Fairchild Semiconductor Corporation

Pravas Pradhan

Jianhong Ju

Shailesh Chitnis

Ethan A Crain

A current mode transfer logic system suitable for driving transmission lines is disclosed. In one embodiment a twisted pair transmission line is terminated in its characteristic line impedance. A signal is formed of two unequal currents, preferably of different polarities as well as magnitudes, that are driven down the two lines. The unequal currents are selectively switched between the two lines creating a logic signal of a differential current drive of unequal current magnitudes. The unequal currents are received and shunted from the distal end of each line via diode connected MOS transistors. The MOS transistors are biased to present a low impedance, but an impedance higher than the terminating resistor. The currents are amplified and converted to useable CMOS voltage levels. In another embodiment the twisted pair is replaced by two parallel transmission lines which are terminated in one resistor, equal to the sum of the characteristic impedances of each line. The terminating resistor is connected between the distal signal carrying conductors of each transmission line. The shields or return paths for each line are tied together at the distal and at the proximate (drive) ends of the line.

Current transfer logic

A folded common cascode circuit with symmetric parallel signal paths from the differential inputs to a single ended output provides a low skew, low jitter, low power, high speed differential amplifier. The signal paths on either side of the differential amplifier are made equal with equal loads along each path. Pairs of complementary NMOS and PMOS transistor pairs with parallel complementary biasing current mirroring stacks on the cascode circuitry have all their gates connected together. The layout maintains symmetrical parallel signal paths and symmetrical amplification and impedance loading from differential input to output. Output inverters provide a higher drive capability.

Low voltage, low power differential receiver

A system and method are described for receiving differential currents in a current mode circuit. When conditions occur where the receiver inputs are floating, undriven, shorted together, or one or both shorted to ground, the output of the system remains stable. Diode connected MOS transistors receive the unequal currents, and current mirrors amplify the received currents. Those amplified mirrored currents are differentially amplified and converted into voltage signals suitable of typical computer and logic systems. The current mode differential nature of the invention provides high common mode current and voltage noise immunity. A threshold for the unequal currents helps provide high differential current and voltage noise immunity.

Failsafe for differential circuit based on current sense scheme

A folded common cascode circuit with symmetric parallel signal paths from the differential inputs to a single ended output provides a low skew, low jitter, low power, high speed differential in/out amplifier. There is a differential input stage followed by a load or current summation stage with all gates tied together, and then a second differential stage. The dynamic voltage range of the second stage allows for lower Vcc operation while providing improved jitter operation. The signal paths on either side of the differential amplifier are made equal with equal loads along each path. Pairs of complementary NMOS and PMOS transistor pairs with parallel complementary biasing current mirroring stacks on the cascode circuitry have all their gates connected together. The layout maintains symmetrical parallel signal paths and symmetrical amplification and impedance loading from differential input to the differential output. Output inverters provide a higher drive capability.

Low voltage differential in differential out receiver

An enveloping curve generator that guarantees one curve will overlap another when both are going high and the other overlaps the first when both are going low. When the input goes high, one steering FET is turned off and the other directs the input signal to drive an output high. That output going high after a delay drive the second output high. The second output being high latches the states of the two outputs high. When the input goes low, the off FET is turned on and the on FET is turned off. The input signal is directed to drive the second output low and after a delay the first output is driven low. The first output going low latches the states of the two outputs low.

Enveloping curves generator circuit

Differential input fail safe circuitry is disclosed that detects missing or too low differential signals combined with a frequency lower than a frequency limit where a final safe condition is detected and signaled. The output signal form the fail safe circuitry is held in a given state that is an invalid representation of the differential input signal. A frequency detector, complementary offsetting auxiliary amplifiers with limit frequency roll offs are used to detect the fail safe condition. In addition a delay circuit is used that requires the fail safe condition to exist for some time before the fail safe circuit is active. Initialization circuitry ensures a proper power up conditions where the circuitry is enabled to detect the fail safe conditions and guarantees a reliable fail safe irrespective of the previous state of the signal.

Failsafe differential amplifier circuit

A folder common cascode circuit with symmetric parallel signal paths from the differential inputs to the differential outputs provides a low skew, low jitter, low power differential amplifier. The signal paths on either side of the differential amplifier are made equal with equal loads along each path. Pairs of complementary NMOS and PMOS transistor pairs with parallel complementary biasing stacks on the output cascode circuitry maintain symmetrical parallel signal paths, amplification and impedance loading from differential input to differential output. Output voltage translating inverters provide a higher voltage level output signal while maintaining low skew and jitter.

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South Portland, ME, United States of America

Pravas Pradhan