Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1999-08-12
2001-12-11
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S112000, C327S157000, C330S010000
Reexamination Certificate
active
06329872
ABSTRACT:
TECHNICAL FIELD
This invention relates to a charge pump circuit, which is used in a phase locked loop (PLL), for instance.
BACKGROUND INFORMATION
A known PLL configuration includes a voltage controlled oscillator (VCO), a phase detector, a charge pump circuit and a loop filter. U.S. Pat. No. 5,675,291 granted to D. Sudjian on Oct. 7, 1997 discloses a PLL for use in a frequency synthesizer.
U.S. Pat. No. 5,334,951 granted to J. G. Hogeboom on Aug. 2, 1994 discloses a PLL. The prior art charge pump circuit shown in
FIG. 2
of the patent is based on a current mirror circuit which includes series-connected field effect transistors (FETs) as switching devices. A VCO signal and an input signal are fed to the phase detector which detects the phase difference between both signals. The phase detector generates two pulse trains responsive to the phase difference. The pulse trains cause the switching FETs to turn-on and -off. In response to one of the pulse trains, the current mirror circuit sources current to the filter via the on-switching FET and, in response to the other pulse train, the current mirror circuit sinks current from the filter via the on-switching FET. As a result, the filter is charged or discharged and the voltage in the filter is fed to the VCO to variably control the frequency of the VCO signal. In some applications of PLLs (e.g., a frequency synthesizer) including the charge pump circuit, the VCO output frequencies are controllably varied and different frequencies require different charge pump circuit output voltages. The source and sink currents are required to be matched to each other at any voltage of the charge pump circuit output. Although the charge pump circuit is designed to have high output impedances to minimize the mismatch, it is difficult to have infinite output impedances and the mismatch is imminent. Any mismatch between the source and sink currents causes undesirable sidebands at the output of the PLL. In a case of the FETs being CMOS (complementary metal oxide semiconductor) devices, the output impedance of the current sourcing FETs becomes very low to provide low output voltage, or voltages near the positive rail.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved charge pump circuit.
According to one aspect of the present invention, there is provided a charge pump circuit for sourcing current to and sinking current from an external circuit, the charge pump circuit comprising: first transistor means comprising first and second transistors, each having an input control electrode and two other electrodes, the other two electrodes of both transistors being connected in series, the series-connected first and second transistors causing a first current to flow therein upon application of operating voltage to the first transistor means which develops a first voltage in response to the first current; second transistor means comprising third and fourth transistors, each having an input control electrode and two other electrodes, the other two electrodes of both transistors being connected in series, upon application of the operating voltage, the second transistor means causing a second current to flow in the third transistor and a third current to flow in the fourth transistor, a second voltage being developed across the external circuit in response to the second or third current; first feedback means for providing a first signal in response to the first and second voltages, the first signal being fed to the input control electrode of the first transistor; second feedback means for providing a second signal in response to a reference voltage and a third voltage, the second signal being fed to the input control electrode of the second transistor; first feeding means for feeding the first signal to the input control electrode of the third transistor in response to a first input signal; second feeding means for feeding the second signal to the input control electrode of the fourth transistor in response to a second input signal; and start-up means for initializing the first transistor means so that a start-up current flows therein.
The first transistor means responds to the difference between the first and second voltages and provides the first signal to the first transistor. In the series-connected first and second transistors of the first transistor means, current flows in response to the reference voltage and the first signal. Current flowing in the second transistor means is mirrored current which is proportional to the current flowing in the first transistor means. The mirrored current is sourcing current from the second transistor means to the external circuit connected to the charge pump circuit, or is sinking current from the output circuit to the second transistor means. Because the source and sink currents depend upon the voltage difference between the first and second transistor means, mismatch between the source and sink currents is reduced.
For example, the first transistor means comprises an operational amplifier. Its non-inverting and inverting input terminals are connected to the junction of the first and second transistors and the junction of the third and fourth transistors. The voltage difference between the two junctions is detected by the operational amplifier and the first signal is provided from the output terminal of the operational amplifier. Because the operational amplifier has a large input impedance, a large and definite input impedance is presented at the charge pump circuit when current sourcing and sinking. The first voltage is variably controlled in response to the first and second signals so that the first voltage matches the second voltage and hence, the mismatch of source and sink currents is reduced at any output voltage of the charge pump circuit.
The start-up means initializes first transistor means. For example, the first transistor is turned-on with initialization and a start-up current flows in the first transistor means. Once initialized, the charge pump circuit operates in current source and sink modes.
REFERENCES:
patent: 4814726 (1989-03-01), Byrd et al.
patent: 5220294 (1993-06-01), Ichikawa
patent: 5334951 (1994-08-01), Hogeboom
patent: 5420545 (1995-05-01), Davis et al.
patent: 5675291 (1997-10-01), Sudjian
patent: 5886551 (1999-03-01), Narahara
patent: 5945855 (1999-08-01), Momtaz
patent: 6107889 (2000-08-01), Strange et al.
patent: 0 440 405 B1 (1991-08-01), None
patent: 0 641 082 B1 (1995-03-01), None
“An Improved Tail Current Source for Low Voltage Applications”, F. You et al., IEEE Journal of Solid-State Circuits, vol. 32, No. 8, Aug. 1997, pp. 1173-1180.
Callahan Timothy P.
de Wilton Angela C.
Englund Terry L.
Nortel Networks Limited
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