Q's: BJT Phase Shift Oscillator

Discussion in 'General Electronics Chat' started by 3n2323, May 18, 2012.

  1. 3n2323

    Thread Starter New Member

    Apr 8, 2012
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    hi all,

    i breadboarded a simple and basic phase shift oscillator as shown in the schematic. it oscillated at about 1.5KHz with a 6V DC wall adapter, but when i switched to 4 AA batteries, no oscillation. these batteries meassured 5.3V.

    [​IMG]

    so i stepped the supply voltage down little by little from 6V in LTspice, and it stopped oscillation at 5.1V.

    questions:
    1) why supply voltage has such a big impact on the oscillator? anyway to make it work for a wider voltage range?

    2) my readings say the amplifier gain is 8 for oscillation of a 3 RC section circuit. what in this schematic sets the gain at 8? how would i control the gain if that becomes necessary?

    3) the output peak to peak voltage is about 2.4V, how can i make the output amplitude vary, say from 10's of mV's to this max of around 2.4V?

    4) how can i make the frequency vary, and what would the limitting factors be to the range of such an adjustable frequency range?

    thank you for helping!
     
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  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Last edited: May 18, 2012
  3. 3n2323

    Thread Starter New Member

    Apr 8, 2012
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    wow, that's fast reply, thank you very much Ron!

    8 v.s. 26, such a big difference! thank you for the pointer!

    however the question on how this schematic sets a correct gain and how to control this gain still remains.

    could the oscillation stopped at 5.1V because of insufficient gain?

    all i know about the BJT i used is its beta, 268 something. with supply voltage at 5.1V and 6V, base current are about 5.9uA and 7.1uA repectively per simulation. so the BJT is working with a beta around 200 and with a 5.1V supply, can't figure out why oscillation stopped at 5.1V.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Certainly the gain decreases as the supply voltage drops. The circuit is difficult to analyze for several reasons:
    1. The amplifier's output impedance ≈2.2k, which gets heavily loaded by the phase shift network, and also contributes to the phase shift.
    2. The amplifier's input impedance is low (a few kΩ), which loads the phase shift network, and also contributes to phase shift.
    The transistor amplifier also generates a lot of even harmonic distortion, due to the fact that the instantaneous gain and input impedance are functions of the instantaneous signal level on the base.
    If you want to play with a nice op amp based phase shift oscillator, check out the attachments.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    I made a phase-shift oscillator that uses a quad opamp. Each opamp buffers its low-pass (not high pass) RC phase-shifter so the distortion is reduced, not increased.
    One opamp clips a little but the following three RC low-pass filters cut the distortion to almost nothing.
     
  6. 3n2323

    Thread Starter New Member

    Apr 8, 2012
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    i see, thank you Ron!

    i'll go find an opamp and try your oscillator out. LTspice schematic provided, man, just great! i imagine when you manage projects you beat competition every time.
     
  7. 3n2323

    Thread Starter New Member

    Apr 8, 2012
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    thank you Audioguru! Ron also suggested opamps instead of BJT's, i suppose i'll be looking for quads then, i can use that both ways. :)
     
  8. The Electrician

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    Oct 9, 2007
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  9. Wendy

    Moderator

    Mar 24, 2008
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    TI also has a pretty good sheet on it. I have yet to build it, but the Bubbu oscillator is very interesting. It is a 90° phase shift oscillator with good characteristics.

    Sine Wave Oscillators
     
  10. 3n2323

    Thread Starter New Member

    Apr 8, 2012
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    thank you The Electrician, that's really good link!

    the author says, "It should be noted that many such circuits are sensitive to biasing transistors carefully into their active regions. If a circuit is sensitive to transistor gain, component values and tolerances may result in an incorrect (or even non-working) circuit, even if rigorously calculated. Fairchild, for example, cites hfe (current gain) as 100-300 @ 150mA and 30 (min) @ 500mA. The data sheet may not make it clear what the correct biasing is for your chosen current. If you play around with the circuit with a modelling program, you will find the circuit has a functional window of only about +/-10% of RB (using no Re) in order to provide just the right amount of base current and voltage to be transformed by the transistor gain into a suitable collector DC operating point. "

    in my case, when supply voltage changes from 6V to 5.1V, change in Ib is ~16% something, maybe that's what killed the oscillation. but why, does it boil down to gain? the BJT was still in active region, the gain shouldn't change, right? maybe i'll try biase the BJT for a larger qiescent current and see what happens.

    in any case, i'll need to read the author more closely. understanding what the circuit is doing has a higher priority to me at this point, than merely getting a working oscillator.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The voltage gain of your amplifier (just the transistor and the collector resistor) is ≈Rc*gm, where Rc is the total resistance on the collector (2.2k||150k in your circuit). The transconductance (gm) is ≈Ie/.026. where Ie is the quiescent (DC) emitter current (Amps). The collector current decreases as Vcc decreases. Therefore, the gain will decrease as Vcc decreases.
    Note that voltage gain is independent of beta. However, input impedance IS dependent on beta, and is approximately equal to (β+1)/gm. The phase shift network will be loaded by the input impedance. Beta will typically decrease slightly as Vce decreases. It may decrease OR increase as a function of Ic, depending on the transistor and the current, but when Ic is a few milliamps or less, beta will typically decrease as Ic decreases.

    I realize that the calculated gain here will be >>29. I don't know how to explain the discrepancy. Maybe someone else here can.

    Are you thoroughly confused now?;)
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    Another good oscillator to stucy is the Wien Bridge oscillator. It was what started HP down their path to success. It was simple, easy to build, and precision. And it used a light bulb for the AGC circuit!
     
  13. 3n2323

    Thread Starter New Member

    Apr 8, 2012
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    thank you for your explanation on gain, beta, and input impedance! i'll do some readings on common emitter gain and get the math right. it is not confusing, i think i'm moving towards the right direction of finally find out what makes it tick along this line of analysis, a little bit more effort will get me there i imagine. thank you for the help!
     
  14. 3n2323

    Thread Starter New Member

    Apr 8, 2012
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    thank you Bill for the suggestion! the Wien Bridge is indeed very tempting, i didn't try that because i don't have a suitable light bulb.

    i looked at flash light bulbs in hardware stores, but no info on resistance was mentioned on the package. besides, the wattage and voltage ratings give a current considerably larger than 20mA. so i ruled this option out. maybe that's a mistake?

    small indicator lamps are available on the web, around $1 each, although probably usable, shipping is around $5, and i need one, urh...

    given a light bulb, i'm not able to make judgements on current, resistance, and how fast the ressitance changes with temperature. guess that's what stopped me trying it out, for now.
     
  15. 3n2323

    Thread Starter New Member

    Apr 8, 2012
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    when supply voltage was lowered from 6V to 5.1V, my oscillator stopped oscillating. so i kept 5.1V, and went from there.

    following what the author at this link said, i lowered R2 from 300K to 220K. Ib went from 5.1uA to 5.6uA, about 10% as the author mentioned.

    bang, oscillation! both in LTspice and on breadboard.

    so i lowered the supply voltage from 5.1V to 4.5V, a corresponding 10% roughly. sure enough, it still oscillates. for supply voltages passing the 10% mark, 4.5V that is, even with 4.4V for example, it won't oscillate.

    the author is really right, very good link!

    now, why so? Ron pointed out to me the gain is indepenent of beta, and is related to Ie. i can almost smell what is at work now. i just need to go through my books and get the math right, and confirm that a supply voltage of 10% lower will not provide sufficient gain to sustain oscillation.

    this is just great, i have work to do now, thank you all very much guys!
     
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