# single supply opamps. how to?

Discussion in 'The Projects Forum' started by efe_vural@yahoocom, Dec 25, 2011.

1. ### efe_vural@yahoocom Thread Starter New Member

Dec 23, 2011
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Hello everyone,

In my lab project, i have to gererate a square wave and it is very simple to do with split supplies. However there is a bonus for single supply. I did some work researched etc. and i found out that the method for using single supplies introduce instability to the system and to deal with it one needs to put capacitor zener diodes and several things to several places.

I thought of a different approach. First i take the supply voltage give it to a follower(single supply); then give its output to a inverting circuit(single supply). Introducing a voltage divider and half voltage i plan to use it as my new refereance(i think it is called virtual ground) for the rest of the circuit. Therefore i will have a double supply (+vcc and -vcc) and i hope not to deal with instability of a voltage follower and an inverter. My simulation is below. As i intended i got -VCC and +VCC. However implementing it on a breadboard, i could only got positive voltage..

What might be the problem and what might be a sollution?

Thanks for any help

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2. ### praondevou AAC Fanatic!

Jul 9, 2011
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For your virtual ground you only need 1 OPAMP. Remove U2A and U1A. Take U3A and feed the middle point between the resistors to its non-inverting input. The output voltage will be 10V. This will be your virtual ground.

The simulation itself is showing correct results. Oscilloscope channel B shows the output voltage of U2A which is at the maximum positive voltage the output of the LM can go to. That`s 18.7V measured to the power supply ground reference.

The next OPAMP has an amplification of 10 and is inverted so it is also saturated and goes to the most negative voltage possible, which is 0.3V above ground.

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3. ### efe_vural@yahoocom Thread Starter New Member

Dec 23, 2011
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Thank you very much for your repply i did something in simulation as i understood. Simulation is below. Is it what you said?

However i implemented on the breadboard and with 7.5 volts. And it gives 0.05 volts vor positive voltage level and -7.5 volts for negative voltage level refereanced to the output of the opamp as in simulation.

two days i tried to use single supply and several methods i found and worked fine in simulation. However could not put into practice.

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4. ### praondevou AAC Fanatic!

Jul 9, 2011
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yes

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5. ### efe_vural@yahoocom Thread Starter New Member

Dec 23, 2011
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ironically if i measure taking referance the input of the voltage follower it gives +- 3.5 volts for each voltage. It seems that my voltage follower doesn not follow the voltage

6. ### praondevou AAC Fanatic!

Jul 9, 2011
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It's not clear what you mean by "taking reference". What is the reference?
Virtual ground or ground?

Remember also when doing your experiments on the breadboard that the maximum positive output voltage of the LM358 does not go near the rail supply. The low output goes very near the zero though.

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7. ### efe_vural@yahoocom Thread Starter New Member

Dec 23, 2011
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the red and blue cables on the back left are - and + of the power supply respectively; the two resistors are voltage dividers and power supply is connected across them ; ground of opamp is connected to - of power supply and +vcc of opamp is connected to + of power supply; - input and output is short circuited; + input is connected to the half voltage at the middle of voltage divider, the opamp is LM385N, the pink wire is nothing to do with the circuit, since i am going to use that as my new referance, i short circuited output to bottom line.

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8. ### efe_vural@yahoocom Thread Starter New Member

Dec 23, 2011
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i meant by taking referance is while measuring, touching - knob of multimeter to - input voltage of opamp. I did not understood verymuch from what you said about LM358 Characteristics. In lab we are learning very basics opamp...

9. ### praondevou AAC Fanatic!

Jul 9, 2011
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what's the black wire?

Pin 4 of the LM is -, pin 8 is +. Did you invert your power supply voltage?
Pin 8 is +, pin 4 is -. Blue is commonly used for a negative or ground reference, red for positive voltage.

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10. ### praondevou AAC Fanatic!

Jul 9, 2011
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I meant this (from the datasheet):

The output voltage of an OPAMP can only swing inside certain limits.

• ###### output swing LM358.PNG
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11. ### efe_vural@yahoocom Thread Starter New Member

Dec 23, 2011
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I am very ashamed of myself, i was connecting the wrong nodes of power supply to wrong pins,

I don't know how to thank you.

With this configuration i am going to ground rest of my circuit to the output op of amp and i will take +VCC and - VCC from the top and down of the voltage divider. Right?

12. ### praondevou AAC Fanatic!

Jul 9, 2011
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Yes. There is however a limit to the current the output can sink or source. You have to pay attention to that.

You find useful information also HERE and HERE. These are a single supply OPAMP circuit collection from TI and a list of OPAMP parameters and their meanings.

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13. ### efe_vural@yahoocom Thread Starter New Member

Dec 23, 2011
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and also do i have to use any follower before connecting +VCC and - Vcc in case of loading of the remaining circuit?

14. ### efe_vural@yahoocom Thread Starter New Member

Dec 23, 2011
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thanks i had looked up there but could not implement them although worked fine in the simulation.

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16. ### Audioguru New Member

Dec 20, 2007
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Like I said in your other thread (why so many threads?), your circuit does not need a virtual ground circuit. Instead it needs an LM386 power amp that is already internally biassed. The opamps in the schematic are already biassed at half the supply voltage.

EDIT:
There are many threads for your circuit maybe because all the kids in your class posted the assignment here (or maybe on a similar website forum).

Last edited: Dec 27, 2011