TL084 quad op-amp help - Electronic Circuits Projects Diagrams Free

coolrfid · 9th May 2008, 02:09 PM

I need to know at what input will the opamp trigger while giving 5v to the inverting end?
Im trying to use it as a filter and the FSK input is around 400mV but I am not getting any output from the opamp....so what is the input level required to get an output??
This is the circuit diagram ...it is the receiver part of the 125KHz RFID system design.

spuffock · 9th May 2008, 02:25 PM

where do pins 4 and 11 go?

coolrfid · 9th May 2008, 04:15 PM

Pins 4 & 11 are the voltage supplies to the opamp. ie +/-12v resp

Hero999 · 9th May 2008, 08:05 PM

Aren't you pushing the capabilities of the TL084 a little bit too far?

DigiTan · 9th May 2008, 09:57 PM

This 125kHz sometime goes through AM peak detection before reaching the op-amps. Possibly, coolrfid's creating a bandpass filter for the 12kHz and 15kHz FSK frequencies.

5V or even 400mV will be enough if your ± supply voltage is in the recommended ±5V to ±15V range

Since you're using a quad op-amp , I'll just wager you're using the TL084 to make Sallen Key filters and inverting amps. A lot of Sallen Key filters I find have unity gain. So if you're putting 400mV in, you should be getting almost 400mV out...unless it's outside your passband.

...That's just based on my guesses. First thing, post a diagram so we can see your amps/filters. Then tell us what frequency this 400mV is.

Also keep in mind the TL084 can't produce an output if the load impedance is under 100Ω.

coolrfid · 10th May 2008, 06:08 AM

Quote:

Originally Posted by DigiTan

This 125kHz sometime goes through AM peak detection before reaching the op-amps. Possibly, coolrfid's creating a bandpass filter for the 12kHz and 15kHz FSK frequencies.

5V or even 400mV will be enough if your ± supply voltage is in the recommended ±5V to ±15V range

Since you're using a quad op-amp , I'll just wager you're using the TL084 to make Sallen Key filters and inverting amps. A lot of Sallen Key filters I find have unity gain. So if you're putting 400mV in, you should be getting almost 400mV out...unless it's outside your passband.

...That's just based on my guesses. First thing, post a diagram so we can see your amps/filters. Then tell us what frequency this 400mV is.

Also keep in mind the TL084 can't produce an output if the load impedance is under 100Ω.

The 400mV signal is the FSK modulated signal of the RFID tag around containing the frequencies 12KHz & 15KHz. Ive posted the circuit diagram ontop in d first post....Does the diode 1N4936 make a difference to the voltage level of the signal...can i use diode 1N4148 instead...

What u r saying is right.. im using the TL084 to make Sallen Key filters

DigiTan · 10th May 2008, 06:45 AM

The 1N4148 would probably give you a longer reception range because it has a low forward voltage (Vf). A germanium diode might be even better than 1N4148. They are the kind used in battery-less crystal radios, and take very little energy from your signal.

Probably I can look at this filter again tomorrow and make a test recommendation. For now, you should be able to see a 12kHz to 15kHz square wave on the final output if everything is okay (and possibly some 3rd frequency if there is no input at all). I'd start by inputting more 12kHz and 15kHz to see if there is a one or more Sallen Keys dropping the signal.

Roff · 10th May 2008, 05:17 PM

Quote:

Originally Posted by DigiTan

The 1N4148 would probably give you a longer reception range because it has a low forward voltage (Vf). A germanium diode might be even better than 1N4148. They are the kind used in battery-less crystal radios, and take very little energy from your signal.

Just for the record - 1N4936 has lower forward voltage than 1N4148, due to the larger junction. Look at the datasheets.

ericgibbs · 10th May 2008, 05:29 PM

Quote:

Originally Posted by Roff

Just for the record - 1N4936 has lower forward voltage than 1N4148, due to the larger junction. Look at the datasheets.

hi Ron,
The BAT43 and the old OA47 [Ge] are low Vfwd.

Roff · 10th May 2008, 05:41 PM

Quote:

Originally Posted by ericgibbs

hi Ron,
The BAT43 and the old OA47 [Ge] are low Vfwd.

Hi Eric,
I wasn't making a recommendation. As I said, it was just for the record. I hate letting erroneous info go uncorrected.

ericgibbs · 10th May 2008, 05:45 PM

Quote:

Originally Posted by Roff

Hi Eric,
I wasn't making a recommendation. As I said, it was just for the record. I hate letting erroneous info go uncorrected.

Hi Ron,
I agree, I just thought I would add two more low fwd diodes to the mix.

What are you doing 'nerding' on a Saturday night, you should be out with the guys..

Roff · 10th May 2008, 06:48 PM

Quote:

Originally Posted by ericgibbs

Hi Ron,
I agree, I just thought I would add two more low fwd diodes to the mix.

What are you doing 'nerding' on a Saturday night, you should be out with the guys..

I'm on vacation in Fergus Falls, Minnesota, where it's 1:45PM, raining, and about 4 degrees C outside. If it was warm and dry, I would be outside fishing or hunting coins with my Minelab Explorer SE.
Nerding is my fallback entertainment.

Actually, I guess you could say I'm always on vacation, since I'm retired.

DigiTan · 10th May 2008, 09:09 PM

Either way, I'd look into a Germanium device in the near future. It will work with a 1N4148 or 1N4936, but you might have better luck with Germanium.

Maybe start by probing filters A, D, B, C at their outputs and tell us if the 12kHz and 15kHz signals are getting through. Only filter C seems to be running at unity gain, so all the others should actually be making the 12kHz and 15kHz larger. Do you have an oscilloscope available?

9th May 2008, 08:05 PM	(permalink)
Hero999 Experienced Member	Aren't you pushing the capabilities of the TL084 a little bit too far? __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

9th May 2008, 09:57 PM	(permalink)
DigiTan Experienced Member	This 125kHz sometime goes through AM peak detection before reaching the op-amps. Possibly, coolrfid's creating a bandpass filter for the 12kHz and 15kHz FSK frequencies. 5V or even 400mV will be enough if your ± supply voltage is in the recommended ±5V to ±15V range Since you're using a quad op-amp , I'll just wager you're using the TL084 to make Sallen Key filters and inverting amps. A lot of Sallen Key filters I find have unity gain. So if you're putting 400mV in, you should be getting almost 400mV out...unless it's outside your passband. ...That's just based on my guesses. First thing, post a diagram so we can see your amps/filters. Then tell us what frequency this 400mV is. Also keep in mind the TL084 can't produce an output if the load impedance is under 100Ω. __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About

10th May 2008, 06:45 AM	(permalink)
DigiTan Experienced Member	The 1N4148 would probably give you a longer reception range because it has a low forward voltage (Vf). A germanium diode might be even better than 1N4148. They are the kind used in battery-less crystal radios, and take very little energy from your signal. Probably I can look at this filter again tomorrow and make a test recommendation. For now, you should be able to see a 12kHz to 15kHz square wave on the final output if everything is okay (and possibly some 3rd frequency if there is no input at all). I'd start by inputting more 12kHz and 15kHz to see if there is a one or more Sallen Keys dropping the signal. __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About

10th May 2008, 09:09 PM	(permalink)
DigiTan Experienced Member	Either way, I'd look into a Germanium device in the near future. It will work with a 1N4148 or 1N4936, but you might have better luck with Germanium. Maybe start by probing filters A, D, B, C at their outputs and tell us if the 12kHz and 15kHz signals are getting through. Only filter C seems to be running at unity gain, so all the others should actually be making the 12kHz and 15kHz larger. Do you have an oscilloscope available? __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About

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9th May 2008, 02:25 PM	(permalink)
spuffock Experienced Member	where do pins 4 and 11 go?

9th May 2008, 04:15 PM	(permalink)
coolrfid New Member	Pins 4 & 11 are the voltage supplies to the opamp. ie +/-12v resp

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