Inverted vs. Non-Inverted Input and Rail-to-Rail chips vs. Non

[Guitar Toad]

Please help me understand the difference between Non- and Inverted input signal I'm presently working with the LM386 headphone amp and see the two input options. I'm using the non-inverted input at the moment. When would you use the Inverted input? And what's the deal with Rail-to-Rail transistors?

[Sir H C]

Rail-to-rail deals with the op-amp input and/or output range able to very close (or past in the case of inputs) to the limits of the power supply.  These op-amps were developed when supply voltages started to drop and so you could not waste that 1 volt top and bottom of the supply range.  Problem is that these op-amps are harder to keep stable under all conditions and have some other drawbacks relative to the standard op-amps.

The op-amp needs both the + and - inputs in order to work.  The job of the op-amp is to keep these two node voltages equal and in so doing move the output node accordingly (assuming some feedback path).  So you can not not be using one of the inputs or you will get oscillation.  You might not be feeding signal in, but that is different.

[StickMan]

First off, the LM386 is NOT an op amp, and doesn't work like one.  On the LM386, the choice of which input you use is not that important if you are using it as a power amp.  I remember seeing a compressor circuit that used one because it was simpler than doing the biasing for an opamp.  That one may have needed to worry about the inverting and non-inverting inputs.

The thing to remember about an op amp is that it always tries to keep the voltages on the two inputs identical by putting out an appropriate voltage on the output pin, and faciliated by a feedback circuit.  The sonic magic usually happens in the feedback circuit, so it is highly unlikely that you'll come across a schematic where it's left up to you pick which input you should use going into the op amp.


dave.

[Guitar Toad]

So, there's no real difference with the LM386. Cool. Thanks for the help. And the Rail-to-rail stuff is an "older technology" so I probably won't have to worry about rail stuff if I stay with modern era products, right?

[Sir H C]

Rail to rail is newer technology.  You need it if you want an op-amp to do anything with a 1 volt rail.