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    Hartley oscillator design with opamp

    Hi!
    I made an opamp hartley oscillator using a certain electronics simulator and even though simulation takes place I'm unable to get any oscillation at all. I tried first using Rf and R1 equal to 1kΩ and two inductors both of whch are 50 mH. The capacitor value is 0.01 µF. I've used a generic op-amp. I've attached a snapshot of the circuit. This simulator uses qucs models of components. I checked the tank circuit separately and gave it an instantaneous voltage and it gave oscillations. But its not helping in this circuit. what could be the problem? I even tried giving a dc offset to the output to imitate the dc offset of a real opamp.

    •   Alt5th June 2012, 11:35

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    Re: Hartley oscillator design with opamp

    I know, this circuit can be found in the internet. However, it does not work.
    Why not? It is derived from the original Hartley BJT circuit without considering that the BJT acts as a current source.
    That means: Large output resistance.
    In your circuit and with Rout=0 the inductor L0 is useless as it acts as a load only and has no influence on feedback.
    Thus: Put a resistor (for example: 100 ohms...1k) between opamp output and the LC feedback path. Check/modify the loop gain to meet the oscillation condition - and the circuit will work.


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    Re: Hartley oscillator design with opamp

    Try this version. There's a few important changes:
    The opamp must have some gain, so in my circuit R1 is bigger than R2. In your circuit, R1 must be bigger than R0.
    The tuned circuit was heavily damped. I improved that by making C1 bigger, so the tuned circuit has a lower impedance.
    As LvW says, a resistor has been added between the opamp and the tuned circuit.
    Oscillators often don't start by themselves in a simulator. That's why a short pulse is injected into the opamp - just to give it a kick and get it moving.




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    Re: Hartley oscillator design with opamp

    Click image for larger version. 

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ID:	75447Hi godfreyl!! I tried the circuit with the changes u advised. The good news is that im gettin oscillations but the bad news is that they are not sinusoidal. Any other suggestions?? I tried varying the capacitor and inductor values with no luck.



    •   Alt6th June 2012, 12:03

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    Re: Hartley oscillator design with opamp

    No oscillator is able to produce a clean sinus without an appropriate amplitude control mechanism (FET, Diodes, thermistor).
    Otherwise you always get a clipped sinus.
    More than that, make sure that your pulse generator creates only one single pulse at t=0 (period larger than simulation time).



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    Re: Hartley oscillator design with opamp

    It's time to reveal some details about the involved simulator and OP model. The waveform doesn't look trustworthy.



    •   Alt6th June 2012, 12:23

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    Re: Hartley oscillator design with opamp

    Quote Originally Posted by s.jayaram88 View Post
    ....the bad news is that they are not sinusoidal.
    The spikes are suspicious. I think you are injecting those by mistake. The idea is to put one short pulse into the circuit to get it started. It looks like you are injecting pulses at 1KHz.

    If you can not only inject one pulse, then slow down the pulse generator so it only gives pulses at say 1Hz or 10Hz. That way you can see the oscillator output nicely between the pulses.



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    Re: Hartley oscillator design with opamp

    Well i did give a single pulse. of width 1.5 ms, amplitude 1 V. I used an analysis time of 100 ms.



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    Re: Hartley oscillator design with opamp

    The output you show consists of 2 waves added together.

    One is a (slightly distorted) sine wave at about 270Hz. This is what we expect from the oscillator. The other is a series of sharp spikes at 1KHz. Where do these come from?

    Check the voltage at different points in the circuit to see what is going on. i.e. Check the waveform at output of pulse generator, inputs of opamp, output of opamp etc.



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    Re: Hartley oscillator design with opamp

    Well i did give a single pulse. of width 1.5 ms, amplitude 1 V. I used an analysis time of 100 ms.

    Fine, but the most important information is missing: The repetition time of the pulse sequence.
    For an analysis time of ta=100ms the next pulse must not appear before ta !!
    More than that, an amplitude of 1mV is sufficient. And reduce the width to 1µs or so.



    •   Alt6th June 2012, 14:36

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    Re: Hartley oscillator design with opamp

    Quote Originally Posted by LvW View Post
    Well i did give a single pulse. of width 1.5 ms, amplitude 1 V. I used an analysis time of 100 ms.

    Fine, but the most important information is missing: The repetition time of the pulse sequence.
    For an analysis time of ta=100ms the next pulse must not appear before ta !!
    More than that, an amplitude of 1mV is sufficient. And reduce the width to 1µs or so.
    As u said I used a pulse with width 1s but the oscillations exist for as long as the pulse and then quickly die out!



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    Re: Hartley oscillator design with opamp

    Can you show a picture of your circuit and the output?
    It sounds like the opamp needs more gain, so you must make the feedback resistor bigger.



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    Re: Hartley oscillator design with opamp

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    I even tried increasing the feedback resistance and also increased and decreased the inductor and capacitance. But moreover the output was same

    - - - Updated - - -

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    This is the results i got!! Im quite satisfied Thanx 4 all ur help!!!


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