| 摘要 |
A multistage amplifier with a low noise figure linearly amplifies an input signal having a level which varies over a wide range. The first stage of the amplifier provides a low noise figure and sufficient power gain to ensure overall low noise performance. An interstage amplifier is protected from saturation by first and second variable attenuators, which are controlled by a level detector. The level detector detects the level of the input or output signal of the interstage amplifier and controls the forward bias current of a diode in each of the variable attenuators. The first variable attenuator functions as a level compressor for signals input to the interstage amplifier having a level higher than a preset level. The second variable attenuator functions as a level expander of the output signal from the interstage amplifier to equalize the signal level compressed by the first variable attenuator. By combining the two variable attenuators, it is possible to provide a multistage amplifier with an extended range of linear operation and which has low noise characteristics.
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