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The principle of high efficiency of class C power amplifier is its small through angle , The reason why it is not distorted is ：
Because the load is a resonant loop , Even if the conduction angle is less than 180, But the resonance circuit LC Will store energy . Give the resonant circuit a fixed frequency signal , The output terminal outputs a complete AC sinusoidal signal .
Why should class C power amplifier use resonant circuit as load ?
1. The distorted collector current pulse can be transformed into an undistorted output cosine voltage .
2. By adjusting the of the resonant circuit LC Resonant resistance R Equal to the collector load required by the amplification tube , Achieve impedance matching .
3. Frequency selection , Select the frequency signal that the amplifier needs to amplify .
Class C power amplifier works in overvoltage state , Why are there pits in the current waveform ？
iC When reaching the saturation zone , Will follow uBE It decreases with the increase of .
LC Relationship between oscillator stability and quality factor
Affect the oscillation frequency w Parameters of
Q The bigger it is , same △φ Caused by the △ω The smaller .
As shown in the figure ：Qe’>Qe,△ω’osc1<△ωosc1.
Therefore, increasing the quality factor will increase LC Stability of resonant circuit .
summary ：Q The higher the value , The smaller the frequency change caused by parameter change , The smaller the frequency stability .
Why does the output of high frequency power amplifier have higher harmonics
Because the high frequency power amplifier works in class c , Both the base current and the collector current are cosine periodic pulses （ Not a complete cosine periodic signal ）, It can be expressed by Fourier series , Its Fourier expansion is ：
Therefore, the output will have high-order harmonics . The voltage transmission coefficient of the quality index of the envelope detector Located in vo(t) Given a specific value Vo Under the circumstances ,id The conduction angle is θc： 1
The voltage transfer coefficient is defined as ：
The first figure shows （Vo>>VD）： 1
because id The output is a cosine pulse signal , Fourier expansion decomposition is performed to obtain ：
To get Vo：
however vo(t) It changes over time , So the conduction angle changes at first , Finally, because the circuit is stable , The conduction angle is stable . In the end ：
summary ： According to the Fourier series analysis and broken line analysis of cosine pulse signal , The voltage coefficient obtained is approximately equal to cosθc, And when R>>Rd when ： The voltage transfer coefficient is approximately equal to 1.