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[SOLVED] How to design Opamp Based Hartley Oscilator ?

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icbergn

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Hello,

I have to design Opamp based Hartley oscilator which oscilates at 35 khz(sine wave).Ive spent my 2 days to design this but I failed.It was huge challange.I checked online but no luck and also I use Pspice and Proteus for my designs.It must be opamp based. :-|

Thanks.
 

The Hartley has a tapped inductor. The windings influence each other. They need to be connected so that oscillations are reinforced.

Have you tested both ways of connecting the windings? I use a simulator which has a transformer in the menu. When I wanted to make a Hartley, I placed a transformer, then experimented with the winding arrangement.

Once you start-up, it takes many cycles for voltage swings grow to a noticeable level.
 
With the centre tapped coil the sine wave is anti phase at the ends. So you put your resonating cap across the coil and earth the centre tap. One end of the coil you connect to the output of the opamp and connect the other to the negative input. Because you have excessive gain in the opamp, it iis best to include a high series resistor with the end going to the input, and to increase the Q of the tuned circuit a high value resistor in series with the other end as well. It depends on how much gain your opamp has at the operating frquency.
Frank
 
Icbergn - are you really forced to realize an oscillator with an inductor?
As you know - there are many ocillator topologies based on RC combinations only.

More than that, did you consider the fact that the active element in the classical Hartley oscillator is a current source (BJT) - whereas the opamp is a voltage source?
 
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The low output impedance of the op-amp will load down the LC, so you'll need a resistance in series. This of course reduces the excitation signal, some experimentation is needed.

If I really had to use an LC at these frquencies I'd use a Colpitts (tapped capacitor) circuit. Otherwise a Wien bridge or 3RC oscillator with some amplitude stabilizing circuit is the way to go.
 
One possible way to make an OP based Hartley oscillator

op hartley.jpg
 

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