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Off-Chip Receiver With Variable Threshold

IP.com Disclosure Number: IPCOM000060334D
Original Publication Date: 1986-Mar-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Smith, GE: AUTHOR

Abstract

Circuitry is disclosed for semiconductor applications which provides a variable receiver threshold as a function of receiver output. As a result, the receiver's threshold is lowered or raised in a direction so as to increase its noise margin. A time delay before the threshold is changed produces a hysteresis in the receiver's gain curve as a function of input transition time. The circuit is shown in the figure. D1 and D3 are low Schottky barrier diodes (SBDs); the other diodes are high Schottky barrier diodes. Transistors T1 and T3 along with diodes D1 and D2 plus associated parts form the receiver of a semiconductor chip. D1 acts to isolate the circuit from the transmission line with the off-chip signal connected to IN. The threshold of the input circuit is VBE (of T1) + Vf (of D2) - Vf (of D1).

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Off-Chip Receiver With Variable Threshold

Circuitry is disclosed for semiconductor applications which provides a variable receiver threshold as a function of receiver output. As a result, the receiver's threshold is lowered or raised in a direction so as to increase its noise margin. A time delay before the threshold is changed produces a hysteresis in the receiver's gain curve as a function of input transition time. The circuit is shown in the figure. D1 and D3 are low Schottky barrier diodes (SBDs); the other diodes are high Schottky barrier diodes. Transistors T1 and T3 along with diodes D1 and D2 plus associated parts form the receiver of a semiconductor chip. D1 acts to isolate the circuit from the transmission line with the off-chip signal connected to IN. The threshold of the input circuit is VBE (of T1) + Vf (of D2) - Vf (of D1). T3 acts as a level shifter to bring the output back to normal chip swings. D3 is only used to isolate the base of T2 from the output. The circuit would work without it, but the output would be clamped at about 0.8 V. In operation, if the input, and hence the output, is high, D3 is reverse biased and T2 is turned on. The Vce of T2 is limited to 0.2 V by DC2, a high SBD anti-saturation clamp. When this is the case, the threshold of the circuit is VBE + Vce - Vf(D1) 0.7 + 0.2 - 0.3 = 0.6 V i.e., the input must drop below 0.6 V for the output to change states. If the input and output are low, D3 is forward biased, insuring that T2 is...