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741 Signal amplifier Answered

How would I go about creating an amplifier using an LM741 to amplify a 0mv-400mv signal to a 0v-4v signal? All the circuits from google or textbooks don't work, or produce strange oscillations. The only way I can get it to work is in inverting mode, where I can get -4v-0v out, which isn't very useful. My specific application for this is for a digital thermometer. I intend to amplify the 0-400mv analogue signal from an LM35 linear temperature sensor, then somehow sample that without using loads of logic or transistors and convert it to decimal or binary, then drive a dual 7-segment display to show the temperature. I have a prototype made using 20 comparators and 10 xor gates, and countless diodes, so if anyone could advise me on an easier way to do that, that would be great. And no PICs/AVRs, I know how to use them and have programmers, but I prefer to do "real" electronics using CMOS chips.


Show your circuit - I use LM35s all the time and amplify them with no problem at all. A simple non-inverting amp is all you need.

For display - try the ICL7106 - analogue in, 7 seg out.


I just checked it out, It looks like it will work, but I couldn't find much documentation on it and it is a rather large package. It also looks like it needs extensive external components to set it up. I want to take a 0-400mv signal from an LM35 (0-40 degrees, 10mv increments) and display the temperature on a dual 7-segment display, any idea how I would do this? I usually get to know the required resistor and capacitor values associated with chips like the 555 timer and various transistors, but I couldn't find much on that particular chip. I can see how to connect the outputs and the crystal, but how would I go about setting the threshold and such?

The datasheet contains a reference design to show you how to wire it.  For an LED you need the 7107, for an LCD, the 7106.

Page 5 on has the information you need to select components.

Its available in QFP packages, which are pretty small - nearly all the DPMs you can buy on Ebay will be based on the old 7106.

Thanks, I didn't notice that before. I guess I will just buy one and try and get it working, unfortunately my emulation software doesn't have that chip. I will have to get the DIP package as I would imagine SMD QFPs would be a nightmare to solder, I have only managed to solder individual 0603s before, and they are hard enough. By "large", I didn't necessarily mean a large package size, but it seems there are too many unnecessary pins. I was hoping for one a bit like the LM3914, but with dual BCD output, and doesn't have an inverting output. However, this looks like my only option, so I guess I will buy one and see what happens, they are only £1.80.

Wow, thanks, I will have a look at that, sounds like it is exactly what I need!

Simplest, given what you have working so far, might be to follow your inverting amp with a unity-gain inverting amp to flip it back to positive...

Standard pointer to Jung's IC Op Amp Cookbook -- wonderful resource.

Re "strange oscillations" -- are you sure your circuit isn't simply acting as an antenna and picking up 60-cycle hum from the house wiring? Shielded box and/or ground plane, shielded and/or twisted-pair cable, no loops... (Been there, done that, though in my case the op-amp circuit was a comparator).

Jung is an excellent resource.

andy70707 741 is a very old undesirable high draw Op-Amp
Look to use a dual Cmos version to invert your amp like ork said, and
how are you going to handle your temperature offset ??


I know how bad the 741 is, but I found a whole tube of them and I don't usually use them, so I would preferably like to try to do it with one instead of buying additional components, although I will if I have to.

Fine for playing around with, certainly. The nice think about op-amps is that -- when used in a suitable feedback circuit -- their behavior comes about as close to the theoretical ideal as anything has a right to, so you can often swap in a different one later without much circuit redesign.

I will have a look at that, and the oscillations definitely aren't caused by EMI as I am testing it on my emulation software. The oscillations are far beyond the 50Hz mains cycle and far below the 2.4 and 5GHz wireless signals. It seems it is just the circuit set up wrong and the 741 keeps inverting.