Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Accelerating switching
Reexamination Certificate
1999-04-22
2001-01-09
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Accelerating switching
C327S307000, C327S563000
Reexamination Certificate
active
06172551
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to current switches and, more particularly, to switches in which the switched current varies over a wide range.
2. Description of the Related Art
Although a majority of transistor current switches are used to switch a current having a substantially fixed magnitude, an important need exists for switches that can properly switch variable currents. Conventional current switches may exhibit significant timing errors in high-speed switching applications because their signal transit time varies excessively with respect to the current magnitude.
Transit time through a switching transistor is related to its transition frequency f
T
which is the frequency at which the short-circuit, common-emitter current gain falls to unity. It has been shown (e.g., see Paul R. Gray and Robert G. Meyer,
Analysis and Design of Analog Integrated Circuits,
John Wiley and Sons, New York, 1993, pp. 30-45) that a transistor's f
T
is related to its device capacitances (e.g., base-emitter and base-collector junction capacitances) and its transconductance.
Because transconductance is a function of emitter current, f
T
approaches a constant value at high currents. At low current values, device capacitances dominate and, accordingly, f
T
falls with decreasing current. Other effects (e.g., increased base transit time due to high-level injection) cause f
T
to fall again at very high currents.
These relationships are illustrated in the graph
10
of
FIG. 1
which shows a plot
12
of an exemplary transistor's f
T
as a function of the transistor's emitter current. Because device capacitances are directly related to device size, a switching transistor can be “sized” to optimize its performance when switching a specified current magnitude. Accordingly, the exemplary transistor has been sized to maximize its f
T
at an emitter current of ~100 milliamps and it can be seen that f
T
has fallen to nearly 10% of its peak value at an emitter current of 1 milliamp.
The dynamic range of a variable-current switching device is typically defined as the difference between the minimum and maximum currents that can be switched without exceeding a specified signal transit-time window. In a switching application in which the signal transit-time window is narrow and the minimum current is zero, the wide variation of f
T
in
FIG. 1
indicates that the exemplary transistor will have a limited dynamic range.
SUMMARY OF THE INVENTION
The present invention is directed to current switches that significantly reduce signal transit-time variations when switching current signals that have a wide range of current magnitudes. Accordingly, the invention enhances the dynamic range of current switches.
These objectives are achieved with the realization that transit-time variations can be reduced by shifting the range of the currents that are carried by switching transistors. In particular, a current range is shifted upward to a higher current range in which the variation of the transistors' transition frequency f
T
is reduced.
Switches of the invention switch a signal current between first and second output ports in response to a control signal. They include a first differential pair of first and second transistors, an offset current source that generates an offset current and a current-steering system. Either of the transistors can be selected to steer the signal current to a respective one of the output ports. Simultaneously, the current-steering system steers the offset current along a current path that includes the selected transistor and excludes the respective output port.
Accordingly, the selected transistor conducts a current whose magnitude is at least that of the offset current to shift the transistor's current range upwards and thereby enhance its dynamic range. Preferably, the offset current has a magnitude between the minimum current and the maximum current.
In switch embodiments, the offset current is formed with substantially equal first and second offset currents and the first differential pair also steers the first offset current through the selected transistor to the respective output port. A second differential pair steers the second offset current to a different output port that differs from the respective output port and first and second correction currents are coupled to the first and second output ports. Because the first and second correction currents substantially equal the first and second offset currents, they cancel the first offset current at the respective output port and cancel the second offset current at the different output port.
The teachings of the invention also include waveform analyzers and test interface circuits whose performance is enhanced by inclusion of the invention's current switches.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.
REFERENCES:
patent: 5682119 (1997-10-01), Soda
patent: 5969579 (1999-10-01), Hartke et al.
patent: 6049251 (2000-04-01), Meyer
Paul R. Gray and Robert G. Meyer,Analysis and Design of Analog Integrated Circuits,John Wiley and Sons, New York, 1993, pp. 30-45.
Jasprit Singh,Semiconductor Devices,McGraw-Hill, New York, 1994, pp. 323-334.
Analog Devices Inc.
Callahan Timothy P.
Koppel & Jacobs
Nguyen Minh
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