| 摘要 |
CMOS differential amplifiers having a multiple gain selectable by a gain control signal that may be directly coupled for cascading. The amplifiers have a differential input pair of transistors, each with a load device in its drain circuit and a current source in its source circuit. The differential input is applied to the gates of the transistors and a differential output taken from the drains of the transistors. A third resistor is coupled between the sources of the transistors for a first, relatively low gain setting. A series resistance and capacitance network may be controllably coupled between the sources of the first and second transistors. The capacitance is chosen to have a relatively low impedance at the signal frequencies, effectively coupling the series resistance in parallel with the third resistor to set a second, higher gain. At zero frequency, the capacitance will act like an open circuit, allowing the amplifiers to be cascaded by direct connection for high and accurately controllable gain at signal frequencies without similar amplification of amplifier offsets.
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