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  1. #1
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    simple symmetrical power supply

    Does the mostly used opamp circuits (e.g., inverting amplifier, non-inverting amplifier, differentiator, integrator, difference amplifer, voltage follower, etc) would work if we don't use a symmetric voltage source (i.e., we apply a positive voltage value at the 7th pin of LM741 and ground the 4th pin)?

    I have a single output power supply and I don't want to put an voltage inverter in my every opamp circuit. Is there a simple way to feed opamps from a single power source?

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    how a symmetric power supply works

    1. You can use rail-to-rail OP that have an input voltage range including the supplies and output voltages near to supply.

    2. There are single supply variants of most basic circuits, some with serious disadvantages compared to usual dual supply.

    I suggest to check the possible in- and output voltage ranges for each circuit yourself. If you have e. g. an inverting amplifier, it would clearly need a virtual ground at mid-voltage, how should it be able to invert an input voltage otherwise? The same with usual inverting differentiator or integrator. A voltage follower in contrast can work almost rail-to-rail.


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    •   Alt17th July 2008, 07:34

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    symmetric supply

    Quote Originally Posted by hkBattousai
    I have a single output power supply and I don't want to put an voltage inverter in my every opamp circuit. Is there a simple way to feed opamps from a single power source?
    In addition to the contribution from FvM I like to remind you on the simple transistor stage in common emitter configuration: Here you use a voltage divider to bias the active part (BJT) somewhere in the quasi-linear region of the transfer curve.

    In case of opamps you can do the same: You have to bias the opamp somewhere in the middle of its transfer curve - i.e. approximately at Vdd/2 - with a resistive divider at the positive input terminal
    Only in this case the output voltage can change in both directions.
    This method can be applied to all opamps - not only to the so called "single supply" opamps (these are "only" suited to this kind of operation as far as rail-to-rail operation is concerned).

    However, be aware that you have to use now coupling capacitors (one input cap for inverter configuration and one additional grounded cap in the feedback path for non-inverter configuration).


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    opamp single power source

    Let me ask you this. In the picture below, you see an opamp in 'inverting amplifier' mode. But the -V pin is connected to the ground.



    How do you expect this circuit to work? What is the input-output characteristic?
    Assume that V2=15V. Can you calculate the output voltage (V0) wave form when the input voltage (V1) is

    a) 1V DC
    b) 2*cos(wt) V
    c) 1 + 2*cos(wt) V (input voltage fall into negative region), Is this circuit linear?
    d) 3 + 2*cos(wt) V (input voltage swings in the positive region)



    •   Alt17th July 2008, 22:22

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    symmetric voltage

    Quote: How do you expect this circuit to work? What is the input-output characteristic?
    Assume that V2=15V. Can you calculate the output voltage (V0) wave form when the input voltage (V1) is
    a) 1V DC
    b) 2*cos(wt) V
    c) 1 + 2*cos(wt) V, Is this circuit linear?


    1.) Correction to your circuit diagram: You have exchanged both power supplies !

    2.) Answers:
    a) No change of the output voltage (nearly zero) as it cannot go negative.
    b) half waves (sinusoidal) positive between app. zero and 2*5=10 volts
    c) as b) but with max. amplitude of only 5 volts (as the negative input range is smaller by 1 volt).

    Resumme: unsymmetrical operation, only negative inputs are amplified. For symm. operation biasing of the opamp at app. Vdd/2 is necessary together with coupling capacitors which cause a low corner frequency.


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    •   Alt17th July 2008, 23:16

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    lm741 without simetric power supply

    Most ordinary opamps have inputs with an allowed common mode voltage range. The inputs must be about 2V or more above the negative power supply and 2V or more less than the positive power supply.
    With a single positive supply then an input does not work if it is grounded, it must be at least +2v for it to work properly.

    If you bias the inputs at half the supply voltage then any opamp will work with a single supply voltage. Capacitor coupling might be needed at the input and output.


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    symmetrical power supply

    yes system will work like between 12v and 0 v,also u can use 0v and -12v,the only difference will be that system wull go only in one saturation stage


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    voltage follower symmetric supply

    For example this circuit will work without symmetric power sources.


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    lm741 always saturated

    Quote Originally Posted by jony130
    For example this circuit will work without symmetric power sources.
    Hmm...
    Can you explain the function of the C3 capacitor here?



    From what understand your answers, when we don't use symmetric voltage sources with opamps, for a linear opamp circuit,

    1) We cannot use inverting amplifier mode since the output voltage will fall into negative region which will make the opamp saturated

    2) Opamp will work with no problem if we give the input voltage in the allowable limits and make the feedback such that the output voltage of the opamp does not exceed the supply voltage range.

    Did I understand it right?



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    opamp without symmetric

    Quote Originally Posted by hkBattousai
    Can you explain the function of the C3 capacitor here?
    C3 reduce the DC gain to unite (1V/V) for stable biasing.

    Quote Originally Posted by hkBattousai
    From what understand your answers, when we don't use symmetric voltage sources with opamps, for a linear opamp circuit,
    1) We cannot use inverting amplifier mode since the output voltage will fall into negative region which will make the opamp saturated
    2) Opamp will work with no problem if we give the input voltage in the allowable limits and make the feedback such that the output voltage of the opamp does not exceed the supply voltage range.
    This is a linear not-inventing opamp amplifier. The gain is equal Au=1+(R2/R1).
    The voltage divider R3,R4 bias the opamp on 0.5Vcc.
    So this circuit behave as a symmetric supply opamp whit ±0.5Vcc (±6V) supply voltage.
    And here you have the inverting amplifier operate with a single supply to.



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    virtual ground symmetric power

    Quote Originally Posted by hkBattousai
    Can you explain the function of the C3 capacitor here?
    From what understand your answers, when we don't use symmetric voltage sources with opamps, for a linear opamp circuit,
    1) We cannot use inverting amplifier mode since the output voltage will fall into negative region which will make the opamp saturated
    2) Opamp will work with no problem if we give the input voltage in the allowable limits and make the feedback such that the output voltage of the opamp does not exceed the supply voltage range.
    Did I understand it right?
    In addition to the explanations and figures as given by jony130 I like to give you some more detailed answers to your questions:

    To C3: For DC the gain must be unity (100% feedback for DC) with the aim to transfer the DC potential at the pos. terminal also to the neg. terminal (that means biasing of BOTH inputs at Vdd/2)

    To 1) If the opamp is biased at Vdd/2 you can use it in any mode, because the output can go in both directions around the operating point which is at Vdd/2.
    To 2) This is always the case - independent on symmetric/unsummetrc supply.

    Further questions ?


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    Can op-amps work without symmetric power sources?

    Thank you for your answers.



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