opamp dual power supply simulation

[earckens]

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]

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.

Basic opamp circuits

This question is asked repeatedly, and Audioguru always posts a really helpful graphic showing inverting and non-inverting opamp circuits for both single and dual supplies. So to avoid him doing this every time, I'm placing his graphic here, and making this thread 'sticky'. I've also uploaded...

www.electro-tech-online.com

 

[earckens]

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]

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]

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?

 

[Nigel Goodwin]

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?.

 

[earckens]

Would those two resistors not balance the opamp in between Vcc and ground?

 

[alec_t]

balance the opamp in between Vcc and ground?

What does that mean?

 

[earckens]

The purpose should be to allow input signals without DC offset.

 

[Nigel Goodwin]

The purpose should be to allow input signals without DC offset.

Again you make no sense, opamps already allow signals with no DC offset - the standard connections are shown in the sticky I linked to earlier.

 

[earckens]

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]

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]

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]

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]

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]

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.

 

[earckens]

ok, that was clear. Thank you!

 

[audioguru]

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.