Op Amp Rail Voltages

Thread Starter

SamR

Joined Mar 19, 2019
4,138
"The Op Amp is powered by a 12V rail". Does that mean ±12V (assume so), or only +12V, or ±6V?
 

AnalogKid

Joined Aug 1, 2013
9,981
Any particular opamp? IOW, is that line a quote from a datasheet or app note?

Agree with bob; to me that sounds like +12 V and GND, no -12 V or anything else.

ak
 

ZCochran98

Joined Jul 24, 2018
205
It would appear the article is intending a single-source 12V supply, in this case (based on the context about the 12V supply leading to a 6V threshold on the trigger). The oscilloscope images also swing between 0 and 12V, so that would track with 12V rail = 12V between positive and negative supply pins.
 

sparky 1

Joined Nov 3, 2018
706
If the op amp such as LM358 is a single supply variety the appropriate PS will usually have a single supply rail. It should be VCC and GND. There are plenty of cases where it is configured with +Vs and -Vs as an exception.
The dual supply op amp has two supply rails. We grew up when multiple supply rail power supplies were coming on.

We ask ourselves is it a single supply or a dual supply and go with the appropriate label as you did.
 

Papabravo

Joined Feb 24, 2006
18,152
It's quote from a Circuits Digest article on using Op Amps as Schmitt Triggers. Op Amp being used in this article is the LM741 but could be any dual PS input op amp.
https://circuitdigest.com/electroni...t-using-op-amp-for-high-speed-digital-signals
You would not normally want to use an LM741 in single supply mode. The main reason is the output won't go anywhere NEAR ground in single supply mode. The LM741 should always be used with a bipolar power supply. The output should be able to get 2.5 to 3 volts from either rail. I would suggest ±8-9 volts minimum for using this part.
 

Thread Starter

SamR

Joined Mar 19, 2019
4,138
At this point I'm playing around with LTS using an LM308 instead of the LM741 in the article. I also have the LM308 (and several other models) in stock for when I get to breadboarding. I've done the basics using op amps, but this caught my interest and is leading up to using op amps as an astable multivibrator instead of the NE555. Interesting concept to me. The idea here is to set the midpoint voltage (or any other by adjusting R1, R2, and R3) as a trigger to turn output on/off. New territory for me...
 
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AnalogKid

Joined Aug 1, 2013
9,981
If the op amp such as LM358 is a single supply variety
Actually, it isn't.

Don't let the images in the datasheet fool you. The LM358 is a dual-supply, 36 V opamp. Granted, it has significant performance differences from the other opamps of its day, such as that magical input stage and a TTL-friendly output stage. But it will run on split supplies just fine. In looking for ways to highlight its differences from the pack, National made the very interesting decision to label pin 4 "GND" rather than "V-". Generations of designers have misinterpreted (or been mislead about) what this pin actually is.

Here's a question: Is there or has there ever been a true single-supply bipolar opamp?

ak
 

Ian0

Joined Aug 7, 2020
4,899
"The Op Amp is powered by a 12V rail". Does that mean ±12V (assume so), or only +12V, or ±6V?
I'd understand that as 12V and 0V. If I wanted to say that it was powered by ±12V, I'd say "The op-amp is powered by ±12V rails" (plural). To be perfectly clear, in the 12V/0V case one could say "The op-amp is powered by a single 12V rail". Sometimes complete clarity only requires one extra word!
 

ericgibbs

Joined Jan 29, 2010
15,378
It's quote from a Circuits Digest article on using Op Amps as Schmitt Triggers. Op Amp being used in this article is the LM741 but could be any dual PS input op amp.
Hi Sam,
The CA3140 was introduced as a 741 replacement, it's inexpensive and suitable for many applications.
Also comes in a dual version CA3240
E
Clip:
Features
• MOSFET Input Stage
- Very High Input Impedance (ZIN) -1.5TΩ (Typ)
- Very Low Input Current (Il) -10pA (Typ) at ±15V
- Wide Common Mode Input Voltage Range (VlCR) - Can be
Swung 0.5V Below Negative Supply Voltage Rail
- Output Swing Complements Input Common Mode
Range
• Directly Replaces Industry Type 741 in Most Applications

Added: the CA3240 d/s. Lots of example circuits in both d/s.
 

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Ian0

Joined Aug 7, 2020
4,899
It's quote from a Circuits Digest article on using Op Amps as Schmitt Triggers. Op Amp being used in this article is the LM741 but could be any dual PS input op amp.
https://circuitdigest.com/electroni...t-using-op-amp-for-high-speed-digital-signals
Not all op-amps can be used as Schmitt triggers, some have input protection diodes (NE5534 for instance) which conduct if there is a difference of more than ±600mV between inputs.
What you really need is a comparator.
 

AnalogKid

Joined Aug 1, 2013
9,981
So to use any dual supply op amp as a single supply the - side has to be grounded? Makes sense...
Or the + side. The opamp has no intrinsic opinion. It will run just fine with V+ connected to GND and V- connected to -24 V (or whatever). It all depends on the input and output signal voltage ranges.

ak
 

Audioguru again

Joined Oct 21, 2019
4,539
CircuitDigest.com is produced in a large country on the other side of the world where some electronic circuits are shown connected wrong and are described wrongly.
 
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