op amp buffer into standard op amp config - Page 1
Forum section: electronics / beginners

author: BeatleManiac
author_rank: Contributor
timestamp: January 17, 2016, 02:54:29 pm
content: Hi everyone!I think I already know the answer to this but wanted to check it out with you guys first. Please let me know if the following statement is true:It is not necessary to put an op amp in buffer configuration before another op amp in a "standard" configuration with gain to get current isolation from the source signal. Just using the op amp with gain will give you the isolation you need.The signal coming into the circuit is from a passive guitar with an amplitude around 400mV. I need to boost it to around 2.5V to feed an Arduino, but I want to make sure I isolate the signal.Thanks!Steve HurdFloyds Knobs, IN

author: LvW
author_rank: Frequent Contributor
timestamp: January 17, 2016, 04:51:26 pm
content: Quote from: BeatleManiac on January 17, 2016, 02:54:29 pmWhat is the meaning of "standard configuration"? Most important: Inverting or non-inverting configuration?

author: BeatleManiac
author_rank: Contributor
timestamp: January 17, 2016, 05:13:33 pm
content: Meaning with a feedback resister to have more than unity gain. Sorry for this nube question: Why does inverting/non-inverting matter as long as its an amplyfing configuration? I guess I'd prefer non-inverting unless there's something I'm missing.Thanks for the help!Steve Hurd

author: Audioguru
author_rank: Super Contributor
timestamp: January 17, 2016, 05:47:18 pm
content: You want what isolated from what? I assume the guitar transducer is a magnetic pickup, not a microphone? I also assume the opamp as a preamp drives the high input impedance of a recording device or power amplifier?A guitar magnetic pickup is usually fed to a preamp that has an extremely high input resistance to allow the pickup to resonate at a high audio frequency to produce a guitar "twang" sound. In the olden days an opamp did not have an extremely high input resistance so a Jfet preamp was used as a "buffer". Today some opamps have Jfets in their inputs. Use a very high input resistance for a Jfet-input opamp.

author: dom0
author_rank: Super Contributor
timestamp: January 17, 2016, 05:55:33 pm
content: Quote from: BeatleManiac on January 17, 2016, 05:13:33 pmInverting amps have low input resistance, but have greater abilities when it comes to input and output connections and shaping of the frequency response. They also have larger input errors compared to a non-inverting amp with the same signal gain.Non-inverting amps have very high input resistance, but are lesser in abilities and have less input errors than equivalent inverting amps.The quoted statement above is only true for non-inverting amps, since inverting amps have a low input resistance which loads the source.First stage (input) amplifiers for LF/audio applications are almost always non-inverting for the high and easily adjustable input resistance.

author: BeatleManiac
author_rank: Contributor
timestamp: January 17, 2016, 10:36:02 pm
content: The main thing I'm using the signal for is for a "visualizer" circuit that will drive programmable LED's to animate based on the amplitude/frequency/beat detection of the guitar signal. It wont be used to change the sound of the guitar. It will basically be a read-only circuit with the signal going on to other pedals and my bass amp for the actual audio processing.

author: BeatleManiac
author_rank: Contributor
timestamp: January 19, 2016, 02:25:33 am
content: I'm using an LF 353N dual op amp in this project. Since it has the jfets you mentioned, it sounds like I can just go with one side in the non-inverting configuration and terminate (ie not use) the other side.