在超高频米波段工作的接收机,内部噪声对灵敏度起主要影响,而接收机内部噪声主要决定于高频放大器与变频器.为了提高灵敏度,接收机变频管常采用三极管.但三极管内阻小,过渡电容大,对中频回路衰减大,使变频级的增益和回路选择性降低.为了消除这些不利因素,往往在变频级加中和与正反馈以提高增益与选择性.以下就两种调频广播接收机常用的电路为例来进行分析计算. The receiver operating in the ultra-high frequency m-band, the internal noise on the sensitivity of the main impact, and the receiver internal noise is mainly determined by the high-frequency amplifier and inverter .In order to improve the sensitivity, the receiver frequency conversion tube often use transistor.But the transistor internal resistance Small, large transition capacitance, the IF loop attenuation, the frequency of the gain and loop selectivity reduced.In order to eliminate these unfavorable factors, often in the inverter plus stage and positive feedback to improve the gain and selectivity. The following two FM radio receiver circuit used as an example to analyze and calculate.