TLC272CP Op Amp Leakage?

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Palbert

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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?
 
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.
 
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.
 
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?
 
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.
 
audioguru said:
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.
Click to expand...

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