioguru]

The gain of a linear amplifier is the same at any level even if it is defective. If it is biased wrong or if an output transistor is blown out then it will clip the output which can be seen on an oscilloscope. A meter might wrongly show that the voltage gain is too low.

If a gain setting resistor is out of tolerance then the gain at any level will be wrong but the gain is the same at any level.
If a capacitor value is wrong then an AC amplifier will have a frequency response problem.

You are measuring voltage gain, not current gain. Beta is a transistor's current gain and it has nothing to do with its voltage gain.
If the beta is too low then a transistor will be biased wrong then its output voltage will be wrong and it will clip some of its output waveform.

 

[stanleystan]

The gain of a linear amplifier is the same at any level even if it is defective.

Ok

So Why would a tech want to test the gain at different input levels? what is this really testing? does it test the gain ratios?

Or is this test does nothing at all? its a joke

If it is biased wrong or if an output transistor is blown out then it will clip the output which can be seen on an oscilloscope. A meter might wrongly show that the voltage gain is too low.

Why would a meter show a the voltage gain is to low? because it's RMS AC voltage? how about for DC?

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The gain of a linear amplifier is the same at any level even if it is defective.

If the output gain ratio is incrementing down when incrementing up the input level

What is wrong with the gain circuit? i'm talking about multiple gain stages

A good working Op amp gain:
It should be linear, the input level increments up and the output increments up

 

[Audioguru]

In my opinion, the tech who said to test gain at 3 different levels does not look on an oscilloscope to see if the output is too high and is clipping.

A meter shows level only, it does not show that the output is clipping. If the output of the opamp is too high and is clipping then the meter will wrongly show that the gain is too low. Why don't you understand that if the gain is 10 and the input is 2V then the output should be 20V, but if the supply is only 15V then the meter will measure 15V/2= a wrong gain of 7.5 instead of the correct gain of 10.

If the output level of an amplifier goes down when the input level goes up then the amplifier is not linear and it will show severe distortion of a sinewave on an oscilloscope. Don't use only a meter because a meter is blind.

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Why are you measuring gain anyway? The gain of an opamp is determined by its two negative feedback resistors, and sometimes some capacitors. Do your circuits use lousy resistors and capacitors?

I designed a complicated equalizer for a new speaker. It used 4 opamps, many resistors and many capacitors. I designed a tester for it that had limits so that each passing circuit has high quality. It accurately measured gain, the frequency response of the complicated ups and downs at certain frequencies, maximum output level and noise. I had 15 thousand of them made and tested. Only 2 failed: One had an IC installed backwards and the other had a shorted electrolytic capacitor. Therefore all 60,000 opamps, many resistors and many capacitors worked perfectly. All were sold and after one year not one was returned for being faulty.

 

[stanleystan]

Why don't you understand that if the gain is 10 and the input is 2V then the output should be 20V, but if the supply is only 15V then the meter will measure 15V/2= a wrong gain of 7.5 instead of the correct gain of 10.

The meter will measure the gain wrong because its measuring AC RMS compared to an Oscope it measure true

But the Oscope also is going to measure the gain at 15 volts because of the power supply is at +15 , it can't go any higher because of the power supply

The Oscope is not going to display +20 volts, it will be +15 volts still

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In my opinion, the tech who said to test gain at 3 different levels does not look on an oscilloscope to see if the output is too high and is clipping.

True, but if you use the Oscope to set gain at 3 different levels what kind of gain test is this? you're checking the gain ratios at 3 different levels right?

If the stages are linear then all the gain ratios at 3 different levels should be the same and linear?

 

[Audioguru]

The meter will measure the gain wrong because its measuring AC RMS compared to an Oscope it measure true

You are silly. If the voltage gain is 10 for a peak voltage then it is also 10 for an RMS voltage because then the input and output are both peak or are both RMS.

But the Oscope also is going to measure the gain at 15 volts because of the power supply is at +15 , it can't go any higher because of the power supply

The Oscope is not going to display +20 volts, it will be +15 volts still

But the o-scope will show that the output is wrong because it is clipping. A meter is blind and does not show clipping.

if you use the Oscope to set gain at 3 different levels what kind of gain test is this?

It is simply a waste of time.

If the stages are linear then all the gain ratios at 3 different levels should be the same and linear?

Of course.

 

[PrescottDan]

But the o-scope will show that the output is wrong because it is clipping. A meter is blind and does not show clipping.

The Meter and O-scope will have the same voltage

The only difference is that the Oscope you can see the clipping of AC waveforms

Yes on a Oscope you can see the noise, ripple, ringing, clipping, distortion

A meter will not show you all this

But both a meter and Oscope will have the same voltage

 

[Audioguru]

If the gain is 10, the input is a 2V peak sinewave and the supply voltage for a rail-to-rail opamp is 15V then its output will be saturate at 15V because its supply is too low for its output to go to 20V.
The meter reading will be wrong but the 'scope will show the clipping on one-half of the waveform.

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If the gain is 10, the input is a 2V peak sinewave and the supply voltage for a rail-to-rail opamp is 15V then its output will be saturate at 15V because its supply is too low for its output to go to 20V.
The meter reading will be wrong but the 'scope will show the clipping on one-half of the waveform.

This website freezes frequently.

 

[stanleystan]

If the gain is 10, the input is a 2V peak sinewave and the supply voltage for a rail-to-rail opamp is 15V then its output will be saturate at 15V because its supply is too low for its output to go to 20V.
The meter reading will be wrong but the 'scope will show the clipping on one-half of the waveform.

Yes true, but the meter reading will be 15volts and the Oscope will be 15volts also

Unless i'm missing something

Not sure why the meter reading would be less only when a waveform is saturated or clipping , 15volts is 15 volts

 

[Audioguru]

The 'scope shows the clipped waveform but the meter shows the RMS amount which is almost an average. The RMS or average measurement is low (wrong) because half of the clipped waveform is missing.
The 'scope shows that the top part is clipped but the bottom part is normal. Then the bottom part can be used to determine the gain.