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放大器采取串联或并联反馈形式时,为了与信号源和负载阻抗匹配,在放大器的输入端和输出端,应分别接入一个与信号源内阻抗和负载阻抗相等的电阻.输入端的这个电阻,将使放大器的信噪比降低0.4～0.7奈培,这是因为电阻的接入,使传输到放大电路(即K电路)输入端的信号降低了0.69奈培,而放大电路的固有噪音电平并无变化的结果.输出端的这个电阻,将与负载吸收相同的功率,放大电路的输出功率要增加一倍,谐波衰耗也要变坏.如果把两端接入的匹配电阻看作是放大器的组成部分,这种放大器就是电阻混合负反馈放大器.所以,通信方面使用的高质量定阻抗放大器,多采用变量器混合负反馈电路,且由于输入和输出端对地平衡,变量器内侧混合负反馈用得较多.
When the amplifier adopts the form of series or parallel feedback, in order to match the signal source and the load impedance, a resistance equal to the internal impedance and the load impedance of the signal source should be respectively connected to the input and the output of the amplifier. The resistance of the input The signal to noise ratio of the amplifier is reduced by 0.4 to 0.7 Nieber because the resistor is connected so that the signal transmitted to the input of the amplifier circuit (ie, the K circuit) is reduced by 0.69 Nie, while the inherent noise level of the amplifier circuit does not change The output of this resistor will absorb the same power with the load, the output power of the amplifier circuit to double, harmonic attenuation also worsened. If both ends of the matching resistor access as the composition of the amplifier Part, this amplifier is a resistor mixed negative feedback amplifier.Therefore, the communication used high-quality constant impedance amplifier, the use of variable hybrid negative feedback circuit, and because the input and output to ground balance, the negative feedback inside the mixing device More.