CMOS amplifiers with multiple gain setting control

Details CMOS amplifiers with multiple gain setting control CMOS amplifiers with multiple gain setting control

2000-03-23

2001-11-13

Lam, Tuan T. (Department: 2816)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S278000

Reexamination Certificate

active

06316997

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of CMOS differential amplifiers.
2. Prior Art
CMOS differential amplifiers having a multiple gain selectable by a gain control signal are well known in the art. Such amplifiers include amplifiers having a differential input pair of transistors, each with a load resistor 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, with a third switching transistor coupled across the third resistor to controllably short out the resistor. With the third transistor off, the gain of the amplifier is proportional to the ratio of the resistances of the load resistors to the third resistor. With the third transistor on, effectively connecting the sources of the first and second transistors, the differential amplifier will have a high gain, dependent on the product of the transconductance of the first and second transistors.
Amplifiers of the foregoing type work well when used alone, but cannot be easily cascaded. In particular, such amplifiers amplify not only the signal, but the DC offsets also, so that if one tries to cascade multiple amplifiers for substantial gain by direct connection, the DC offsets would be amplified also, likely saturating the latter stages. AC coupling could be used between stages, but that would not only require coupling capacitors between stages, but also biasing circuits between stages to bias the gates of the next stage.
BRIEF SUMMARY OF THE INVENTION
CMOS differential amplifiers having a multiple gain selectable by a gain control signal that may be directly coupled for cascading are disclosed. 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 one, 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 gain at signal frequencies without similar amplification of amplifier offsets.


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patent: 5650747 (1997-07-01), Chen
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patent: 5668502 (1997-09-01), Rijns

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