Op Amp Circuit Math - Page 1
Forum section: electronics / beginners

author: dB
author_rank: Newbie
timestamp: January 28, 2016, 04:06:51 pm
content: Hello everyone, longtime listener, first time caller...I have a simple circuit I've built up to experiment with opamps. I've included a crappy picture of it. It's a basic non-inverting op amp circuit.I have a 10K resistor at R1, and a 2K2 resistor at R2. 10/2 + 1 == 6. Which is the gain I expect. However, I'm only getting a 1.2 gain, as if the resistors are switched. If I switch the resistors so I have 2K2 at R1 and 10K at R2 I get a gain of six! I'm measuring the gain with my Rigol Oscope, sorry I didn't include any screen shots.I know I'm doing something wrong, because every opamp calculation says R1/R2 +1 for a non inverting circuit, I'm getting just the opposite. WTH!!!PS the opamp is a TI tlo82 dual opamp if that matters, I've checked the pinout many times...Thanks,Brian

author: w2aew
author_rank: Super Contributor
timestamp: January 28, 2016, 07:47:48 pm
content: What are your power supply rails?What exactly is your input signal?My guess is that you're not obeying the input common mode voltage limits of the op amp. For the TL082, the input common mode range only extends to within a few volts of either supply rail.

author: dB
author_rank: Newbie
timestamp: January 28, 2016, 09:54:01 pm
content: The op amp rails are being fed from +10V and -10V from a bench power supply through a TLE "railsplitter". I'm feeding in a 2V p2p sine wave from an app on my phone, since I don't have a proper signal gen.I'll attach a photo of my scope, the Yellow is the orig signal and the Blue is taken from the output pin of the opamp.

author: newbrain
author_rank: Super Contributor
timestamp: January 28, 2016, 11:08:31 pm
content: Have you checked the resistor values? From the (quite out of focus) pic, it would seem 1k ohm (brown, black, black, brown) rather than 10k ohm (br,bk,bk,red).That would give a gain less than 1.5 in one configuration and just above 3 in the opposite one.

author: Lee Leduc
author_rank: Regular Contributor
timestamp: January 28, 2016, 11:52:38 pm
content: Sorry, I'm a little confused here. If your power supply provides +10 volts and -10 volts, why are you using a TLE "railsplitter"?

author: Audioguru
author_rank: Super Contributor
timestamp: January 29, 2016, 02:19:42 am
content: Maybe this and maybe that.We need a detailed schematic showing resistor values that were measured with a multimeter and power supply voltages also measured.Maybe the power supply is a 10V supply with two terminals marked positive and negative.Where is the detailed and labelled schematic of the TLE rail splitter?Is the opamp a cheap Chinese fake from ebay?

author: w2aew
author_rank: Super Contributor
timestamp: January 29, 2016, 02:40:28 am
content: Are you using 10x probes, and properly accounted for the probe attenuation in both channels?

author: dB
author_rank: Newbie
timestamp: January 30, 2016, 01:16:22 pm
content: Thanks for all the responses...I've checked the resistors with my DMM. I'm not proficient on color codes, so that's how I pick them, with my meter...I've double-checked the probes and o scope, they are both set for 10x .My bench power supply is set to output 20 Volts. The railsplitter gives +10v and -10v. (Measured with trusty fluke 87V)Op amp is straight from Mouser.I've put together a schematic in Eagle, I hope it's clear enough...

author: dB
author_rank: Newbie
timestamp: January 30, 2016, 01:37:46 pm
content: I'll add some bonus pictures in to try and help you help me!!!

author: Brumby
author_rank: Unknown
timestamp: January 30, 2016, 01:40:53 pm
content: The output (pin 1) of the TLE2426 should be your 0v (virtual ground). This is where the bottom of R2 should go.The positive of your supply becomes the +10v rail and the negative becomes the -10v rail. You need to change the +ve supply connection to the op-amp.

author: dB
author_rank: Newbie
timestamp: January 30, 2016, 02:25:49 pm
content: Of course you're correct. An error in drawing up the schematic.Here's an updated version.

author: Brumby
author_rank: Unknown
timestamp: January 30, 2016, 02:42:25 pm
content: Looking better.Where is your scope earth connected and where is your input signal earth connected?

author: Brumby
author_rank: Unknown
timestamp: January 30, 2016, 02:52:51 pm
content: Ahhhh - hang on...That's a1Kresistor!As stated above:Quote from: newbrain on January 28, 2016, 11:08:31 pm

author: dB
author_rank: Newbie
timestamp: January 30, 2016, 03:16:03 pm
content: Quote from: Brumby on January 30, 2016, 02:52:51 pmWell, that's only a LITTLE embarrassing!!!Sure enough, when I replace the 1K resistor with a 10K resistor, the math works!!!!10 / 2.2 = 5.54This is NOT the lesson I wanted to learn from this experiment, but apparently it was neccessary!!!Thank you all for the help, I'll go back to lurking now...The proof!

author: Brumby
author_rank: Unknown
timestamp: January 30, 2016, 03:19:55 pm
content: Quote from: dB on January 30, 2016, 03:16:03 pmThat's OK. We've all had moments like that - but we try and avoid doing so on a global stage.

author: newbrain
author_rank: Super Contributor
timestamp: January 30, 2016, 07:52:38 pm
content: Quote from: Brumby on January 30, 2016, 03:19:55 pmYes, but sometimes the only way to see the obvious is through someone else's eyes!Glad to have been of some help.