high frequency
It's Xiao Hou 2021-08-09 16:32:58

This is my participation 8 The fourth of the yuegengwen challenge 8 God , Check out the activity details :8 Yuegengwen challenge

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
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Q The bigger it is , same △φ Caused by the △ω The smaller .
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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 :
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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.

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2021-08-09