Does LM358 need a negative supply voltage? - Page 1
Forum section: electronics / beginners

author: awesomechapro
author_rank: Contributor
timestamp: January 04, 2022, 09:51:53 am
content: Hi, I am currently making an ac signal inverter with an lm358. I was wondering if I need the -5v supply voltage or if I could just connect it to ground instead? This is the circuit I am using:

author: magic
author_rank: Super Contributor
timestamp: January 04, 2022, 09:59:17 am
content: Hard to tell without knowing what the input signal is going to be, but as a matter of fact, an opamp cannot output voltages lower than its negative supply and that will be a problem if the input goes below ground.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2022, 10:09:30 am
content: One of the 2 OPs will need the negative supply to have a negative output. So this circuit would need a negative supply.

author: awesomechapro
author_rank: Contributor
timestamp: January 04, 2022, 10:10:30 am
content: The input signal is an audio signal, and does go below ground. So do I need to connect pin 5 of the lm358 to -5v?

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 04, 2022, 11:52:04 am
content: Quote from: awesomechapro on January 04, 2022, 10:10:30 amyes... unless you set input to 1/2 DC supply and use capacitors at input and output

author: Terry Bites
author_rank: Super Contributor
timestamp: January 04, 2022, 02:47:02 pm
content: It does.Heres an atricle on what you can do with a single suplly if thats all you have.https://www.ti.com/lit/pdf/sloa030?keyMatch=SLOA030

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2022, 02:53:10 pm
content: For the given circuit it does not matter which signal (except fixed 0) - it would allways need a negative ouput from one of the OPs. The signal just decides which OP would need the negative voltage. So the output would be essentially always be near zero.For audio the LM358 is a really bad choice, as it shown lots of cross over distorion. With a negative supply one could as well use an RC4558 or similar.

author: rstofer
author_rank: Super Contributor
timestamp: January 04, 2022, 02:58:26 pm
content: Google for 'rail splitter' or 'rail splitting' and you'll get something like this:https://www.maximintegrated.com/en/design/technical-documents/tutorials/4/4344.htmlNotice that there are physical devices designed as 'rail splitters'. The advantage is lower output impedance than you might want to get with low resistor values as a voltage divider.Be careful when you select the voltages for the rails. Check "Output Voltage - High Limit" (page 4) and you'll see that for a 5V single rail design, you can't get above 3.3V and for "Output Voltage - Low Limit" you can't get below 20 mV.https://www.onsemi.com/pdf/datasheet/lm358-d.pdf

author: awesomechapro
author_rank: Contributor
timestamp: January 06, 2022, 01:39:28 pm
content: Although if I added a dc offset to the signal wouldn't I need to then subtract that dc offset again at the end with another op amp? That would still require a negative voltage input? I would prefer to have only a positive supply voltage but I need an ac signal output. So what op amp should I use for an ac output?

author: rstofer
author_rank: Super Contributor
timestamp: January 06, 2022, 02:50:41 pm
content: That's why you see single rail amplifiers with capacitor coupled input and output. You set up a DC bias on the input to center the output and then couple in AC signals through a capacitor. You remove the DC in the output with another capacitor before the next stage (or whatever).

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2022, 10:07:11 pm
content: The op-amp needs to be biased, at half the supply voltage.

author: rstofer
author_rank: Super Contributor
timestamp: January 07, 2022, 02:31:44 am
content: That's an excellent page, thanks for posting it!