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opamp dual power supply simulation

earckens

Member
Could a dual power supply for opamps be simulated by using a single DC source and 2 1k resistors, one from DC source to +Vcc, and one from -Vcc to ground?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, and it often is - it's a standard configuration - but comes with numerous limitations.

If you look in the stickies of this very forum, there are examples.

 

earckens

Member
Thank you for those very useful references. However these schematics use a voltage divider to create an offset on the inputs. My question was rather about balancing the entire opamp between Vcc and ground by using two resistors in the power supply lines (Vcc and ground). A 741 needs about 3mA, so what about 2x 100 Ohms or lower? Would that be electrically correct?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thank you for those very useful references. However these schematics use a voltage divider to create an offset on the inputs. My question was rather about balancing the entire opamp between Vcc and ground by using two resistors in the power supply lines (Vcc and ground). A 741 needs about 3mA, so what about 2x 100 Ohms or lower? Would that be electrically correct?
I'm sorry, but I've no idea what you're on about? - could you post a diagram of what you're trying to do?.
 

earckens

Member
Schematic attached. This is for a filter testboard. My issue is with the use of R1 and R2: would that be an acceptable means to create an offset for incoming AC signals on Vin?
 

Attachments

Nigel Goodwin

Super Moderator
Most Helpful Member
Schematic attached. This is for a filter testboard. My issue is with the use of R1 and R2: would that be an acceptable means to create an offset for incoming AC signals on Vin?
Sorry, still makes no sense - as your schematic doesn't - all it shows is 47 ohms in the power connections?.
 

alec_t

Well-Known Member
Most Helpful Member

earckens

Member
The issue is that when I apply an AC voltage (with reference to "ground") to an input of an opamp with +Vcc connected to the supply pin, and "ground" connected to the groundpin, then only half wave signals appear on the output (the negative part of the AC wave being suppressed by the opamp connected to "ground").

So besides the options described in the Texas Instruments Application Report SLOA058 I would like to clamp the opamp halfway between Vcc and ground.

And therefor allow the opamp output to fully swing, for the full AC input wave, between Vcc and ground.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The issue is that when I apply an AC voltage (with reference to "ground") to an input of an opamp with +Vcc connected to the supply pin, and "ground" connected to the groundpin, then only half wave signals appear on the output (the negative part of the AC wave being suppressed by the opamp connected to "ground").
I refer you again to the sticky which shows both AC and DC connections, along with single and dual supplies.
 

earckens

Member
I refer you again to the sticky which shows both AC and DC connections, along with single and dual supplies.
Hi Nigel, yes I understand what you say; my question however refers to a possible alternative way of obtaining a full wave output when using a single power supply; besides the solutions described in this sticky.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hi Nigel, yes I understand what you say; my question however refers to a possible alternative way of obtaining a full wave output when using a single power supply; besides the solutions described in this sticky.
And how do you expect leaving out all the required parts that make a circuit is going to help?.
 

earckens

Member
For a -suppose- first order high pass non-inverting active filter, a capacitor C1 between AC source and +IN is required, and a resistor R1 between +IN and ground. R2 and R3 provide negative feedback (out to -IN) and between -IN and ground. Classical layout.

So when applying a single supply to Vcc, and the ground pin to ground then the output would be half wave signals only.

Now my theory is that if Vcc is connected to the opamp Vcc pin but with a (for example) 47R resistor in series; and the opamp ground pin connected to a 47R resistor in series to ground then the opamp should in theory be clamped halfway between Vcc and ground therefor an AC signal on the non-inverting input would be amplified to a full wave output signal?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Now my theory is that if Vcc is connected to the opamp Vcc pin but with a (for example) 47R resistor in series; and the opamp ground pin connected to a 47R resistor in series to ground then the opamp should in theory be clamped halfway between Vcc and ground
No, to be blunt.

That would lift the ground pin by a minuscule amount with most opamps, and equally offset the positive supply pin a fraction below the actual supply.

The voltage across each resistor is purely proportional to the current through it; it "clamps" nothing.

Edit - to clarify one of your other comments:
The signal is not "suppressed" by the ground connection. The opamp can only drive its output pin within its supply voltage range, so no more negative than the negative supply and no more positive than the positive supply; or significantly less, not even close to the supplies with some older opamps like the 741.

The circuit DC bias must be designed to keep the output within the device working range at all times, if you want a linear output.
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
I have made thousands of opamp circuits that use a single supply voltage. Of course the + input was biased at half the supply voltage and negative feedback was used.
 

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