Abstract
A MESFET automatic gain control (AGC) amplifier for a direct broadcast satellite IC tuner that has wide gain control range and a flat frequency response is described. The GaAs IC for the tuner consists of an RF AGC amplifier stage, a mixer circuit, and an IF AGC amplifier stage. The mixer converts the amplified broadcast satellite IF (0.95 approximately 1.8-GHz) signal to the second IF (480-MHz) signal with a variable oscillator (1.43 approximately 2.28-GHz) signal. This signal is amplified and output to the demodulator through the IF bandpass filter. The AGC amplifier uses a differential amplifier with FETs whose source terminals are connected together. The FETs operate as a variable resistor controlled by the gate voltage. Since V/sub ds/ and the V/sub gs/ of the amplifier FET vary little during the gain control, the amplitude frequency response can be flat. Furthermore, if the resistor consisting of the connected FETs has a high value when the broadcast satellite IF signal level is high, the amplifier can also yield a low distortion characteristic.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
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