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Inverted op amp

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fleur_

New Member
Earlier I wrote:


You have a table of output voltages at various frequencies.
You are looking for the gain of the amplifier, so you need to know the input voltage.
The gain is then the output volts divided by the input volts.
The gain can then be converted to dB using dB = 20Log10(V/v).

You do not state the input voltage so I am going to make the assumption that it was 200mV (0.2 volts).
Here is my interpretation of your results:

View attachment 96626

Using this table I produced this graph:

View attachment 96627


That is not too bad is it?

JimB
Thank you very much I also used a logarithmic scale base 10 for the frequency on the x axis as well as the gain because I forgot to mention that to you.So i just changed that myself. and ended up with this
upload_2016-1-7_9-32-58.png
 

fleur_

New Member
The discussion section in this report requires you to write make Comments on: The validity of any assumptions made. Practical problems encountered. Mistakes made. Any areas requiring more investigation.
I am just not too sure on how to do this bit
 

fleur_

New Member
Hi fleur_,

Your calculation is correct. In an inverting amplifier the gain is determined by the ratio of Rf/Rin. If you measured a gain of 10, instead of -10, then your meter probes were probably backwards.
Does that mean I can say that the difference between the theoretical voltage gain and actual gain is 20 and would the percentage error be 50%
 

audioguru

Well-Known Member
Most Helpful Member
You are mixing up the voltage gain of 10 times with the voltage gain of 20dB which are the same. The negative voltage gain is not a loss of gain, instead it is the output signal that has become inverted.
A voltage gain of 10 times= +20dB, 100 times= +40dB, 1000 times= +60dB, 10,000 times= +80dB etc. A voltage loss of 10 times is a loss of -20dB.

One assumption that you made was that the output resistance of the signal generator is zero ohms.
A second assumption that you made is that the signal level is much lower than the 2Vp-p input shown on your schematic so that the poor slew rate of the lousy old (47 years old design) 741 opamp does not cut and distort the high frequencies.
A third assumption that you made is that you think all 741 opamps are the same which is not true. You tested only one and the datasheet shows only a typical one. But they are not all the same, some are worse and some are better.
 

fleur_

New Member
For the conclusion Key achievements and results. • Report on how closely the objectives have been met. What recommendations have been made
 

DerStrom8

Super Moderator
Does that mean I can say that the difference between the theoretical voltage gain and actual gain is 20 and would the percentage error be 50%

Since you were dealing with an AC input, I expect you failed to notice that the output was 180 degrees out-of-phase with the input. The output should have a -10x gain (the negative means it's 180* out of phase).
 

fleur_

New Member
I am trying to calculate the quantities labelled in this schematic Vr1,Vr2,ir3 and ir4 so i have to use ohms law but combine it with the rules for series and parallel resistors.
What I did so far is for the first two resistors in series I=V/R therefore 10V/100k+47 not sure if this is the correct way of doing it ....
upload_2016-1-7_21-17-6.png
 

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DerStrom8

Super Moderator
Be careful about your order of operations. Saying 10V/100k+47k won't give you the same answer as 10V/(100k+47k). In this case you need to make sure you add the resistors first.

That being said, if you're trying to find Itotal first (which I recommend), then you need to combine all of the resistors. Start with the series resistors and get one equivalent resistor, then combine the parallel resistors and get one equivalent resistor, and then you'll see that you simply have two resistors in series after that. Simply combine them, divide the 10v by the Rtotal you calculated by combining R1, R2, R3, and R4, and you have Itotal. That should give you a good start.
 

audioguru

Well-Known Member
Most Helpful Member
R1 and R2 are in series so their total resistance is 147k ohms.
R3 and R4 are in parallel so their total is 31.972789k ohms. Then the total resistance is 147k + 31.972789k= 178.97278k ohms and the total current and current in R1 and R2 is 10V/178.97278k= you figure it out.
The total current is known then the voltage across R3 and R4 can be calculated and their currents can be calculated.
Why did your teacher select 47k that gives very long numbers?
 

fleur_

New Member
Be careful about your order of operations. Saying 10V/100k+47k won't give you the same answer as 10V/(100k+47k). In this case you need to make sure you add the resistors first.

That being said, if you're trying to find Itotal first (which I recommend), then you need to combine all of the resistors. Start with the series resistors and get one equivalent resistor, then combine the parallel resistors and get one equivalent resistor, and then you'll see that you simply have two resistors in series after that. Simply combine them, divide the 10v by the Rtotal you calculated by combining R1, R2, R3, and R4, and you have Itotal. That should give you a good start.

Yeah that's what i did initially I found i total but for some reason I thought it was wrong.So i did 10v/100k+47k+100k+47k because ohm's law is I=V/R so now I have I which is 1/294000A
But I am stuck here.
 

DerStrom8

Super Moderator
Yeah that's what i did initially I found i total but for some reason I thought it was wrong.So i did 10v/100k+47k+100k+47k because ohm's law is I=V/R so now I have I which is 1/294000A
But I am stuck here.

The values of parallel resistors do not add. Only series resistors add up.
 

audioguru

Well-Known Member
Most Helpful Member
Read my post #29 again. Two resistors in parallel have a value that is less than either one.
 

JoeJester

Active Member
How does one jump from having difficulties in the post 1 problem to having difficulties in the post 27 problem?

Did he schooling not begin at the basics?
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Hi fleur_,

Your calculation is correct. In an inverting amplifier the gain is determined by the ratio of Rf/Rin. If you measured a gain of 10, instead of -10, then your meter probes were probably backwards.

Matt's post was a little off, I think. The gain is -10. let's go back to the definition of "inverting"? There should be a (-) sign somewhere.

With sine wave in and a sine wave out, it's kinda tough to see.
 

DerStrom8

Super Moderator
Matt's post was a little off, I think. The gain is -10. let's go back to the definition of "inverting"? There should be a (-) sign somewhere.

With sine wave in and a sine wave out, it's kinda tough to see.

I was saying that it should be -10, and if the measured value was +10x the input then the probes were probably crossed (though when I learned the OP was putting in AC I mentioned the output would actually be 180* out of phase with the input).
 

DerStrom8

Super Moderator
That's not how it came across to me. No big deal.

I guess I don't see how it could have come across otherwise....?

so for the theoretical voltage gain i got -10
Your calculation is correct. In an inverting amplifier the gain is determined by the ratio of Rf/Rin. If you measured a gain of 10, instead of -10, then your meter probes were probably backwards.
Since you were dealing with an AC input, I expect you failed to notice that the output was 180 degrees out-of-phase with the input. The output should have a -10x gain (the negative means it's 180* out of phase).
 

fleur_

New Member
Does anyone have some spare time to check my lab report for me? it would be appreciated.
I can send you the report via message or email because it wont let me attach it on here.
 
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