Oscillator distortion

Discussion in 'General Electronics Chat' started by MrShhh, Jul 7, 2009.

  1. MrShhh

    Thread Starter Member

    Jul 7, 2009
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    Hi everyone :)

    I've built a FET stabilized wein bridge osc that outputs a sinewave at 1.5v rms from pin 1 of a TL071. The TL071 is a gain stage fed from a TL072 (the wein bridge IC).

    I'd like to energize a small coil (R = 1.2ohms) from the output of the TL071 but the act of adding the coil across ouput & ground badly distorts the original sinewave :confused: I can get around this by adding a 2k resistor in series with the ouput and the coil but then I'm getting insufficient voltage to the coil.

    Is there a way to get the whole 1.5v rms to the coil without distorting the signal coming from the TL071?

    Many thanks :):):)
     
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    More details would be helpful. For example, what is the output frequency of the oscillator, and what is the inductance of the coil. You mention R=1.2 ohms for the coil. This sounds like the DC resistance, but we also need to know the AC reactance. The best thing is to post the schematic to get the most useful feedback.

    It sounds like you are trying to drive too much current from the OPAMP into the coil, but without more information it's hard to be sure. If this is the problem, then you need a current boosting stage or use OPAMPs that can provide the current.
     
  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
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    Oscillators normall drive things thru buffer amplifiers. That way the oscillator is not effected by the load.
     
  4. MrShhh

    Thread Starter Member

    Jul 7, 2009
    24
    0
    Thanks Steve,

    f = 10Hz.

    The coil is bi-filar, wound so it radiates no magnetic field. I don't have an inductance meter but I believe this kind of coil is non-inductive.

    I'll post the schematic as soon as I have made it.
     
  5. MrShhh

    Thread Starter Member

    Jul 7, 2009
    24
    0
    Russ, there is already a buffer stage. The TL072 output feeds the TL071 which has a fixed gain of 3.

    I'm not disturbing the TL072 sinewave signal, I'm taking the signal from the TL071.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
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    Well I can't see a TL07xx driving a load of 1.2 ohms with anything like 1.5 volts rms. That's 2.25 watts.

    Steve is correct you need an output driver stage.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Do you realize how much current you're asking that little TL071 opamp to output?

    I=E/R, or Current = Voltage/Resistance, so 1.5V/1.2 Ohms = 1.25 Amperes. :eek:

    Even with a 2k load on it's output, the TL07x family's output voltage is reduced. By the time the load is down to around 250 Ohms with Vcc=15, maximum output voltage is 7.5v, or half, and the opamp output is sourcing 30mA.

    1.2 Ohms appears to the opamp output as nearly a dead short.
    [eta]
    If you really want to supply that much current to your coil, you could use an emitter follower/buffer circuit like in the attached schematic. It's nothing fancy; parts were chosen based on ease of availability and low cost, rather than high performance.

    Q1 and Q2 could be replaced by a TIP120 Darlington or the like, which would likely be less expensive than the 2N3055.
     
    Last edited: Jul 7, 2009
  8. MrShhh

    Thread Starter Member

    Jul 7, 2009
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    Thanks very much everyone :) I am still learning and this is really helping. I have just learned how to use a "Max peak output voltage vs load resistance" chart! (Cheers SgtWookie :D for peaking my curiosity.)

    I don't need 1.25a through the coil! I have been confusing myself. 10ma max is all that is required.

    So let me get this straight:

    V=IR so 1.5/0.01 = 150 ohms. So I need a 150R in series with the coil to reduce the current to 10mA?

    It seems the TL071 buffer is a poor choice, I should try some other op amps with a greater output current capability. Any recommendations? Would an NE5534 suffice? It know it has a greater drive capability than a TL071.

    What op amp can furnish 10ma with a 150 ohm load resistance?

    Silly question: What is the figure I need to look for in op amp datasheets? e.g. The OPA134 datasheet says it has a max output current of +/- 35ma but this figure is not given for the TL0xx family or the NE5534 in the datasheets I have looked at. Is it called something else maybe?

    Thanks again.
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    There must be a good reason for this.

    What is the purpose of a coil which is non inductive and has no inductance?
     
  10. MrShhh

    Thread Starter Member

    Jul 7, 2009
    24
    0
    I'm exploring the creation of the electrostatic scalar potential and the magnetic vector potential. They become apparent in a bi-filar coil with self-canceling magnetic fields. It is also possible to wind a coil so it has only the electrostatic scalar potential (Mobius coil).
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I don't think the OP actually means Bifilar. As I understand it Bifilar winding is when you have two distinct lengths of wire (4 ends) and wind them together, rather than one on top of the other.

    What I think he means is a single length of wire (2 ends) doubled over and wound onto a former so that the current in each half of the wire is going in opposite directions and the respective magnetic fields oppose and cancel out.

    This method is used in making high power non inductive resistors, the coil shape simply a compact method of placing a longish length of wire.


    Over to you MrShhh

    Edit I see you have replied whilst I was composing this.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I see - so, since your coil resistance is still 1.2 Ohms, and you only need 10mA current, and E=IR (or Voltage = Current x Resistance) so 1.2v x 10mA = 1.2 x .01 = 12mV (0.012V). Alternatively, you could use a current limiting resistor, and/or reduce the amplification in the opamp circuit.

    That would work, and 150 Ohms is a standard resistor value. You could also reduce the gain of the TL071 and use a smaller value resistor.

    If you only need 10mA current through your coil, the TL071 with a 1-transistor (NPN) voltage follower will work just fine. Since you already have the TL071 on hand, adding just a transistor will be a cheap option for you. You could use a wide variety of general purpose NPN transistors.

    If you really wanted more current output, you could use something like an L272 or L2722. They're dual power opamps capable of 0.7A output. They're pretty slow, but fine for your purpose. More expensive than the TL071 with a voltage follower though.

    Take a look at the "output voltage swing vs current" in the OPA134 datasheet .
    Compare that to the "Maximum Peak Output Voltage vs Load Current" in the Texas Instrument datasheet. Since I=E/R (Current = Voltage / Resistance) and R=E/I, you can interpret between the two charts.
     
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