Op amp gain and resistors

In summary, the conversation is about a circuit designed to amplify ultrasound signals using two operational amplifiers. The first stage of the circuit has a problem where the signal is only amplified by a factor of 5 instead of the expected 150. The second stage works correctly. The conversation discusses possible reasons for this issue, such as limited bandwidth and incorrect biasing. Suggestions are made to try using a different op amp model and adjusting the feedback resistor values.
  • #1
nadersb
28
0
hello,
I have a strange problem!
I wanted to amplify my signal and i used 2op amps, each of them has a feedback resistor between (-) pin and output( pin 2 and 6 in 741) the ratio of resistors(r2/r1) is 150, but my signal only amplify 5times! I change the R2 resistor but nothing changed! i put different resistors but all of them had the same results! I changed my op amp but still the same results! the second op amp worked perfectly but the first stage has this problem!
does anybody have an idea what's wrong with the op amp?! you can see the circuit in the link below:( the value of capacitors are 0.47uf and the first feedback resistor is not 1M it is 1.5M)
http://www.2shared.com/photo/a_JmZITS/Untitled.html
 
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  • #2
With no signal input, what is the DC voltage at each of the op amp outputs?
 
  • #3
after a few second they become zero.
 
  • #4
nadersb said:
hello,
I have a strange problem!
I wanted to amplify my signal and i used 2op amps, each of them has a feedback resistor between (-) pin and output( pin 2 and 6 in 741) the ratio of resistors(r2/r1) is 150, but my signal only amplify 5times! I change the R2 resistor but nothing changed! i put different resistors but all of them had the same results! I changed my op amp but still the same results! the second op amp worked perfectly but the first stage has this problem!
does anybody have an idea what's wrong with the op amp?! you can see the circuit in the link below:( the value of capacitors are 0.47uf and the first feedback resistor is not 1M it is 1.5M)
http://www.2shared.com/photo/a_JmZITS/Untitled.html

I don't see anything wrong of the circuit. Basically it is powered by 9V and the +ve input biased at 4.5V midway. All stages are AC coupled so no DC offset problem to talk about.

1) First posibility is: My question is what is you frequency input to see only gain of 5. Remember 740 has gain bandwidth of 1MHz, if you have first stage gain of 150, the -3dB bandwidth is only 1MHz/150 which is less than 10KHz. The circuit talk about Ultra Sound signals, what is the frequency?

The second stage has closed loop gain of 10, the frequency response using a 741 is about 100KHz so it is very different.

Give me the input frequency before we talk more.

2) What is the output impedance of the detector/receiver at the left side of the schematic that drive the first op-amp? It is a current devices like a detector diode or photo detector? If so, your analysis is wrong. That is a current devices and the first stage is actually a TRANSIMPEDANCE amp with the transimpedance gain of 1M ohm or 1uA per volt. There is inconsistency with this theory as you change the R2 and nothing happened. But don't dismiss this just yet because it might still be a combination of bandwidth limited and detector characteristic. Please give me the kind of detector you use to be sure.
 
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  • #5
If your output becomes zero after a few seconds then perhaps your 4.5 V bias is the problem. Can you check pin 3 on both op amps and see if they have 4.5 V on them? I suspect your circuit isn't wired exactly like the diagram.

Once you are able to get a constant 4.5 V at the output of each op amp, why don't you try a gain of 15 instead of 150 and see what happens?
 
  • #6
I use ultrasound transducer, and input signal is 40khz with 30 mv amplitude(pick to pick)
there is a 33ohm resistor parallel with it as you see in the image that i have sent.
what if I change the op amp, and try different model that is compatible with this frequency?! do you have any suggestion which model would be suitable?!
 
  • #7
I double checked my connections, and I don't think that (+) pin's bias has any problem, but I will check it again and I will inform you ;)
 
  • #8
nadersb said:
I use ultrasound transducer, and input signal is 40khz with 30 mv amplitude(pick to pick)
there is a 33ohm resistor parallel with it as you see in the image that i have sent.
what if I change the op amp, and try different model that is compatible with this frequency?! do you have any suggestion which model would be suitable?!

How did you decide on 33 ohms? What happens to your gain if you replace it with 100k ohms? What values are your capacitors? If your signal is 40 kHz, try using a 270k feedback resistor for each op amp.
 
  • #9
because we use signal generator instead of transducer most of the time and its resistor is 50 ohm so we chose a resistor that is near to 50 ohm .
each capacitor is 0.47uF.
I didnt change the value of input resistor, I will change it and inform you what happens;)
 
  • #10
I understand now about the 33 ohm resistor. Still I would prefer a value of 100k or higher at least when using a transducer. The lower the value, the more it will lower the Q and sensitivity of the transducer. A gain higher than you need can cause the op amp to oscillate if the right phase shift occurs. A good rule of thumb is that 60 dB of gain is the most you can achieve on the same board at the same frequency. It is possible to get positive feedback through the power supply, ground or other connections with gains that high.

It still is important that you have 4.5 V at the output of each op amp with no signal in.
 
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  • #11
nadersb said:
I use ultrasound transducer, and input signal is 40khz with 30 mv amplitude(pick to pick)
there is a 33ohm resistor parallel with it as you see in the image that i have sent.
what if I change the op amp, and try different model that is compatible with this frequency?! do you have any suggestion which model would be suitable?!

I don't see 33ohm, can you double check, I cannot see the entire picture of the signal detector.

40KHz is too high for 741 with gain of 100. Maybe try something like OP-37 or others that has higher gain bandwidth product. But I think you have bigger problem than that.

Did you say you read 0V at the output of the both op-amps(pin 6)? If so, that is wrong, you should get 4.5V on both. I think Skeptic2 is doing a good job helping you, I don't think you need two people telling you what to do. Good luck.
 
  • #12
I will check pin 3, but what if pin 3 is 4.5?! what else could be the problem for zero output?!
and last thing sir, what do you mean by "bigger problem"?! what is it?!
thank you
 
  • #13
i know this sounds dumb but ---

National Semconductor's engineers used to field this problem so often they put it into an application note...

"how to" schematics usually don't show power connections to the IC
and often newbies don't connect power because it's not shown

and i notice that circuit linked doesn't show power to pins 4&7
and it's easy to mis-wire a round can opamp when you're looking at it from the bottom and using a top view wiring diagram
http://en.wikipedia.org/wiki/File:LM741_Pinout_Round.svg

dont be offended by this dumb question its just i have no idea as to your level of expertise.
i'm not offended if you ignore this post. You can even resent me it's okay.


lastly what is your source for 9V?
If it's a small battery place a few microfarads across it - reason is opamps demand a low-impedance power supply which a BL006P battery is not.

as yungman observed skeptic is doing a good job with technical advice.

but I don't mind asking the real dumb questions.
hope it doesn't offend you guys
and i won't meddle further

old jim
 
  • #14
:D
dont worry I connected 7 and 4 to Vcc and ground;) I am working with a Dc voltage source.
I will be thankful for your ideas ;)
 
  • #15
nadersb said:
I will check pin 3, but what if pin 3 is 4.5?! what else could be the problem for zero output?!
and last thing sir, what do you mean by "bigger problem"?! what is it?!
thank you

What I meant is you have some basic wiring problem. You should get 4.5V at the output on both of the op-amp. If it float to 0V, something is very wrong. I suggest you check your wiring and check for cold solder joint. Get the 4.5V on both output before you worry about the gain. Measure pin 3 is 4.5V, pin 2 should follow pin 6 because output is connect to pin 2 through R2.

Measure and verify pin 7 is 9V and pin 4 is 0V. After you get the correct DC reading, if Skeptic is not around, I'll keep helping you.
 
  • #16
I checked it today and pin 6 is around 4.5 in both op amps;) so this problem is solved;) I also changed the input resistor( 33ohm) and it worked, I mean when I used 33ohm ac voltage of pin 2 was about 10 mV pick to pick but now we have much more better signal and its about 25pick to pick, so thank you for your suggestion, but I didnt change the op amps yet, when I do it I will inform you;)
thank you
 
  • #17
With the new values how much signal are you getting out of the op amps?
 
  • #18
about 4 V pick to pick, but the problem is we amplify noise too, and i think we should add a filter to our circuit!
 
  • #19
nadersb said:
I checked it today and pin 6 is around 4.5 in both op amps;) so this problem is solved;) I also changed the input resistor( 33ohm) and it worked, I mean when I used 33ohm ac voltage of pin 2 was about 10 mV pick to pick but now we have much more better signal and its about 25pick to pick, so thank you for your suggestion, but I didnt change the op amps yet, when I do it I will inform you;)
thank you

I really don't understand what is pick to pick! When the op-amp is working, you really should not see any signal on pin 2. It is a summing junction. What I want you to tell me is what is the input to R1, what is the output at pin 6 of the first op-amp.

If your input frequency is 40KHz, if you are using 741 of gain bandwidth product of 1MHz, the -3dB point is about 10KHz. Above 10KHz, the gain will roll off at 6dB per octave. 40KHz is two octave above 10KHz, so your gain will go down 12dB which result of a gain of 25 instead of 100. This mean if you have 1mV at the input of R1, you will have 25mV at pin 6 of the first op-amp.

I do not see the 33ohm in your diagram, so I don't know that part. One thing is the way your circuit is, you are assuming that the output impedance of the detector is very low impedance. If the output impedance is high, your gain is

[tex] A_{cl}=\frac{R_2}{R_1+R_{det}} \;\hbox { instead of }\; \frac {R_2}{R_1}[/tex]

So the whole thing is not as simple as you think. First you have to show me what kind of detector you use as I asked on my first post. And show me the complete circuit where is the 33ohm.
 
  • #20
What I fail to understand is why we haven't switched to a better op-amp than the 741. Get a good op-amp and then switch back and forth. It will be a good lesson on frequency response.
 
  • #21
I am not convinced that he is really running a gain of 100. I need to see what kind of detector first as a lot of these kind of detectors are current output and you cannot look at it as a voltage source. Yes, I think OP37 will work a mile better than 741. I am sure there are newer and better ones as I used OP37 like 20 years ago. It is a much lower noise amp also design for higher close loop gain. I want to make sure his circuit is really doing what it supposed to do before sending him out to buy another op-amp.
 
  • #22
would it be meddling to offer a simple, non-intrusive test?
I said i wouldn't meddle
but old troubleshooters habits die hard.

I always check the dumb things first - it's less embarassing to find them one's self.
And if it's not one of the dumb things, well, you don't have to admit to anybody that you were dumb enough to have looked.
so here's something you can check quietly

>>Reasoning:<<
As Yungman and Skeptic observed, 741 is not a real fast opamp.

it is asking a lot of the 741 to amplify 40 khz by gain 150
though by datasheet it looks like it should do it

1 volt sinewave at 40 khz has dv/dt of 0.25 v/usec
and you are trying to make 15mvX 150 = 2.25 v
which is dv/dt of 0. 565 v/usec

National's datasheet
http://www.national.com/ds/LM/LM741.pdf [Broken]
near bottom of page 3
states 741 'slew rate' is 0.5 v/usec
so you're using it all
but that datasheet is written for +/- 15 volt supply not your +/- 4.5

Your 741 might be short of "oomph" at reduced supply
see this appnote
http://www.national.com/an/LB/LB-19.pdf [Broken]
bottom of page 1:
"The slew rate limit is the maximum rate of change of the amplifier’s output voltage and is due to the fact that
the compensation capacitor inside the amplifier only has finite currents available for charging and discharging...
..For example the full-power bandwidth of the LM741 with a 0.5V/μs Sr is approximately 6 kHz ..."

At your reduced power supply voltage, you may be just asking too much of it --

>>Suggestion:<<
You could test that thought by simply raising your power supply to 30 volts while watching output with an o'scope

if it doesn't help - you don't have to admit you looked.
But either way you'll learn whether "Slew Rate" is affected by supply voltage.

old jim
 
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  • #23
Not meddling, the more the merrier!:approve:
 
  • #24
Nadersb, so you're getting 4 V peak to peak out with the signal generator set at 30 mV and with 25 mV at pin 2 of the op amp. Is that 4 V using both op amps or just one?

You should check the power supply for noise. The noise may disappear once you start using the ultrasonic transducer. It in itself acts as a pretty good filter. If that's not enough, there are some pretty good active filter circuits that will also give you gain.
 
  • #25
Mr.yungman, sorry about the pick, I meant peak;) it was mistake!
I am working with an ultrasonic transducer( I attach the picture of it)
about 33ohm: do you see the resistor that is parallel with the transducer?! that is 33ohm, but after mr.skeptic comment I changed it to 100k.
Am I answer all of your questions sir?!
thank you for your attention
 

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  • #26
mr.avreagesupernova, I am busy these days so I couldn't buy new op amps, but I will on saturday;)
 
  • #27
mr.jim hardy, thank you I will test it;)
 
  • #28
nadersb said:
Mr.yungman, sorry about the pick, I meant peak;) it was mistake!
I am working with an ultrasonic transducer( I attach the picture of it)
about 33ohm: do you see the resistor that is parallel with the transducer?! that is 33ohm, but after mr.skeptic comment I changed it to 100k.
Am I answer all of your questions sir?!
thank you for your attention

You mean the one in parallel with not value or refence? Yes it's better to change to 100K, don't make sense to be 33 ohm.
 
  • #29
nadersb said:
about 4 V pick to pick, but the problem is we amplify noise too, and i think we should add a filter to our circuit!

Is this from the output of the first amp like skeptic asked? Tell me the output of the first amp. What is the value of R1 and R2 now to make sure. If the R2 is still 1.5M, I want you to change to 150K and report back how much pick to pick do you get on the first stage.

The reason I ask is because after I reduce the gain by 10 times, you should get the full response. From that, I can back calculate what is the output from your detector. In turn to make sure the amp is not frequency limited.

You should not get a lot of noise from the electronic. The gain is only 1000, even 741 is reasonable. But if you change to OP37, it is a much lower noise amp. But if the noise is from the detector input, then you might have something elso going on.
 
  • #30
"""about 4 V pick to pick, but the problem is we amplify noise too, and i think we should add a filter to our circuit! ""

see figure "voltage swing vs supply voltage " on page 4 of this link:
http://www.national.com/ds/LM/LM148.pdf [Broken]

it's the datasheet for quad 741 and it has all the good ol' information that used to be on single 741 datasheet. Print a copy to share with your colleagues.
What that graph shows is that opamps need "headroom"
that is, they only guarantee it can drive output within about two and a half volts of supply,
so with 9 volt supply and 2.5 v headroom at each end the output might only swing about 4 volts peak to peak ...

25 millivolts p-p at inverting input?
well it's sure not holding summing junction at zero.
which could be from lack of headroom
or lack of slew rate
or scope probe capacitance upsetting amplifier
or simply out of gain
might you be overdriving it in attempt to get full output ?

I think Yungman is on to something...
see same datasheet page 5 figure "Open Loop Frequency Response" top right of page...
...at 40 khz it's only got open loop gain of ~30 db = gain of 31.6 if i remember logarithms
with open loop gain of 31 , i wouldn't expect a higher closed loop gain of 150


so i repeat - if you're using an adjustable bench supply, do a quick gain check at higher supply voltage to look for headroom and slew rate limiting
and if you're using a signal generator do a quick gain check at 400hz then 4khz to look for frequency response limiting.

and be aware it's generally risky to connect test equipment to the summing junction particularly if feedback resistance is large.


Glad to see somebody is really bujilding stuff instead of simulating it.
congratulations, sir.
old jim
 
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  • #31
jim hardy said:
"""about 4 V pick to pick, but the problem is we amplify noise too, and i think we should add a filter to our circuit! ""

see figure "voltage swing vs supply voltage " on page 4 of this link:
http://www.national.com/ds/LM/LM148.pdf [Broken]

it's the datasheet for quad 741 and it has all the good ol' information that used to be on single 741 datasheet. Print a copy to share with your colleagues.
What that graph shows is that opamps need "headroom"
that is, they only guarantee it can drive output within about two and a half volts of supply,
so with 9 volt supply and 2.5 v headroom at each end the output might only swing about 4 volts peak to peak ...

25 millivolts p-p at inverting input?
well it's sure not holding summing junction at zero.
which could be from lack of headroom
or lack of slew rate
or scope probe capacitance upsetting amplifier
or simply out of gain
might you be overdriving it in attempt to get full output ?

I think Yungman is on to something...
see same datasheet page 5 figure "Open Loop Frequency Response" top right of page...
...at 40 khz it's only got open loop gain of ~30 db = gain of 31.6 if i remember logarithms
with open loop gain of 31 , i wouldn't expect a higher closed loop gain of 150


so i repeat - if you're using an adjustable bench supply, do a quick gain check at higher supply voltage to look for headroom and slew rate limiting
and if you're using a signal generator do a quick gain check at 400hz then 4khz to look for frequency response limiting.

and be aware it's generally risky to connect test equipment to the summing junction particularly if feedback resistance is large.


Glad to see somebody is really bujilding stuff instead of simulating it.
congratulations, sir.
old jim

Amen to this! You never learn from simulation. People need to learn to think through the circuit.

Back to the circuit. I think there is a lot of questions op did not share with us to help him.

1) The detector characteristic. We need a data sheet to know exactly what it is. Is it a current out like a photo detector, a voltage output or what. What is the output impedance?

2) What signal the circuit suppose to detect. Is it a wide band from low freq to 40KHz or is it mainly 40KHz. I designed a 64 element ultra sound scanner medical instrument. The signal is a pulse concentrated at one specific frequency. That make it so much easy to reduce noise than wide band. I actually use a parallel tank circuit right at the input to limit the band. And we had multi pole filter all over.

3) OP did not give info on where the noise come from. Is it from the detector, or from noise in the surrounding that picked up by the detector. He need to isolate this. I don't believe the op-amp is the problem because it is only 60dB gain. We did 110dB gain before. Then you really see noise. My suspicion is the detector pick up noise from the surrounding. Is it white noise or noise with specific frequency components.

4) Sooner or later, OP need a better op-amp!

I think we are all trying to guess with very little information. Maybe the OP don't want to give out too much info, this is already on the second page and show very very little progress. So far, we know the op-amp finally have the correct DC biasing and does have some sort of signal going through. Not much more. For any of us to help, OP need to provide more info. I have a suspicion the situation is a lot more complicated than just having the op-amp running correctly. At this point, what he is looking for is more a system question.
 
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  • #32
Nadersb, have you checked your power supply for noise? There are circuits for active filters with gain that you could incorporate into your op amps.
 
  • #33
First let me thank all of you, all of you are very kind too me;) and sorry for my bad english;)
as I said before I am busy a little bit and I can't work on my project these pas days, I will continue on Saturday, so please give some time to test your ideas and tell you the results;)
and about mr.yungman's comment, let tell you about the circuit, (I don't want to hide anything, I appreciate your help) the circuit is an ultrasonic receiver, we have another circuit that is transmitter, it works perfectly and it sends ultrasonic waves,it has a transducer too
receiver circuit detect these ultrasonic waves(so its input is only 40 khz not a band) first we amplify input,then with two diodes and one capacitor we convert it DC voltage(I am not sure that convert is right verb for here!) after that we have a comparator, CA3140, it compares pin 2 with 3 and if 3 is bigger than 2 its output(pin6) becomes 6.8V .after that we have 2stages of transistors to amplify current because we want to drive a relay so we need high enough current.
about the transducer, it does not have datasheet, I asked someone and he told me this, so I can't help you with this. is there any question yet?!
 
  • #34
skeptic, I am sorry but I don't understand what you mean by power supply noise?! how can I check this?!
 
  • #35
everyone, I will tell you results and more information on saturday, mr yungman I will collect some information about the transducer and I will tell you;)
again thank you all, you are very kind indeed.
 
<h2>What is an op amp and how does it work?</h2><p>An op amp, short for operational amplifier, is an electronic component that amplifies the difference between two input signals. It has a high gain and high input impedance, making it useful for a variety of applications such as amplification, filtering, and signal conditioning. It typically consists of an inverting and non-inverting input, an output, and a power supply.</p><h2>What is the gain of an op amp and how is it calculated?</h2><p>The gain of an op amp is the ratio of the output voltage to the input voltage. It is typically expressed in decibels (dB) or as a unitless number. The gain can be calculated using the formula: G = -(Rf/Ri), where Rf is the feedback resistor and Ri is the input resistor. For example, if Rf = 10kΩ and Ri = 1kΩ, the gain would be -10 or -20dB.</p><h2>How do resistors affect the gain of an op amp?</h2><p>Resistors are an essential part of an op amp circuit and play a crucial role in determining the gain. The feedback resistor (Rf) controls the amount of output voltage and the input resistor (Ri) sets the level of input voltage. Changing the values of these resistors can alter the gain of the op amp. A larger feedback resistor or a smaller input resistor will result in a higher gain, while a smaller feedback resistor or a larger input resistor will result in a lower gain.</p><h2>What is the difference between inverting and non-inverting op amp configurations?</h2><p>In an inverting op amp configuration, the input signal is connected to the inverting input and the output is taken from the output of the op amp. This results in a negative gain. In a non-inverting configuration, the input signal is connected to the non-inverting input and the output is taken from the inverting input. This results in a positive gain. The choice of configuration depends on the desired output and input signals.</p><h2>What are some common applications of op amps and resistors?</h2><p>Op amps and resistors are used in a wide range of electronic circuits and systems. Some common applications include audio amplifiers, signal filters, voltage regulators, and oscillators. They are also used in instrumentation and control systems, such as in sensors, data acquisition, and feedback control loops. Additionally, op amps and resistors are used in communication systems, power supplies, and many other electronic devices.</p>

What is an op amp and how does it work?

An op amp, short for operational amplifier, is an electronic component that amplifies the difference between two input signals. It has a high gain and high input impedance, making it useful for a variety of applications such as amplification, filtering, and signal conditioning. It typically consists of an inverting and non-inverting input, an output, and a power supply.

What is the gain of an op amp and how is it calculated?

The gain of an op amp is the ratio of the output voltage to the input voltage. It is typically expressed in decibels (dB) or as a unitless number. The gain can be calculated using the formula: G = -(Rf/Ri), where Rf is the feedback resistor and Ri is the input resistor. For example, if Rf = 10kΩ and Ri = 1kΩ, the gain would be -10 or -20dB.

How do resistors affect the gain of an op amp?

Resistors are an essential part of an op amp circuit and play a crucial role in determining the gain. The feedback resistor (Rf) controls the amount of output voltage and the input resistor (Ri) sets the level of input voltage. Changing the values of these resistors can alter the gain of the op amp. A larger feedback resistor or a smaller input resistor will result in a higher gain, while a smaller feedback resistor or a larger input resistor will result in a lower gain.

What is the difference between inverting and non-inverting op amp configurations?

In an inverting op amp configuration, the input signal is connected to the inverting input and the output is taken from the output of the op amp. This results in a negative gain. In a non-inverting configuration, the input signal is connected to the non-inverting input and the output is taken from the inverting input. This results in a positive gain. The choice of configuration depends on the desired output and input signals.

What are some common applications of op amps and resistors?

Op amps and resistors are used in a wide range of electronic circuits and systems. Some common applications include audio amplifiers, signal filters, voltage regulators, and oscillators. They are also used in instrumentation and control systems, such as in sensors, data acquisition, and feedback control loops. Additionally, op amps and resistors are used in communication systems, power supplies, and many other electronic devices.

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