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一、中频放大电路中放电路由三级中放组成。它的作用是将高频头输出的图象中频信号(37MHz)进行放大(三级中放的放大增益大于50 dB),并保证有足够的通频带。为提高图象信号的选择性,在中放输入端和三中放b极分别加有Q_1、Q_3、Q_4和Q_5等四个串联谐振的吸收回路,吸收频率分别为29.2MHz、30.5MHz、38.5MHz、39.7MHz。其中Q_4(30.5MHz)的作用是将伴音第一中频抑制在图象信号幅度的5%,防止伴音信号干扰图象。Q_2和C_3(20P)是串联谐振于37MHz的匹配回路。正向AGC电压同时控制第一和第二两级中
First, the intermediate frequency amplifier circuit in the discharge route composed of three levels. Its role is to amplify the IF signal (37MHz) output from the tuner (amplification gain of more than 50 dB in three stages) and to ensure sufficient passband. In order to improve the selectivity of the image signal, four series resonant absorption circuits Q_1, Q_3, Q_4 and Q_5 are respectively added to the input end of the amplifier and the b-pole of the third amplifier. The absorption frequencies are respectively 29.2MHz, 30.5MHz and 38.5 MHz, 39.7 MHz. The effect of Q_4 (30.5MHz) is to suppress the first intermediate frequency of the audio signal by 5% of the amplitude of the video signal to prevent the audio signal from interfering with the video. Q_2 and C_3 (20P) are the series resonant resonant circuit at 37MHz. The forward AGC voltage controls both the first and second stages