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# High Performance FET Crosspoint Switch Utilizing Current Steering Technique

IP.com Disclosure Number: IPCOM000060969D
Original Publication Date: 1986-Jun-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 62K

IBM

## Related People

Larsen, TA: AUTHOR

## Abstract

The crosspoint function is illustrated in Fig. 1. The function of the crosspoints is to provide selected horizontal to vertical connectivity. For example, A1 can be connected to B4 by allowing C1 and C4 to be on, and all other crosspoints to be off. Fig. 2 illustrates the FET current steering switch and the use of the principle of current-steering to propagate a signal from A1 to, say, B4 as above. First, convert the logic voltage signal to a current source utilizing a voltage-to-current converter. This can be achieved by using a differential amplifier with a current source at the common source node (Fig. 2). To route the signal, turn on C1 and C4, as shown, with all other FET devices connected to C1 and C4 biased off. The voltage conversion can be achieved by an N or P load device connected to drain of C4.

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High Performance FET Crosspoint Switch Utilizing Current Steering Technique

The crosspoint function is illustrated in Fig. 1. The function of the crosspoints is to provide selected horizontal to vertical connectivity. For example, A1 can be connected to B4 by allowing C1 and C4 to be on, and all other crosspoints to be off. Fig. 2 illustrates the FET current steering switch and the use of the principle of current-steering to propagate a signal from A1 to, say, B4 as above. First, convert the logic voltage signal to a current source utilizing a voltage-to-current converter. This can be achieved by using a differential amplifier with a current source at the common source node (Fig. 2). To route the signal, turn on C1 and C4, as shown, with all other FET devices connected to C1 and C4 biased off. The voltage conversion can be achieved by an N or P load device connected to drain of C4. The "on" resistances of C1 and C4 can be 200 ohms or less with reasonable dimensions (width/length ratio at 1 micron). Similarly, the output device of the current source can also have "on" resistance less than 200 ohms. As a result, the maximum voltage at any source of a selected crosspoint transistor is less than 600 mV. Assuming a 5 V power supply and threshold voltage of 1 V, the net gate-to-source drive will be 3.4 V. A further improvement, at the cost of another mask, is to make the crosspoint transistors "zero Vt" device
(i.e., threshold voltage is approximately zero). The gate t...