Oscillators – Ring oscillators
Reexamination Certificate
1999-07-26
2001-06-26
Mis, David (Department: 2817)
Oscillators
Ring oscillators
C331S034000, C331S175000, C331S17700V, C331S185000
Reexamination Certificate
active
06252467
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage controlled oscillator and, more specifically, to a voltage controlled oscillator having a frequency corresponding to a control voltage and generating a clock signal having a predetermined amplitude.
2. Description of the Background Art
Recently, as the speed of internal operation of a semiconductor integrated circuit device has been improved, a PLL (Phase Locked Loop) circuit for generating an operation clock of a higher frequency has been required. In order to implement such a PLL circuit, a voltage controlled oscillator (hereinafter referred to as VCO) operating at a high frequency is necessary.
A differential VCO using a differential buffer has been proposed as a VCO operating at a high frequency (See Japanese Patent Laying-Open No. 9-214299).
The differential VCO includes, as shown in
FIG. 14
, a ring oscillator
51
and a bias circuit
52
. Ring oscillator
51
includes a differential buffer train
51
a
including a plurality of differential buffers connected in series. An output of the differential buffer at the last stage of differential buffer train
51
a
is inverted and input to the differential buffer of the first stage. Delay time of each of the differential buffers varies in accordance with the control voltage VC, and amplitude of an output clock signal from each of the differential buffers varies in accordance with a bias voltage VB. Therefore, ring oscillator
51
oscillates at a frequency in accordance with the control voltage VC and outputs a clock signal of which amplitude corresponds to the bias voltage VB.
Bias circuit
52
includes a replica circuit of the differential buffer, and generates and applies to each differential buffer a bias voltage VB so that the amplitude of the output clock signal of the differential buffer does not vary dependent on the control voltage VC but has a prescribed amplitude in accordance with the reference voltage VR.
Therefore, the differential VCO enables setting of the amplitude of the clock signal to be smaller than the power supply voltage, and hence enables generation of a clock signal at a higher frequency.
In the conventional differential VCO, however, if a clock signal of a high frequency is to be obtained, the differential buffer comes to have smaller gain, which causes difficulty in causing oscillation of ring oscillator
51
.
Further, control voltage VC and bias voltage VB are adapted to vary gradually to ensure stable operation of the differential VCO, and therefore at the time of power on, for example, a certain time period is necessary until the control voltage VC and the bias voltage VB attain to the normal values. In that period, however, it is more difficult for the ring oscillator to oscillate.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a voltage controlled oscillator which facilitates oscillation.
According to an aspect, the present invention is provided with a ling oscillator including a plurality of differential amplifiers connected in a ring, a first bias circuit including a replica circuit of at least a part of the differential amplifiers, and at least one differential gain increasing circuit for increasing differential gain of at least one differential amplifier. Therefore, even when the differential amplifier comes to have smaller gain as a result of obtaining a clock signal of higher frequency, the differential gain of at least one differential amplifier is increased by the differential gain increasing circuit, and therefore, oscillation of the ring oscillator is facilitated.
Preferably, the differential amplifier includes a first differential transistor pair, a first current source causing current flow in accordance with a first bias voltage, and a first pair of variable resistance elements, control value of each of which varies in accordance with the control voltage. Thus, the differential amplifier can be formed in a simple manner.
Preferably, the differential gain increasing circuit includes a cross coupled second differential transistor pair, a second current source causing current flow in accordance with the first bias voltage, and a second pair of variable resistance elements, resistance value of each of which varies in accordance with the control voltage. Here, it is possible to provide the differential gain increasing circuit having similar structure as the differential amplifier.
Preferably, the differential gain increasing circuit includes a cross coupled second differential transistor pair, a second current source causing current flow in accordance with a second bias voltage, and a second pair of variable resistance elements the resistance value of each of which is set at a predetermined value. Further, a second bias circuit for generating the second bias voltage is provided. Here, it is possible to generate the second bias voltage regardless of the oscillating frequency, and therefore degradation of gain increasing performance of the differential gain increasing circuit is prevented.
Preferably, the differential gain increasing circuit includes a differential comparator, a first capacitor connected between an output node and an input node thereof, and a second capacitor connected between an inverted output node and an inverted input node thereof. Thus, the differential gain increasing circuit can be formed in a simple manner.
Preferably, a switch circuit for selectively connecting or disconnecting the differential gain increasing circuit to and from the corresponding differential amplifier is further provided. Here, as an unnecessary differential gain increasing circuit can be disconnected from the differential amplifier, power consumption can be reduced.
According to the further aspect, the present invention is provided with a ring oscillator including a plurality of differential amplifiers connected in a ring, a bias circuit including a replica circuit of at least a part of the differential amplifiers, and a start up circuit for controlling at least one of the control voltage and the bias voltage such that oscillation of the ring oscillator starts with ease, for only a prescribed period from an instruction to start oscillation. Therefore, even in the initial state where the control voltage and the bias voltage have not yet attained the normal values, oscillation of the ring oscillator starts smooth.
Preferably, the start up circuit includes a first switching circuit for selectively applying the control voltage or a first reference voltage for facilitating start of oscillation of the ring oscillator to a first node, and a first capacitor coupled to the first node, and delay time of each differential amplifier varies in accordance with the voltage at the first node. Here, the, first reference voltage and the control voltage can be switched easily in a smooth manner.
More preferably, the start up circuit includes a second switching circuit for selectively applying either the bias voltage or a second reference voltage for facilitating start of oscillation of the ling oscillator to a second node, and a second capacitor coupled to the second node, and the amplitude of the output clock signal of each differential amplifier varies in accordance with the voltage at the second node. Here, the second reference voltage and the bias voltage can be switched easily in a smooth manner.
Preferably, the prescribed period is from the turn on of the power supply voltage to stabilization of the power supply voltage. Here, generally, a semiconductor integrated circuit device is provided with a circuit for outputting a reset signal in the period from turning on of the power supply voltage until stabilization. Therefore, the start up circuit can be controlled in a simple manner utilizing this signal.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 5191301 (1993-0
McDermott & Will & Emery
Mis David
Mitsubishi Denki & Kabushiki Kaisha
LandOfFree
Voltage controlled oscillator including a plurality of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Voltage controlled oscillator including a plurality of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Voltage controlled oscillator including a plurality of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2540067