Study Of Of The Stability Of RF Power MOSFET Oscillator

With the development of science and technology, oscillator has been widely used in various electronic products. And the development of oscillator is presenting the diversified trends, among which oscillator with high-power and high frequency is an important development direction. This kind of oscillator can produce higher RF energy that can be used in many fields, like exciting of plasma source, heat treatment device, high-powered transmitter and etc. Oscillator's function to transit DC to AC energy form is realized by RF power amplifier circuit, which is also presenting high power, high frequency and high efficiency trend, which put forward higher request to the core power device. This paper take the power MOSFET as the power device, which is widely used in medium and high frequency range and can not be replaced by other devices in the frequency range over the MHz frequency.The main task of this thesis is to design a RF power MOSFET oscillator which has the stable amplitude function. To realize the stability of oscillator output amplitude, the method of negative feedback is usually adopted, which changes oscillator's output amplitude into the corresponding control signal, and realizes the stability of output amplitude by feedback controlling. Though, the negative feedback circuit can improve the performance of the oscillating circuit, it can emerge the phenomenon of self-excited oscillation which could make the output of amplitude more unstable than the circuit without negative feedback, make the oscillator amplitude wave output similar to the waveform of modulation wave, and cannot reach the purpose of steady amplitude output. Confronted with this problem, the whole system of oscillating circuit should be analyzed for stability, finding out the method of adjusting circuit to reach the purpose of steady amplitude output.The circuit is mainly analyzed and designed from three aspects in this article. Firstly, a high-power oscillator with frequency 13.56MHz and output power 100W is designed. Through setting the stable working position, the working state of the oscillator is transited from class-AB at starting-oscillating moment to class-C at stable oscillating moment. Secondly, using an automatic gain control circuit, through transiting the output amplitude of the oscillator into the corresponding controlling parameter, controlling the gate-source voltage of the power MOSFET, it can make the output amplitude of RF power oscillator stable. Thirdly, by analyzing of the whole designed circuit's transfer function and adopting Royce criterion, it can judge that whether the circuit is stable or not. According to the judgment, by adjusting transistor emitter resistance's value in feedback circuit, self-excited oscillation can be avoided in AGC loop, which makes the whole circuit work in stable state, and further realizes the purpose of stable amplitude output of the oscillator.