Wien Bridge Oscillator HELP!

Discussion in 'Homework Help' started by kfc123, Jun 21, 2010.

  1. kfc123

    Thread Starter New Member

    Jun 21, 2010
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    Hello,

    I am making a Wien Bridge Oscillator, however, I have a few problems:

    1) One uniqueness of Wien Oscillator is that it has no energy source: Since the energy source comes from capacitor, the sine wave can't be maintained permanently, meaning we have to charge the capacitors everytime they are fully discharged, how to overcome this?

    2)Since f=1/2piRC => frequency depends on R and C, how to select appropriate R,C to get my desirable frequency, lets say 20kHz?

    3) There can be resistors connected to inverting input (-) of an op-amp in Wien Oscillator, what are their function? Since frequency is determined by R,C at non-inverting input, so why are there resistors at the inverting input? How and why to determine their values? And sometimes there can be diodes and 3/4 resistors at the inverting input, in this case, how to determine their values again?

    4) How to control the peak amplitude of the sine wave? Lets say the wave oscillates between -5V and +5V.

    Thanks a lot !!! I really appreciate your help. :D
     
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  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    All Wien bridge oscillators use some form of controlled current feedback, sometimes as simple as a small light bulb. Rather than reinvent the wheel, I'd look for some existing schematics for wien bridge oscillators. They've been around for about 75 years.

    Eric
     
  3. stancomm

    New Member

    Jun 21, 2010
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    Hi, Mr. kfc123,
    Actually the energy source for the circuit comes from a power supply, controlled or modulated by the amplifier... ya gotta have gain somewhere or as you stated, the oscillation will die out.
    As regards circuit values, the resistors at the negative input form the gain setting and by that, the amplitude and distortion levels and are to an extent dependent on the characteristics of the particular op-amp you choose to use..
    Best I can offer is this app note found on the Wiki site, http://focus.ti.com/lit/an/sloa060/sloa060.pdf ,
     
  4. Audioguru

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    Dec 20, 2007
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    A Wien bridge oscillator will produce clipping distortion unless a level-stabilizing circuit is used.
    A light bulb or Jfet can be used to control the output level.
     
  5. kfc123

    Thread Starter New Member

    Jun 21, 2010
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    Thanks for all responses !!

    But I still have a question that I can't figure out even though after many searchings in the net and book. Which pin (op-amp) does the dc power supply have to be connected to? the Vcc and Vee pin? But then why is input often referred to + terminal (non-inverting terminal) (as said by many books and online resources)? I mean how does the input voltage at Vcc find its way to + terminal and also maybe remained unmodified? Please help !! Thanks !!!
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    This will depend much on the OPAMP that you are using. Some OPAMPs must have dual rail power supply (like +/- 15 volt) But other can use a single supply voltage. It is a common beginner mistake to not take this into consideration. Can you post your schematic and tell us which type of OPAMP you plan to use, and also what voltage(s) you will use for power supply
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    What you are asking is basic op amp theory. Op amps typically (but not always) use negative feedback to balance voltages where the two voltages at the + and - inputs are the same.

    The AAC book has a section devoted to them.

    Chapter 8: OPERATIONAL AMPLIFIERS
     
  8. kfc123

    Thread Starter New Member

    Jun 21, 2010
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    I am using single power supply: 24 VDC, so the preferred op-amp could me lm358(not sure if this is a cheap 1, all I need is an op-amp which works with single power supply of 24 VDC, best if it is as cheap as possible and also reliable). In addition, I heard that lm741 could work with single power supply, is this true?

    My schematic is pretty simple: A positive feedback (R + C in series with R || C) at + terminal and a network of resistors with diodes at - terminal. Please refer to attachment.(actually this schematic is extracted from somwhere in this forum...)

    As far as I know, what I need to do is to maintain the gain of op-amp at 3. For this reason, diodes are implemented in the design. Well, I derived an expression relating R3,R4,R5, using the condition that gain of op-amp must be 3. Since the expression involves more than one unknown, it is not possible to determine the resistance, so is it ok to select any resistances as long as the expression is satisfied?

    Another issue that I'm facing is the output must be a sine wave with +-5V (10V peak-to-peak). How to achieve this from a 24VDC single power supply? Did I miss out an expression in determining the amplitude of sine wave output? Please advise :D

    Thank you !!!
     
  9. kfc123

    Thread Starter New Member

    Jun 21, 2010
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    Thanks !
    If I use a LM358 or LM741 op-amp, and if I use a 24VDC single power supply, how to determine the input voltage at + and -? Are they actually equal to 24VDC? How to make sure that the input voltage is going to + and not -? Thanks again !
     
  10. kfc123

    Thread Starter New Member

    Jun 21, 2010
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    Sorry, here's the attachment.
     
  11. Audioguru

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    Dec 20, 2007
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    Any opamp will work from a single-polarity power supply if its inputs are biased at about half the supply voltage. The resistor to ground in the negative feedback needs a coupling capacitor to ground so that its DC voltage is not shorted. The output might need a coupling capacitor.

    The LM358 has two opamps inside. It and the LM741 opamp are old and have poor performance. I use the TL071 single audio opamp that is low noise, has very low distortion and has a bandwidth to 100kHz.

    The circuit oscillates when its gain is more than 3. The diodes limit the output level by reducing the gain (and cause some distortion when they begin conducting) when they have a signal of about 0.6V. Change the resistors in the negative feedback loop to increase the output to 10V p-p.
     
  12. kfc123

    Thread Starter New Member

    Jun 21, 2010
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    Thanks ! Can I have a schematic involving coupling capacitor in the negative network and also in the output?
     
  13. Audioguru

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    In my sketch of the oscillator with a single-polarity power supply, the coupling capacitor to ground for the negative feedback divider resistor that went to ground on a dual-polarity power supply oscillator is the 22uF capacitor connected to R10.

    If you want to block the DC at the output but pass the AC signal then connect a capacitor in series with the output of the oscillator circuit. Its value must be high enough to pass the lowest frequency you want into the lowest load impedance that you will use.
     
  14. kfc123

    Thread Starter New Member

    Jun 21, 2010
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    why there are two extra resistors (10khom each) and one 47microF capacitor in the single supply case?
    why block DC signal at the output? I thought that the output should carries only AC signal?
    what do u mean by putting a coupling capacitor in the negative network so that DC voltage of the resistor to ground is not shorted?

    hehe...too many questions...sorry and thanks !!
     
  15. Audioguru

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    Dec 20, 2007
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    The resistors make "half the supply voltage" and the 47uF caspacitor filters it./ it is the bias voltage for the opamp.

    The output of the opamp is at half the supply voltage (a DC voltage) so it can swing up and swing down to make AC. the DC voltage might upset a circuit it feeds so a coupling capacitor is used to block the DC but pass the AC.

    the 22uF capacitor prevents R10 from being shorted to ground for DC.
    If R10 is shorted to ground then the opamp will try to amplify the "half the supply voltage" at its non-inverting input. Then the output of the opamp will be saturated near the positive supply voltage and it will not do any useful work.
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Creating a Virtual Power Supply Ground

    A simple voltage divider and capacitor is all that is needed for most op amp applications, but the above article is meant for more power hungry beefy applications.
     
  17. Prof78

    New Member

    Dec 24, 2010
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    A fuller understanding of the design and operation of the Wien bridge oscillator may be found in the reference book (and other references therein):
    An Analog Electronics Companion by Scott Hamilton (Cambridge University Press, ISBN 978-0-521-68780-5), p449. The discussion is too extended to reproduce here.
     
  18. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    [​IMG]

    That is the simplest configuration of a Wein bridge.
     
  19. Audioguru

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    Dec 20, 2007
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    The light bulb "variable resistance" in the Wien Bridge oscillator causes the output level to bounce up and down then smooth when the circuit is turned on and sometimes when the frequency is quickly changed.
     
  20. Fraser_Integration

    Member

    Nov 28, 2009
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    Don't know if this is what you were asking but it is a key point about Wein Bridge oscillators:

    In a perfect circuit, the oscillator would not oscillate until there was some input at one of the op-amp terminals. However, there is noise present in any real circuit, and this minor increase in the terminal voltage sets up the differential voltage which drives the oscillations.
     
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