Duty cycle integrator with tracking common mode feedback...

Details Duty cycle integrator with tracking common mode feedback... Duty cycle integrator with tracking common mode feedback...

2001-05-25

2003-06-03

Ton, My-Trang Nu (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific input to output function

By integrating

C330S260000

Reexamination Certificate

active

06573779

ABSTRACT:

TECHNICAL FIELD
This invention relates to differential amplifiers and to clock duty cycle integrators.
BACKGROUND
Differential amplifiers are used for a variety of purposes. One use of a differential amplifier is in a circuit that measures and corrects for duty cycle error in a clock signal.
A clock is typically desired to have a 50% duty cycle. A duty cycle integrator is used to produce a differential output voltage that represents the integral of the duty cycle error on a clock signal over time. That voltage is then used to correct the duty cycle of the signal back to 50%.
FIG. 1
shows a simplified prior art duty cycle integrator
10
. As input, the duty cycle integrator accepts complementary clock inputs CLK+ and CLK−, and produces a differential output voltage on nodes OUT+ and OUT−.
Duty cycle integrator
10
comprises a differential pair of transistors M
1
and M
2
. The transistors in this example are n-channel MOSFETs whose sources are connected in common to a lower supply voltage V
ss
. The drain of each transistor is connected to an upper supply voltage V
dd
through a load circuit
12
. The load circuit typically provides a bias and preferably exhibits high differential mode impedance to facilitate integration of the duty cycle error.
A capacitance C is connected to integrate the output current and generate voltages OUT+ and OUT−.
The gates of transistor M
1
and M
2
receive the complementary clock inputs CLK+ and CLK−, and the transistors switch on and off in response. Because of the complementary nature of the clock inputs, one transistor is on while the other is off. If there is a 50% duty cycle, each transistor is on for same duration, and the integrated outputs equal each other. If there is other than a 50% duty cycle, however, one of the transistors is on for longer durations than the other transistor. Over time, this causes the integrated outputs OUT+ and OUT− to become unequal, producing a differential signal. The rate of change of that differential signal is indicative of the duty cycle error on the clock inputs.
FIG. 2
shows a prior art duty cycle integrator
20
having an active load. Transistors M
1
and M
2
are connected as in
FIG. 1
, except that a current source I is connected between V
ss
and the common sources of the transistors. The load comprises two pairs
21
and
22
of p-channel MOSFETs. Each pair has its drains connected in common to the drain of a corresponding one of transistors M
1
and M
2
, and its sources connected in common to V
dd
. Within each pair, the gate of one transistor is tied to the drain of the corresponding one of differential transistors M
1
and M
2
, while the gate of the other transistor is tied to the drain of the other, opposing one of differential transistors M
1
and M
2
. For example, of the pair
21
, one transistor has its gate tied to the drain of differential transistor M
1
, while the other transistor has its gate tied to the drain of differential transistor M
2
.
This load is advantageous in certain situations because it exhibits a very low common mode impedance and a very high differential impedance.
A load such as this is described in U.S. Pat. No. 5,422,529, entitled “Differential Charge Pump Circuit With High Differential and Low Common Mode Impedance,” which states that the described load eliminates the need for common mode feedback.


REFERENCES:
patent: 5422529 (1995-06-01), Lee
patent: 5451898 (1995-09-01), Johnson
patent: 5914638 (1999-06-01), He
patent: 6281751 (2001-08-01), Maulik
patent: 6326846 (2001-12-01), Brandt
patent: 6356152 (2002-03-01), Jezdic et al.
patent: 6359510 (2002-03-01), Ishii et al.
patent: 6384653 (2002-05-01), Broome

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