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

[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.