Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
1997-09-19
2001-03-27
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C327S478000, C327S482000, C327S374000
Reexamination Certificate
active
06208195
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to signal switches and in particular, to a fast transmission gate switch, particularly useful for switching digital logic signals.
With the advent of very large-scale integrated circuits, the size of devices manufactured has been shrinking and the speed of the devices has continually increased. Most of the efforts, however, have been directed to designs where many circuits and the connections between them are made in the same medium such as a silicon chip. Since all the circuit components being fabricated are done in the same medium, devices may be made smaller by improved techniques such as improved lithography. By reducing the sizes of the devices and the lengths of the connections between them, the speed of the devices is increased due to a decrease in the inductances, resistances and capacitances of individual devices and of the connections between them.
Board level designs have not kept pace with the above-described development in very large-scale integrated circuits. Printed circuit board designs frequently have large capacitances which slow down signal propagation. Thus if two high speed logic chips are connected through a slower device, the overall speed of the system is determined by the slowest component, namely, the slow connecting device. Passive switching devices such as transmission gates have been used in printed circuit board level designs for switching signals between digital logic devices. The slow speed of this type of switches determines the speed of signal transmission even though the two logic devices may operate at much higher speeds. It is therefore desirable to provide a stand-alone switching device which is much faster than the conventional passive transmission gate.
In one type of improved switching circuits that is frequently used, instead of a passive switch, an active device such as a logic buffer is used. While a buffer used as a switch causes a delay that is less than that caused by the conventional passive transmission gate switch, the delay caused by such buffers may nevertheless be excessive and undesirable for some high speed applications. It is thus desirable to provide a high speed switch that causes less delay than the above described switches.
SUMMARY OF THE INVENTION
This invention is directed towards a stand-alone switch for switching signals between two nodes, where the two nodes are connected to conductors such as conducting traces on a printed circuit board, so that a signal propagating from one node to the other will experience an external capacitance C
b
. The switch comprises two input/output pins and a switching pin, and a first transistor having a control terminal and a first and a second input/output terminal. Each input/output terminal of the transistor is connected to one of the nodes through an input/output pin for passing signals between the nodes through the transistor when the transistor is turned on and disconnecting the two nodes when the transistor is turned off. The switch further comprises means responsive to a switching signal at the switching pin for applying a control signal to the control terminal of the transistor for turning it on or off. The delay of signals passing through the switch is at least equal to the RC delay, which is the product of the resistance R
i
of the transistor between the two input/output pins and the sum of the internal capacitance C
i
of the switch and the external (e.g., bus) capacitance C
b
. The values of R
i
and C
i
are such that, for a given value of C
b
, R
i
(C
i
+C
b
) is less than the delay through a typical buffer, e.g. 6.5 nanoseconds. Hence, replacing the conventional buffer by the switch of this invention will reduce delay in signal propagation between the two nodes.
In the preferred embodiment, the applying means and the capacitance between the two input/output pins and the control terminal when the transistor is turned on are such that time required for the switching signal to turn on or off the transistor through the applying means is less than the time to turn on or off a typical buffer, e.g. 7 nanoseconds.
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Bever Hoffman & Harms LLP
Cunningham Terry D.
Integrated Device Technology Inc.
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