Re: where to buy 14 pin pkg tl082??

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From: Steven Weber (kd1jv@moose.ncia.net)
Date: Sat Sep 06 1997 - 12:00:46 EDT


>replacements for the more common LM324N bipolar device commonly used >in
QRP audio filters. I was hoping to get quieter performance, but they just
>didn't work - even they are pin compatible with the LM324N. Must be due
>to the higher J-FET input impedance in the TL084P device.

The TL084 and LM324 have very diffrent internal circuit configurations.
Although the pinout of a quad or dual op amp are almost always the same,
that does not mean they are interchangeable. The reson there are so many
diffrent types of op amps is that they are designed to do diffrent jobs.

The TL084 is a better audio op amp than the LM324. It has better frequency
responce, less distortion and lower noise. It, however, needs a higher
supply voltage than the LM234 to work. Other factors are the output swing
of the 084 is less than the 324 and the input common mode voltage is more
restricted.

Therefore, if a circuit was designed for a LM324, and took advatage of it's
ability to use input signals that go to ground (negative supply), its'
lower operating voltage and greater output voltage swing, than subbing a
084 will not work.

Another factor is the 324 will not drive heavy capacitive loads. It can
oscillate if you try and do so. The 324 can source current better than it
can sink current. In audio applications, a load resistor on the 324 output
to ground can be a help. It should be 1K to 2.2K ohms. This resistor helps
reduce cross over distortion in the output.

 A small, 47 to 100 ohm resistor is series with the output can help
stability when driving large capacitive loads. (This is not generally
needed if the large cap is simply a coupling cap driving into a high
impeadance input)

So, in general, try to use the op amp specified in a design. If the
designer took advantage of some special characteristic of the device
specified, subbing another type probibly will not work.

73,
Steve, KD1JV....In the White Mountains of New Hampshire

"Melt Solder"


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