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The Minimum voltage requirement for an RC Oscillator

  1. Jan 13, 2017 #1
    I am trying to run an RC oscillator using a 2N3904
    (Datasheet for reference: http://www.kynix.com/uploadfiles/pdf8798/2N3904.pdf)
    The circuit looks like the following.

    I want to run it at 0-5v but I failed, I assume it is because the voltage is not high enough?

    What would you expect the minimum voltage requirement to be for this type of circuit.
  2. jcsd
  3. Jan 13, 2017 #2


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    Did you DESIGN the circuit based on some assumptions/requirements for th DC operational point?
    Did you recalculate this operational point? Do you know (from oscillator theory) how much gain is necessary?
  4. Jan 13, 2017 #3


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    Hi leehyong. :welcome:

    Possibly it could be insufficient supply voltage.

    Two independent set of conditions must be met. The first involves DC biasing: if you remove all of the capacitors from the circuit then what you are left with must set the transistor's operation comfortably within its active region (linear amplifying).

    ⏩ Are you able to analyse the circuit with all capacitors missing to determine the expected DC voltage at its 3 nodes?

    What do you plan to use the oscillator for?
  5. Jan 13, 2017 #4


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    That`s for sure.
    A rough calculation of the base voltage (forgetting the base current) gives only app. VB=5*10/110<0.5 volts.
  6. Jan 13, 2017 #5


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    The 100k : 10k voltage divider sets the transistor base bias voltage.
    It will need at least a 5V supply before the transistor begins to turn on.
    I would remove the 10k between the base and ground then find a minimum voltage.
  7. Jan 13, 2017 #6


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    With a little tweaking of component values this should easily run on 5 volts.
  8. Jan 13, 2017 #7


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    Yes - of course. Therefore my question in post#2 if the OP has designed the circuit.
  9. Jan 13, 2017 #8


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  10. Jan 13, 2017 #9


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    There are a number of problems with this circuit, all of which conspire to prevent oscillation. One is the time it takes to start, if it ever does. Spice simulation shows it does not start until the supply voltage reaches about 8V.

    Another problem is the gain of the transistor amplifier needed to overcome the losses in the phase shift network at the frequency of operation. The emitter capacitor needs to be increased to raise the gain at lower frequencies. Scaling the frequency down by increasing the capacitors in the feedback circuit necessitates increasing the emitter capacitor to restore that gain. That was not done with the OP circuit.

    The emitter resistor needs to be reduced to enable the transistor amplifier to drive greater loads on the output without killing the oscillation.
  11. Jan 14, 2017 #10


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    I am always surprised when - as in the present case - somebody seems to have a problem and is asking for help, but is not willing to give requested additional information. Even worse - sometimes it seems that he is not interested anymore. Really a bad behaviour.
  12. Jan 14, 2017 #11


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    Measure the volts across the 390R in the Emitter. It should be more than Zero, which it will be if the transistor does not turn on. (due to wrong base bias volts)
    If you disconnect the 10k resistor in the base, the Emitter Volts should go to a non zero value. From the diagram, it looks like a simulation.
    Please take note of the above post (from LvW), if you want any more help . . . . . ever.
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