riddle me this

[MrBinns]

Ok so I have my guitar running into a basic op amp buffer set up. The voltage at the (+)input is ~0v but the ouput and (-)input is at ~8.4V.
I'm using TL072 power with +9V and +0V.

The other side of the op amp is used as a buffered half voltage source and it works as expected.

edit before posting: I added a cap between the guitar and the buffer input. now everything is at the same voltage. 

Still not sure what the problem was though.

[tubegeek]

I think it may have been latching to the + rail in the absence of a voltage ref for the inputs.

[MrBinns]

O shit 

*walks away*

[amptramp]

Shouldn't your (+) input voltage be 4.5 volts as produced by the half-voltage supply rather than ~0 volts as you have said?  During linear operation, the (+) and (-) input of the op amp should be nearly the same voltage so the (-) input should also be ~4.5 volts.

[samhay]

TL07x's are quirky like that. To expand on tubegeek's comment, if the input hits one of the rails, the output often swings to the other rail.

[MrBinns]

huh that's interesting.

[bool]

This is a well-known TL06x, 07x, 08x series (mis)behaviour.

[R.G.]

Let's use this riddle as a teaching opportunity.

As you've just learned, opamps are not perfect. And in addition, each opamp can and usually does have quirks and limitations depending on its internal circuitry.

One of the quirks of many opamps is a limitation on the input common mode range. This means that the opamp cannot do normal amplification if both inputs are pulled too close to one or the other power supply. When this happens, the opamp does one of a number of unpleasant things. It's up to you, the user, to make sure that the inputs can never be put in this compromising position.

In fact, this behavior is so common that there is a whole class of opamps specified as "rail to rail input" devices, denoting that these special devices will NOT do ugly things if their inputs are pulled to either power supply rail.

If it's rail-to-rail, they'll make a big deal about it. If the datasheet does not make a big deal about it, it's not a rail-to-rail type.

And the moral of the story is - read the datasheet. 

[duck_arse]

while we are on the topic, with an expert watching, this version of the pulsar trem, as an example, [mis-]uses a dual opamp, TL072 specced. the circuit only shows half of same, but the pcb has pin 5 connected to ground.

http://www.tonepad.com/getFile.asp?id=29

and the question is : what is the long term effect on the IC itself when connected thusly?

[R.G.]

while we are on the topic, with an expert watching, this version of the pulsar trem, as an example, [mis-]uses a dual opamp, TL072 specced. the circuit only shows half of same, but the pcb has pin 5 connected to ground.
...
and the question is : what is the long term effect on the IC itself when connected thusly?

In the immortal words of Scotty on Star Trek "Unknown, Captain."

It depends on what is inside the IC. It may have little effect, especially since the output is connected to the inverting input on that half of the opamp. It may cause the opamp to exceed its own differential mode input limits if the output goes high and stays there, pulling up the inverting input. In any case, using an opamp like that is something you get away with, not something you ought to do by choice. That connection would not be something that would pass a peer design review, with good values of "peer".

It's much more predictable to tie that pin 5 to anyplace at all that keeps it within the common mode range and just let its output follow that voltage, as the output is not connected to anything. It then just sits there counting its own toes. 

[Transmogrifox]

but the pcb has pin 5 connected to ground.

On the same topic:
I am currently working on a proximity controlled wah.  I was using 1/2 an LM358 to amplify the rectified/filtered CV coming off the proximity detection circuit.  Out of laziness I left the other 1/2 floating.

Noise in the circuit was out of control, but went away when I disconnected power from the proximity detector (and I expected this).

The next thing I did because I was lazy is similar to what you described.  I tied pins 1,2 and 3 to pin 4 (-Vcc) because it was easy to make a solder bridge across the whole line of pins.   The whole proximity wah became pleasantly absent of hiss and modulated high-pitched noises. Now the circuit draws about 20 mA when I expect it to be using significantly less power.

When I get it perfected and all that I will probably find a way to make use of the extra op amp.  At the very least the final product will just turn the extra op amp into a unit gain buffer and tie its input to the CV output or something.  I might just end up using an R2R device instead.

So...

what is the long term effect on the IC itself when connected thusly?

#1) In the general sense:

"Unknown, Captain."

#2) You might drain your batteries more quickly than expected.
#3) You might damage the device from thermal stress.
#4) It might go unstable internally and couple noise into the other 1/2 or onto the power supply rails.
#5) Any combination of 1-4.

[R.G.]

I was using 1/2 an LM358

The LM358 datasheet says that the input common mode range includes ground (i.e., Vminus) and it's OK for such single-power supply use.

[Transmogrifox]

I was using 1/2 an LM358

The LM358 datasheet says that the input common mode range includes ground (i.e., Vminus) and it's OK for such single-power supply use.

The fact that 1,2,3 AND 4 are all tied together may be related to the excess current condition in my case.  If I were to simply tie only pin 4 to ground, then 1 to 2, it would have been ok.  Looking at the internal schematic in the datasheet, I see that if both inputs are tied to -Vcc, then the output at the active load is at the mercy of differential input matching (which would be very good, but not perfect).  When the output is shorted to ground it may more or less provide current of a magnitude similar to VOL/Rlimit.  Some might have this high current problem while others may not.

[tubegeek]

a peer design review, with good values of "peer".

Dude, you aren't working for HAL anymore. Get over it.

[R.G.]

Sorry - it's just this leftover twitchiness I get. 

[Ice-9]

In the words of Scotty (star Trek)

"Aye Captain , the power is oot, it'll need to be sorted before this thing will take off again."

Or in the words of the Doctor (star trek again)

"Jim, I'm a Doctor, not a plasterer"

You have just discovered what DC decoupling is.

[MrBinns]

nerds

[tubegeek]

nerds

I appreciate the compliment, but I fancy myself more of a geek.

[MrBinns]

*golf clap* that is awesome

[tubegeek]

*golf clap* that is awesome

It was a present from my wife - SHE is the awesome one!