13-11-2015, 04:46 PM
That Hartley oscillator had me up all night. I was a bit baffled by the description in the textbook. I think it was the phase of the circuit that had me foxed since the diagram was supposed to illustrate positive feedback.
There were two current blocking capacitors in this Hartley circuit. One was the grid leak. It prevented any current flow back to cathode except by the grid leak resistor alone. That did make sense from the outset but the phase part wasn't explained in a way I could clear it up.
In a simple valve circuit, the signal will come between the grid and cathode. When the signal is + amplitude on the grid, current will increase at the anode and the anode and cathode voltage will drop across the anode load resistor. So the load drop is not in phase with the signal and the drop can be made to pass back to the signal voltage and oppose the phase. In American circuits they just use a series resistor.
In the Hartley circuit as shown I can only gather the anode load would be the HF choke which is shown at the anode of the tube. I finally made sense of it when I noticed another DC blocking capacitor further in the anode circuit and noticed the anode voltage was being fed back in phase with the signal - positive feedback. It would be clearer to show the actual diagram I had in the book.
There were two current blocking capacitors in this Hartley circuit. One was the grid leak. It prevented any current flow back to cathode except by the grid leak resistor alone. That did make sense from the outset but the phase part wasn't explained in a way I could clear it up.
In a simple valve circuit, the signal will come between the grid and cathode. When the signal is + amplitude on the grid, current will increase at the anode and the anode and cathode voltage will drop across the anode load resistor. So the load drop is not in phase with the signal and the drop can be made to pass back to the signal voltage and oppose the phase. In American circuits they just use a series resistor.
In the Hartley circuit as shown I can only gather the anode load would be the HF choke which is shown at the anode of the tube. I finally made sense of it when I noticed another DC blocking capacitor further in the anode circuit and noticed the anode voltage was being fed back in phase with the signal - positive feedback. It would be clearer to show the actual diagram I had in the book.
