Why Does My Op Amp Only Amplify by a Factor of 5 Despite Changing Resistors?

[minijh]

@Jim:
Nice points you mentioned abt pin 2 of opamps indeeeeeeed! But I have this question: when you say "signal on pin2 means our opamp isn't in its operational state", How strong a signal should be to say so? I mean is a signal with p-p of 10 mvs considered as "no signal"? :D
About what you asked about our measurements, as you asked us to measure pin 2 we did so,directly on pin2! :D Reading before the resistor is just equivalent to reading pin6 of the previous opamp in ac analysis ?! Correct me if I’m wrong :)
I can’t say anything but I just loved what you said & I hope the misunderstanding is over:
“I hope you guys find this is indeed a mis-communication because i am interested in learning from all of you.
That's another advantage of building something instead of simulating iit, you develop real world skill with real world test equipment.”

 

[jim hardy]

" Reading before the resistor is just equivalent to reading pin6 of the previous opamp in ac analysis ?! Correct me if I’m wrong :)""
"

welll for all practical purposes, Yes.
i see a capacitor in between pin 6 and 10K resistor so the DC value could be different on pin 6 side of that cap, but in your circuit only by a few millivolts. EDIT: And you did say ac analysis... my bad.

""About what you asked about our measurements, as you asked us to measure pin 2 we did so,directly on pin2!"
oops we should have said "signal to pin 2 measured at junction of 10K and cap"



""But I have this question: when you say "signal on pin2 means our opamp isn't in its operational state", How strong a signal should be to say so? I mean is a signal with p-p of 10 mvs considered as "no signal"? ""

let's think for a moment.
Thinking is exact and measurement is approximate.
The basic principle of the opamp is that it will hold the two inputs equal.
That is true for both inverting (your arrangement) and non-inverting (where input signal goes to pin 3 not 2).

In an exact world, your circuit would hold pin 3 at exactly 4.5 volts, with no variations because of the RC filter 10K and the capacitor.
The opamp would attempt to hold pin 2 at exactly the same voltage.
In a real world there is likely some teeny bit of signal frequency showing up at pin 3 from parasitic capacitances or ground loops or something. The opamp will obediently try to reproduce that at pin 2.
So look at pin 3, it's okay to put the scope there because of that big cap already on pin 3. If you see 10 mv there the opamp is entitled to put 10 mv on pin 2 as well. But you can't look at pin 2 for reasons already mentioned.
I will be surprised if there's as much as a millivolt at pin 3, let alone ten millivolts.

Now more directly to your question:
return to thinking mode:
What is open loop gain of opamp?
something like 10^6 at low frequency.
So in an inverting configuration, the signal at pin 2 should be [output signal] / 10^6
which is going to be in microvolts.
Remember you can only get 4v of signal in your circuit, and to produce that takes only 4/10^6 which is 4 microvolts at pin 2.
If you have more, the circuit is failing to be an ideal inverting amplifier.

As you raise frequency the opamp's gain falls off so the voltage at pin 2 required to produce same output swing becomes larger. Read that appnote on slew rate limiting.

i too hope the misunderstanding is over.

Let us know how it works measuring on upstream side of 10K at pin 2.
And whether gain is affected by supply voltage.



Good luck with your project.
Yungman and i both think your 741 is not quite up to the task of high gain at 40khz.
Here's a faster opamp, two to a package and widely available from hobbyist sites:
http://www.national.com/profile/snip.cgi/openDS=TL082
be aware you may need to raise power supply to 12 volts.
should give you better gain. There's way faster ones yet out there.

We learn by doing. Hats off to you guys.

And thanks for the kind words.

 

[minijh]

1) Thanks a lot for clarification Jim, I'll test the circuit just the way you said & I'll inform you.

2) "Now more directly to your question:
return to thinking mode:
What is open loop gain of opamp?
something like 10^6 at low frequency.
So in an inverting configuration, the signal at pin 2 should be [output signal] / 10^6
which is going to be in microvolts.
Remember you can only get 4v of signal in your circuit, and to produce that takes only 4/10^6 which is 4 microvolts at pin 2.
If you have more, the circuit is failing to be an ideal inverting amplifier."

What you just said shot the 741 in head! :D we thought it would be better to get rid of him as soon as possible! Poor 741! so we went for some other ops including op37 as yungman advised. Do we have to change anything? I mean R1 or R2 or..? because when we substitute 741 with the new fellow, the circuit wrecked badly!

3) By the way, after your advice we tuned the dc supply up to 20 volts, with 30mvs of signal generator as the input, we got near 6 volts out!
4) Nice technics Skeptic & Yungman said abt searching for the source of noise! we checked them all, as Yungman wisely predicted it was the environment. We also tried to detect its frequency with oscope, it was some 200Hz. What do you suggest for out LC Tank filter? For maximum efficiency of the circuit-maximum board-, what would be the best frequency to put the bandwidth around? To Strongly pass 40ks & kill 200s?

5) Hats off to you Jim! You've been extraordinary helpful! :)

P.S: @Skeptic: Sry for my delay, Now that everything got crystal clear I'll check for all the pins of the circuit & get you know of it. Our laboratory was closed these days,I have to wait till tomorrow. :)

 

[skeptic2]

What you just said shot the 741 in head! :D we thought it would be better to get rid of him as soon as possible! Poor 741! so we went for some other ops including op37 as yungman advised. Do we have to change anything? I mean R1 or R2 or..? because when we substitute 741 with the new fellow, the circuit wrecked badly!

3) By the way, after your advice we tuned the dc supply up to 20 volts, with 30mvs of signal generator as the input, we got near 6 volts out!

4) What do you suggest for out LC Tank filter? For maximum efficiency of the circuit-maximum board-, what would be the best frequency to put the bandwidth around? To Strongly pass 40ks & kill 200s?

What do you mean, "...the circuit wrecked badly"? What did the circuit do?

Is the 6 volts out at the output of the first op amp or the second? If the second, what is the output from the first op amp? Are the outputs clean sinewaves or are they distorted or noisy?

You might start with a tank circuit with 100uH and 160nF. My preference is to strongly pass 40 kHz, that way you eliminate everything you don't want.

 

[minijh]

@Skeptic2:
I meant there was no amplification, like when you use ops out of order or something like that. :-?? We checked for some 2 more op37 but still same result. So the problem couldn't be the op-amps themselves.

6volt out the circuit, output of the second one. I'll check tomorrow for sure, I mean in less than 12 hours! :D

Thanks for your comment on filtering.

Tomorrow!The big day!
:)

 

[skeptic2]

The only problem with using a tank circuit is its low impedance. For instance with the values I gave you the Q of the inductor will be under 30. Since the reactance of the inductor is 25 ohms, the maximum impedance of the tank circuit would be 750 ohms. That would mean to get a gain of 25 you'd need to replace the 10 K resistor at the input with a 30 ohm resistor.

 

[minijh]

Finally, I got into laboratory & took this data out the circuit before they threw me out of there! :D

@Jim:I checked for pin 3 in ac status, there was some like 0.1 mv p-p! as you had told me so, earlier :)

op741/ DCSup=20v/ Vin(p-p)=30mv/ R(feedback)op1=150k

op1:
pin2(B4 Resistor):30 mv (As expected! :D)
pin3:10 v
pin6:320 mv (ac)
=> gain=10.6

op2:
pin2(B4 Resistor):320 mv (as expected)
pin3:10 v (not directly measured)
pin6:5 v (ac)
=> gain=15.62

 

[minijh]

What takes attention is the difference in gain of each stage. It seems stage 1's got bugs down there! :D

 

[jim hardy]

To see your progress is very heartening. Thanks!


first stage gain is ten , lower than desired;; and
second is fifteen, higher than desired ?

okay first things first
First stage
i'd wager first stage is slew rate limited.
Will your signal generator allow you to drive with a square wave instead of sine?
Try that and look at output. The more it resembles a sawtooth od triangle wave, the more you are seeing the opamp limited by how fast it is capable of moving its output.
Repeat but with a 400 hz squarewave not 40 khz, output should be a more faithful reproduction.

Second stage has more input swing so can drive output at a faster slew rate.
But why so much gain? are you still using 100K / 1K there?
any green stripes on those second stage resistors?
brown black orange = 10K
brown black yellow = 100K
brown green yellow = 150K
brown black green = 1meg
brown green green = 1.5 meg

and if you are using 1% resistors there's a third stripe for third digit before the multiplier band...
------------

did you say the circuit 'wrecked' with op37?
that looks like a wonderful opamp.
http://www.analog.com/en/all-operat...amplifiers-op-amps/op37/products/product.html
Fast opamps are sensitive to capacitance at their inverting input and board layout(long leads) can get you.

See if a few picofarad capacitor in parallel with your 1 meg feedback resistor settles it down.
Tack one across the .47 that's on pin 3 also - capacitors that are made from a rolled up foil have some parasitic inductance so need a high frequency bypass. I don't know what style is your 0.47uf .

old jim

 

[skeptic2]

I suspect that if you are using a 741 for the 2nd op amp and are getting 5V pk-pk out that you are seeing a triangle wave instead of a sine wave. That is a sign you are slew rate limited. That shouldn't be true of op amp 1 though. You must have some other problem with op amp 1.

 

[minijh]

Good news every one! Using your advice, we checked OP-37 for our circuit- but we had to change resistors- and we got 9 volt out!cheers! :)
(what we did was playing with resistors, triggering the signal and here are the results:)

DC Sup=12/OP-37/Vin(p-p)=25 mv

OP1
R1=10k/R2=180k
pin2=25 mv
pin3=6v
pin6=0.5 v

OP2
R1=100k/R2=1.8M
pin2=0.5v
pin3=6v
pin6=9v

As we have a relay to be turned on, we took 9v dc out with two diodes and a capacitor. Regarding this, Is it important to check for slew rate or something?
*by the way as Jim said, I checked the signals for they were sines, just as signal generator produced.

What we have achieved in our project up to now:
1. Producing 40KHz pulses with p-p 0f 5v and send via a transducer(transmitter),
2. Receive the transmitted signal within a second transducer (receiver),
3. Amplifying with gain of some360,
4. Rectifying the signal,
5. Detecting the signal using a comparator3140,
6. Amplifying current the relay needs,
7. Tik Toks on relay! :)
8. Do whatever you want we relay's output...

There is only one thing left for better efficiency, and that is filtering. As you have suggested before it is better to do so before amplification; using a band-pass filter. We need low image impedance there -Correct me if I’m wrong- We checked the Rin of our circuit with an ohmmeter turning off the supplies & we got some 22Ks, Isn’t it a little big for our purpose?
Thanks every one
:)

 

[skeptic2]

1. You could make 1 op amp a low pass active filter with gain and the other one a high pass active filter with gain.

2. You could use a Twin T notch filter in the feedback loop of the 1st op amp which would give you a bandpass characteristic.

3 There are probably other active bandpass filter designs on the internet.

 

[jim hardy]

congratulations on your success !

Hmmmm - Active filters are fascinating because they are such a direct application of math.
They are a whole field unto themselves.
My experience is limited, Yungman is FAR more conversant than i.

Once upon a time i needed sharp filters with gain
and stumbled across this IC
and built my filters (22 and 27 khz) , measured Q almost 100 had to tweak down to ~50 to get bandwidth to cover the FM signals i was decoding. (Telephone touch tones FM'd onto ultrasonic carriers. )

In other words:
"Here's a great filter that can be built by an amateur"
http://www.national.com/ds/LM/LM359.pdf
the Biquad on page 22/23
and someplace National has an appnote on active filters with a discussion of this one.
AN72 mentions it but i think there's a better one somewhere in their library.
http://www.national.com/an/AN/AN-72.pdf

it's called 'biquad' because both numerator and denomoinator of transfer function have a quadratic in them
so there's poles and zeroes galore to tinker with.
The formulas in the datasheet work.
I hand picked my resistors and capacitors to achieve the values called for by formulas.
worked quite well.

Just a thought...

old jim

ps keep your eyes peeled for those old 1970's catalogs - paperbacks several inches thick.
National, Signetics, RCA, TI et al
Packed with information, they are. You find them in junkshops and Ebay.