hartley oscillators

Posted on Feb 4, 2014

Hartley oscillator are inductively coupled, variable frequency oscillators where the oscillator may be series or shunt fed. Hartley oscillators have the advantage of having one centre tapped inductor and one tuning capacitor. This arrangement simplifies the construction of a Hartley oscillator circuit. Here I`ll present the schematic for my old favourite, together with a buffer stage and an amplifier stage which

hartley oscillators
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should deliver about 5V P/P into a 50 ohm load. We`ll discuss each relevant stage and produce some rule-of-thumb design info. Because the consensus comes down in favour of FETS and I`m big enough to lay aside my prejudices in the noble cause of advanced education we`ll use a FET oscillator. Nothing to do with a few friends who might belt me up! Now I chose a 2N4416A FET purely because I bought a big bag of them years ago and have them on hand. You could use any general purpose JFET you can readily obtain. Note the 2N4416A is a metal can and the case is grounded. Note: I have been asked a number of times the function of C4 in this circuit. Capacitor C4 is to reduce the loading on the tuned circuit components. It may be as small as possible consistent with being able to provide sufficient drive to the succeeding buffer amplifier stage. Often the home constructor will often make C4 a trimmer. The other components are bog standard. The two resistors, silicon diode and zener diode need never change, capacitor C5 is about right for this frequency. C6 can be selected to give higher / lower output to the buffer amplifier. Smaller C6 values give lower output and conversely higher values give larger output. The silicon diode I`ll explain later, the zener diode is to give a regulated 6. 2 volt supply. Now there is NOTHING sacred about my frequency determining capacitor combination O. K. Too many people look at these kind of circuits...

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