Op amp voltage question

Banjo · Posted: Sat Jun 16, 2012 10:24 am Post subject: Op amp voltage question

When taking a voltage reading of the output from a bipolar op amp, I noticed that sometimes I would get a reading with a negative voltage or a positive voltage depending on whether I had the meter set for volts or millivolts. It has only been within the past few days that I started to delve into the electronics side of DIY instead of just copying my circuit from the schematic and possibly switching component values here and there. I prefer to research the answers to my questions before I ask them here, but sometimes the answers can take hours to find and decipher, when that time could be spent building, playing or cleaning up the work area. So I have two questions.

What is the proper way to determine the voltage output from a bipolar output. I assumed that you could just put the black lead to ground, and red to the output.

Question two.

Is there any "standards" or rules of thumb relating to voltages being sent to other modules or circuits? If the signal is to be used as an audio signal it should be x volts, if it is to be fed into a gate or a cv.... I suspect that the answer is not that simple and is determined on a case by case basis. Perhaps I should be asking are there upper and lower bounderies that should be stayed other than the obvious don't go beyond what the component is rated for.

As always thank you for your replies.

Blue Hell · Posted: Sat Jun 16, 2012 11:03 am Post subject: Re: Op amp voltage question

Blue Hell · Posted: Sat Jun 16, 2012 2:02 pm Post subject:

frijitz sent me a correction on this :

Banjo · Posted: Sat Jun 16, 2012 6:24 pm Post subject:

Ok, so let's say we have 10vpp. Bipolar that would be +5v to -5v, right? Is possible for that to be +7v to -3v? I suspect that it is, but I have been wrong before in my assumptions.

This latest breadboard project is really pushing me into new territory as far as my understanding of electronics goes, and the community here has helped me immensely. Thanks again.

Tim Servo · Posted: Sat Jun 16, 2012 8:24 pm Post subject: Op amp voltage question

Hey Banjo,

Welcome to the weird world of Volts, Ohms, and Amps! If you're trying to measure an AC voltage (like the output of an oscillator) with a voltmeter set to read DC Volts, that may explain the odd readings you're getting. Even if the meter is set to measure AC Volts, a slow moving 'LFO' signal will be pretty much impossible to accurately measure. For this sort of thing, you'll be doing yourself a HUGE favor to grab an oscilloscope. Even an older model will work fine for most audio work, since you won't need to measure anything over 20kHz very often.

You may well be getting a signal that runs from +7 to -3V, especially if your power supplies are off (ideally you're using something like +15 and -15V supplies). Op amps can also have a slight bias or 'offset' in their outputs that will need to be adjusted out in precision applications, but I can't imagine the +2V offset you're seeing is coming just from the op amp. The offset voltage us usually more like 0.1V or less.

As far as 'standards,' well, there are several to chose from Wink

A commonly used standard published in Electronotes years ago includes
Audio and LFO waveforms: +5 to -5V
Envelopes: 0 to +5V
Triggers: +5V

Of course, some synths go with slightly different numbers. My ARP produces 0 to +7V waveforms, and takes at least +10V to trigger. Still, one big thing to remember is that you want your circuitry to be protected and not fry itself if someone puts in a larger signal. If you want to interface with modular gear from a variety of sources, you really should expect anything up to +15 to -15V (right up to the limits of the power supply) although signals greater than +10 to -10V are rare. Most modular designs you see around are fairly well protected, with large value resistors (10K, 50K, and 100K at the inputs being most common) on the inputs, and smaller resistors (100 Ohms to 1K) for short circuit protection on the outputs.

Hope this helps!

Tim (electric shorts) Servo

Banjo · Posted: Sun Jun 17, 2012 7:12 pm Post subject:

Thank you for the response Tim. I picked up a scope two weeks ago at a yard sale cheap. It's about 35 years old, but does what I want it to. Still learning some of it's finer limited details. My question about the 2 volt offset was just curiosity, but your response filled in some of the grey areas in my limited knowledge. Your standards, we're what I was looking for. Thank you again.

What are the advantages and disadvantages of running at 15 volts rather than say 5v or in my case 9v? One of these days, I need to build a power supply, instead of running on batteries.