Hi - I was wondering - if I were to use an op-amp as a buffer, with only a positive input (thus the output would also always be positive) - would I need to connect -Vcc to the negative biasing terminal? (ie pin 4 on a 741 DIP package). The reason I ask is that I only have a positive power supply - so I was hoping I could get away with just connecting it to ground. That way - I would think that essentially it would think it was seeing +- Vcc/2 - and would function entirely normally. Also - I was looking at the datasheet for the 741 (http://focus.ti.com/lit/ds/symlink/ua741.pdf) and I noticed that at the end of the triangle symbol on page 1 - there is a circle on the output. What exactly does this mean? I mean normally that would mean that the signal is being inverted - but a 741 is a standard op-amp - I don't think the output is being inverted? Thanks!
The 741 can only swing to about 1.5V from either power rail, so you can only get a zero output (or any output between 0V & 1.5V) with split-rail (or a virtual earth). With some other opamps (eg LM324) you can do it as you describe - in fact, that's how U1d is connected here.
And to add, op-amps commonly have the negative supply tied to ground in single power supply systems. Also, they are commonly used as analog buffers as you described since the input resistnace is so high, and the output resistance so low. In other words, a variable resistor voltage divider can be used and the op-amp can buffer this output so that the load will not adversly effect the voltage divider circuit.
Ah - I hadn't realized that would be a problem. So the 324 would have very similar characteristics to the 741? Reason I ask is that I'm used to working with the 741 - but not so much used to any other op-amps out there. That's exactly the plan But I was looking at the 741 - and the resistance isn't as large as I had hoped - it says typical value is 2Mohms. That's small enough that a voltage divider circuit would have to use fairly small resistors for the divider circuit to not be affected by the op-amp. Now - would there be any problem with me putting a large resistor between the voltage divider circuit and the op-amp input?
Get hold of NS Application Note AN-116, it compares LM358 (a dual for single supply) and LM1458 (which I think is similar to a dual 741) and gives loads of circuits if you want high performance. 741 is past its sell-by date. If you want a pin-compatible, CA3140 mosfet will swing down to ground. Watch it doesn't become part of the gain formula like R7/R6 & U1c here...
Just to throw another suggestion at you, the TLC2272 is quite a bit faster than the lm324, is true rail-to-rail, and has a FET input so the input resistance is in the TerraOhm range. Just incase these parameters are important to you. But be aware that the supply voltage is only 16V instead of 32V for the LM324. As for only wanting to deal with the 741... you'll find that a very large percentage of opamps out there have the same pinouts, depending on if its single dual or quad. And most can be used interchangably, you just pick the characteristics thats important to you and plug 'er in. Besides, after a LONG successful run, it's about time they retired the 741, its been around 30years or something hasnt it? Oh and Cpemma is right... putting a resistor inbetween the voltage divider and opamp is bad practise.
