Beginner Op Amp Question - Page 1
Forum section: electronics / beginners

author: iamgrazzi
author_rank: Newbie
timestamp: January 19, 2014, 03:07:15 am
content: Hi everyone! I'm learning electronics on my own so I may be missing some vital information here, which is why this might be a very silly question, sorry!So this video helped me understand op amp circuit analysis:and I've been using this technique to go through and analyze/build the circuits in "Timer, Op Amp & Optoelectronic Circuits & Projects" (Forrest Mims) but there's something I don't understand. Let me explain with the difference amplifier...Here's the circuit:http://mfleisig.files.wordpress.com/2010/10/mims-circuit.jpgSo say there's +0.9V on IN 1 and +5V on IN 2. Because of the voltage divider, there's +2.5V at the non-inverting input. To get +2.5V at the inverting input, the op amp puts +1.6V across the 100K input resistor and current flows toward the +0.9V input. The feedback resistor also has +1.6V on it due to the current flow and equal resistor values. So, the output is +4.1V as expected (IN 2 - IN 1) and because of what the video showed (input voltage + input resistor voltage + feedback resistor voltage), but this is what confuses me. Why is the +0.9V input voltage added when determining the output? From what the video shows, current flows into the input to get the voltage boost on the input resistor, but does this mean that the output of the op amp is applying 0.9V to the input? I'm confused as to why the output is producing the input voltage and where it's going...Sorry if this is a confusing question, and I'm sorry if it's stupid! :\ Thanks!

author: IanB
author_rank: Super Contributor
timestamp: January 19, 2014, 03:44:13 am
content: One of the rules of a perfect or ideal op amp is that the input terminals have infinite impedance. So in your diagram no current can flow in or out of terminals 2 or 3.So to get +2.5 V at terminal 2 you have to look at the potential divider formed between the output at terminal 6 and the input at IN 1. If you solve this potential divider with IN 1 = +0.9 V it follows that OUT must be at +4.1 V in order for the mid-point voltage at terminal 2 to be +2.5 V. A simple way of solving this divider is to compute the sum:(0.9 + 4.1) / 2 = 2.5Hope this helps?

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: January 19, 2014, 04:00:19 am
content: Voltages don't sum like that. Let's simplify the part of the circuit that's giving you trouble. We can just simulate that in spice to see the answer, but where's the lesson in thatThis is a single loop with two supplies in it. If we guess that current will go from the 1.6V supply to the 0.9V supply (which seems like a good guess), we get -0.9V -100k*I -100k*I + 1.6V = 0. a little algebra later, and I = 3.5uA. So the voltage drop across each resistor is 350mV. Double-checking, 0.9+.35+.35 does indeed = 1.6. So the voltage between the resistors is 0.6+0.35 = 1.25, not 2.5!In fact, if you have to voltages joined to a node through identical resistors, the resulting voltage at that node is the average of the voltages. So to get 2.5, you use the formula IanB described.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: January 19, 2014, 04:03:49 am
content: Incidentally, the output is IN2-IN1 (the inverse of what's written on that paper.)

author: Rudane
author_rank: Regular Contributor
timestamp: January 19, 2014, 05:07:14 am
content: It's because you are applying Kirchoff's Voltage law to determine the output Voltage, and in order to do that you need to consider a path. The path you have chosen is from the input to the output, so every voltage along the path is entering into the equation.If you want to be able to examine op amp circuits quickly and with more understanding, learn how to apply the Superposition Theorem to each input.

author: w2aew
author_rank: Super Contributor
timestamp: January 19, 2014, 03:47:46 pm
content: Quote from: iamgrazzi on January 19, 2014, 03:07:15 amThere are no stupid questions, only stupid answers, and we'll all do our best not to provide those.Everyone has their own tricks/techniques that they use to analyze circuits and understand what is going on. So, I'm glad to see a few different approaches here. So, hopefully one of these, or a combination of them will resonate with you.For this circuit, your analysis is right on track. The divider sets up +2.5V on the non-inverting input. Assuming that amplifier is able to properly close the loop, this means that the voltage difference between the inverting and non-inverting input will drive the output to a voltage that brings the voltage at the inverting input to be "nearly" equal to the voltage at the non-inverting input. Since the voltage gain op amp itself is HUGE, the voltage difference between the op amp inputs is minuscule - this is why we say they're equal. Since the inputs ideally draw no current, we're left with a simple series circuit. 0.9v at the IN of the circuit and 2.5 at the op amp inverting input. Since no current flows into the inverting input, the current in the feedback resistor and input resistor are the same. Thus, the same voltage drop appears across the feedback resistor.The output can be thought of an adjustable voltage source that is controlled by the op amp to make the inputs equal. So, we simply add the voltages around the loop. From ground we go up 0.9V with the applied input. Then, we add the 1.6V that appears *across* the input resistor, and add the 1.6V that appears across the feedback resistor, and reach the conclusion that output must be supplying 4.1V in order for the loop to be satisfied.

author: iamgrazzi
author_rank: Newbie
timestamp: January 21, 2014, 01:38:21 am
content: Thanks for all the replies! I've been processing all the information while getting settled back at college and this is what I've come up with, I hope it's right! :]So say I want to find the output voltage of this op amp. I know that +2.5V is at the positive input, so there will be +2.5V on the inverting input as well. I can redraw the inverting input, feedback resistor, and op amp output like this:https://drive.google.com/file/d/0Bwvp6QWTpSVaS0RKSURUNFBwd0E/edit?usp=sharingI know that to get to +2.5V at the input, I must go up 1.6V on the 100K input resistor (and 16uA flows through the resistor towards the +0.9V source). Since I know the current through that resistor, I can use it to figure out the voltage across the feedback resistor (in this case it's the same but just in case it was different).Then my equation to solve this circuit would look like -0.9V - 1.6V - 1.6V + output of the op amp = 0. Then mathematically, I can solve this and find the op amp's output to be +4.1V.Is this correct? How did I do? This is what I took from all of your replies (haven't learned the Superposition Theorem yet) and I feel pretty good about it!

author: w2aew
author_rank: Super Contributor
timestamp: January 21, 2014, 03:09:07 am
content: Looks like you've got it.From a practical standpoint, this holds true only if the power supplies and the output swing capability of the particular op amp used permits it to reach +4.1V in order to close the loop.