TLC272CP Op Amp Leakage?

[Palbert]

So I am building a simple inverting op-amp amplifier with feedback. My input is a bass guitar coming in through a .1uF cap and a resistor. The resistor feedback ratio is 10k/1k, so a gain of 10. It worked okay when I powered the chip with 9V (obviously still fuzzy due to no filtering yet) but when I turned the power to the chip off and played, I still got output. A whole lot of output. The project is breadboarded and if I take away power and ground, it still works rather well. I wonder why this is? My bass has a pre-amp so maybe it is using some of that battery power?

 

[ronsimpson]

Build a new amplifier, (with no TLV272) add a 1k and 10k resistor from input to output. Because your pre-amp drives hard (100 ohms) and the next amplifier has a high input impedance (100k or more), the signal will pass straight through.

 

[Palbert]

So you mean just cascade another amplifier from the output of the first 272 or just use a new op-amp?

 

[ronsimpson]

No,
You asked about, "when I turned the power to the chip off and played, I still got output" why?
Answer, a signal will go from input to output when the OP-amp is removed or un-powered. You have 10k+1k=11k from input to output of the amplifier you built.

 

[Palbert]

Ahh, I gotcha. That makes sense. Thanks!

 

[Palbert]

Will signals go through transistors like that when they don't have power? I'm assuming they won't.

 

[Palbert]

Sorry for the multiple posts. But, if I understand correctly, since the signal is passing straight through with a high input impedance device, should I use an amplifier with a low input impedance? Like a common base BJT amplifier?

 

[audioguru]

Your amplifier is inverting so it has two resistors in series from input to output.
If you make a non-inverting amplifier then it does not have resistors from input to output.

 

[Palbert]

Your amplifier is inverting so it has two resistors in series from input to output.
If you make a non-inverting amplifier then it does not have resistors from input to output.

Hmm, I just switched it to non-inverting and I still got the same problem. Noise with no power.

 

[Palbert]

We now have liftoff. When I made the non-inverting amplifier, I still had the resistor feedback setup. Instead, I have my input signal coming into the non-inverting inut and the resistor feedback going to the inverting input and ground. Thanks for the advice.

I.e. (albeit, very badly drawn),

Input--- | + R1
---------- | - > ----/\/\/\---|---------------Output
|------ | --------------------|
|
|
/
\ R2
/
|
GND

 

[SgtWook]

We now have liftoff. When I made the non-inverting amplifier, I still had the resistor feedback setup. Instead, I have my input signal coming into the non-inverting inut and the resistor feedback going to the inverting input and ground. Thanks for the advice.

I.e. (albeit, very badly drawn),

Input--- | +                R1
---------- | -    >    ----/\/\/\---|---------------Output
       |------ | --------------------|
               |
               |
               /
               \   R2
               /
               |
            GND

If you use the CODE tags, then it will preserve your formatting. It helps to lay out your ASCII text in Notepad first, or a word processor with a monospace font like Courier New.

Try using Reply with Quote to see what I mean about the CODE tags.

 

[unclejed613]

yes very badly drawn since the forum software reformatted the text (not your fault, just keep in mind spaces disappear when you hit the save button).... i seem to remember Analog Devices publishing something about the unpredictable effects of applying signal to unpowered op amps.
found it... hope you find this helpful

**broken link removed**

 

[Palbert]

yes very badly drawn since the forum software reformatted the text (not your fault, just keep in mind spaces disappear when you hit the save button).... i seem to remember Analog Devices publishing something about the unpredictable effects of applying signal to unpowered op amps.
found it... hope you find this helpful

**broken link removed**

Thanks, this was helpful. I enjoyed their naming of the "Off-Amp" as well