autotransfomer vs tapped coil in a hartley oscillator


Joined Mar 6, 2009
I guess the difference is the basis for using a tapped inductor in the Hartley oscillator.
The goal is to achieve the required phase shift in the oscillator feedback path so that oscillation can actually take place.

The Hartley has a dual topology in the Colpitts oscillator where the phase shift is achieved using a pair of capacitors ("tapped" at a common reference point) in the main tuning network.

This perhaps begs the question...

Would the Hartley work just as well using two magnetically uncoupled inductors, rather than the single element "auto transformer" / tapped inductor topology?

Give it some thought ....
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In the Hartley oscillator the center-tap is held at AC ground (typically) so one end swings in the opposite direction from the other, like a seesaw, providing the phase inversion needed for oscillation. The autotransformer typically has one end grounded and the tap swings in the same direction (same polarity) as the voltage across the entire coil (more like a pry bar).


Joined Apr 24, 2011
what is the diference in:

Autotransformer vs the center tap coil in a the hartley oscillator?
they work the same way? because they look the same.
They are very similar and completely different.

Similar in that they are both taped inductors.

Different in one is for very small signals and the other for AC power.

They do both work on the same set of equations.

Thread Starter


Joined Jan 8, 2013
wow! amazing how complex a coil and capacitor are, they can behave in many different ways .

I would like to study and have a DEEP understanding in this subject. So I would know in the future for example why the tapped coil in a Hartley oscillator is sometimes connected to ground and in other circuit they are connected to power supply. What book/web page you recommend? mathematics is not a big problem for me.

hartley tapped coil connected to ground:

hartley tapped coil connected to power supply


Joined Mar 6, 2009
One of the most important concepts in signal analysis is that the effectively decoupled DC power supply ac potential is equivalent to signal ground.
However, it is not a fundamental requirement that the tap point be ac or signal grounded for the Hartley oscillator to function. Other topologies are possible.
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