Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2001-02-09
2002-05-28
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S376000, C327S377000, C327S427000
Reexamination Certificate
active
06396325
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electronic switches. In particular, the present invention relates to semiconductor switches, including those formed of one or more metal-oxide-semiconductor field effect transistors (MOSFET). More particularly, the present invention relates to semiconductor switches capable of switching at relatively high frequencies, including those frequencies above about one gigahertz.
2. Description of the Prior Art
Developments in semiconductor technology have created the capability to produce low-cost, highly reliable switches that are, effectively, implementations of mechanical relays. They have been found to be of particular use, when implemented, as single pole, single throw, type relays, but are not limited thereto. Semiconductor switches are being used more and more as replacements for the prior mechanical relays, due to the high switching speed available as well as their ability to transfer relatively high currents without failure. These switches are often referred to as transfer gates or pass transistors as they employ the characteristics of transistors—usually MOS transistors—to either permit or prevent the passage of a signal.
It is well known that switches are widely used in many fields. They are used in all variety of large- and small-scale consumer products, including, but not limited to, automobiles and home electronics. They can be and are used as analog routers, gates, and relays. They are used as digital multiplexers, routers, and gates as well.
A generic P-type MOS transistor switch is shown in FIG.
1
. The switch is essentially PMOS transistor M
1
having a source coupled to node A and a drain coupled to node B for the purpose of regulating signal transmission between nodes A and B. The control gate of switch M
1
is enabled by way of a coupling to enable-signal-input node EN from external control circuitry. EN is commonly coupled to the gate of M
1
by way of an inverter chain including one or more pairs of inverters such as inverters IV
1
and IV
2
. Inverters IV
1
and IV
2
are powered by a high-potential power rail identified as Vcc and a low-potential power rail identified as GND. The bulk of the switch transistor is coupled to the high-potential power rail. In operation, a logic LOW applied at EN propagates through the inverter chain to turn on M
1
, thereby allowing a signal to pass between nodes A and B, whether from A to B or from B to A. A logic HIGH at EN turns M
1
off, thereby blocking signal propagation between nodes A and B.
For illustration purposes in order to advance the discussion of the present invention, line resistances R
1
and R
2
are shown, as are parasitic capacitances C
1
, C
2
, and C
3
. Resistances R
1
and R
2
represent the impedances associated with circuitry coupled to the transistor switch circuit. That impedance may be of some expected value; for example, in certain applications, resistances R
1
and R
2
are generally on the order of about 50 ohms. However, it is important to note that the present invention is not limited to any specific load impedances associated with external circuitry.
Continuing the discussion regarding
FIG. 1
, capacitance C
1
represents the impedance associated with the gate-to-source interface of the transistor structure, capacitance C
2
represents the impedance associated with the drain-to-gate interface of the transistor structure, and capacitance C
3
represents the impedance associated with the gate-to-bulk interface (typically a gate oxide layer) of the transistor structure. It is to be noted that an N-type MOS transistor may be employed to perform a complimentary same switching function as that provided by PMOS transistor M
1
, with appropriate modifications in the inverter chain and the coupling of the bulk of the transistor to GND instead of Vcc, and bearing in mind certain differences understood by those skilled in the art in regard to NMOS and PMOS transistors.
MOS transistors are desirable in that they consume very little power to operate. As fabrication techniques have advanced, the supply potentials and switching speeds at which such structures can operate effectively have improved. Nevertheless, it has been determined that most silicon MOS transistor switches configured in the manner shown in
FIG. 1
have significant difficulty in propagating signals between A and B when such signals exceed transmission frequencies on the order of 400 MHz. It may appear to be possible to improve this characteristic by reducing the size of M
1
; however, there is an undesirable trade-off involving an increase in the on-resistance of the transistor. Apart from an overall interest in keeping transistor on resistances low, the net result when evaluating the transfer function of the structure may be little or no gain in frequency performance.
An analysis of the impedances of the switch transistor shown in
FIG. 1
leads to an understanding of the propagation frequency limitation associated with that device. Specifically, as the transmission signal propagation frequency exceeds 300 MHz, for example, the impedances associated with the characteristic of the system identified simply by resistances R
1
and R
2
, and the gate-coupled capacitances C
1
, C
2
, and C
3
begin to dominate the transfer function. As a result, at such a frequency and higher, a shunt or short is established between the transistor's bulk coupled to Vcc and GND (through inverter IV
2
that enables M
1
). The dominating impedance at such frequencies causes an unacceptable attenuation of the signal to be passed. As earlier noted, this cannot be resolved by reducing the gate size of M
1
as that drives up the drain-source resistance undesirably.
For most computing applications, the frequency limitations of MOS transistor switches are of little concern. However, as the drive for increased operating bandwidth capabilities grows, such as in the video transmission field for example, there is a greater need for MOS transistor switches that can pass relatively higher frequency transmissions with minimal losses. Therefore, what is needed is a semiconductor circuit that acts as a switch for digital and analog operations. What is also needed is a semiconductor switch circuit that is operable as a transfer gate or pass gate over an array of expected supply potentials. Further, what is needed is a MOSFET-based switch circuit capable of propagating relatively high frequency signals with minimal attenuation. What is further needed is such a switch circuit that propagates high-frequency transmissions with minimal effect on the on-resistance associated with the transistor circuit.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor circuit that acts as a switch for digital and analog operations. It is also an object of the present invention to provide a semiconductor switch that is a transfer gate or pass gate operable for a broad range of supply potentials. It is a further object of the present invention to provide a MOSFET-based switch circuit capable of propagating relatively high frequency signals with minimal attenuation. Another object of the present invention is to provide such a switch circuit that propagates high-frequency transmissions with minimal effect on the on-resistance associated with the MOSFET-based passgate structure.
These and other objectives are achieved in the present invention by increasing the impedance of the shunting pathway associated with the existing MOSFET structure used to establish the pass gate. Specifically, an impedance element, such as a resistive device, a capacitive device, or a combination thereof, is coupled between the gate of the pass gate transistor and a supply rail. The impedance element serves to decouple the pass gate transistor's gate from the supply rail that determines the gate potential. Additionally, such an impedance element may be coupled between the bulk of the pass gate transistor and the supply rail to which the bulk is coupled, again, to decouple that portion of th
Cesari and McKenna LLP
Cunningham Terry D.
Fairchild Semiconductor Corporation
Nguyen Hai L.
Paul, Esq. Edwin H.
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