Electronic – Understanding Royer oscillator (for wireless power transfer), (specially mosfet gate driver circuit)

It intentionally DOES oscillate.

It goes beyond a regenerative receiver and is called a super-regenerative receiver, invented by Edwin Armstrong in 1922.

The values of R3 and C3 are arranged so that after oscillation starts the voltage builds up on the emitter due to rectification of the oscillations. After some period (10's of microseconds typically) the bias will be such that the oscillations stop. The voltage will then decay with the time constant of R3/C3 until oscillations start again. This typically occurs at a frequency in the range of 10's to hundreds of kilohertz.

Normally in an oscillator the oscillations build up from noise however if any signal at the oscillation frequency is present the oscillations will build up more quickly. This is the signal being received. The average supply current will depend upon how quickly the oscillation starts which depends upon the signal present. Thus the supply current is dependent on the received signal, the output from R1 depends on the supply current.

Usually the transmitter is amplitude modulated at an audio frequency and this can be recovered from C7 with suitable filtering and amplification.

A super-regenerative receiver can get extremely high gain out of a single stage but when no signal is present the output will be very noisy. Because the build up of oscillations is exponential it usually has a logarithmic response.

They are used less these days because they tend to radiate and also have a very broad bandwidth. A common application is for garage door openers.

kevin

Gabric - here are my answers to the 4 questions:

1.) The oscillation starts as soon as you close the switch. This closing causes a sharp inrush transient which contains (among others) also the frequency component that fulfills Barkhausen`s oscillation condition (see point 2).

2.) The oscillation condition requires a loop gain (slighly) larger than unity and a loop phase of -360 deg (0 deg). The transistor in common emitter configuration contributes -180 deg phase shift between base and collector. The remaining phase shift is provided by the feedback loop.

This loop again consists of a third-order lowpass which is able to provide these -180 deg at one single frequency only. The lowpass consists of a second-order lowpass (L1-C) working upon a first-order lowpass (Lq-r,in). Here, Lq is the crystal working as an high-quality inductor and r,in is the finite input resistance of the transistor at the base node.

3.) The primary inductor is tapped to provide a certain kind of ac voltage division in order to limit the gain of the amplifier stage (because it should not be too large, just sufficient to allow self-excitement without severe non-linearities).

4.) The 10k resistor is necessary to provide the correct DC biasing of the transistor; the small emitter resistor provides dc feedback for stabilizing the dc bias point. The 0.05µF capacitor establishes signal ground potential at the pos. supply pin. This is necessary because the capacitor of the first L-C lowpass block is connected to the pos. supply but must be referenced to ac ground.