Patent Publication Number: US-5428310-A

Title: Voltage compensating delay element

Description:
PRIOR APPLICATION 
     This application is a continuation of Ser. No. 08/032,900 filed Mar. 18, 1993, now U.S. Pat. No. 5,352,945. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electronic circuits, and more specifically to circuits used to delay high-speed digital signals. 
     BACKGROUND OF THE INVENTION 
     Most signal-delaying circuits used in various electronic devices such as in the feedback of an oscillator consists essentially of a capacitive element attached between the node where a waveform is to be delayed and ground or other reference point. This type of circuit, however, does not compensate for the fact that the speed of the device tends to increase as the supply voltage rises. 
     It would be advantageous to have a signal-delaying circuit which responds inversely to increases in the supply voltage in order to compensate for the above-mentioned phenomenon. 
     SUMMARY OF THE INVENTION 
     The principal and secondary objects of this invention are to palliate the excessive sensibility of high-speed circuit to variations in the circuit supply voltage; and to provide an implementing circuit suitable for use in integrated microcircuits. 
     These and other objects are achieved by inserting a variable resistance element between a node where a waveform is to be delayed and a delaying capacitor, and biasing said variable resistance element with a voltage proportional to the power supply of the device. As the power supply voltage decreases and has the effect of slowing down signals through the device, the resistance of the variable resistance element is increased, thus masking the effect of the circuit delaying capacitor and inhibiting its delaying effect. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic of the prior art; 
     FIG. 2 is a a schematic of the preferred embodiment of the invention; and 
     FIG. 3 is a schematic of an application of the invention to a ring oscillator circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
     As illustrated in FIG. 1, a waveform may be delayed at a certain node point 1 in an electronic circuit by inserting a capacitor 2 between the node point 1 and ground 3 or other reference point. However, this type of signal-delaying capacitor circuit does not provide any compensation for the effect of variations in the voltage supply on the speed of the circuit. It is well known that as the voltage supply increases the speed of a signal through a circuit under the voltage supply tends to increase. 
     FIG. 2 illustrates an improvement in the signal-delaying capacitive circuit particularly suitable for implementation in microelectronic integrated devices. 
     In the first place, the capacitor of the prior art 2 has been replaced by a metal-oxide semiconductor (MOS) wired to act as a capacitor. The source 5 and drain 6 of the MOS device have been wired together and connected to the reference ground 3. The gate 7 of the MOS device is connected to the source 8 of a second MOS device, the drain of which 10 is connected to the node 1 where a waveform is to be delayed. The gate 11 of the second MOS device is biased by connection to a voltage-divider circuit 12 made of resistors R 1  and R 2  between the supply voltage Vcc and the reference ground 3. 
     It is easy to understand that as the power supply voltage Vcc decreases and the speed of a waveform through the overall electronic device powered by said supply voltage tends to decrease as a result of said Vcc voltage drop, the biasing signal on the gate 11 of the second MOS device 9 is reduced. The source-drain resistance of this second MOS device increases. In the extreme case, where the biasing voltage derived from the voltage divider circuit 12 falls below the threshold of the second MOS device 11, the capacitive circuit created by the first MOS device 4 is totally masked and the waveform is not subject to any delay at node 1. By contrast, if the supply voltage Vcc increases the second MOS device 9 becomes more conductive and the signal-delaying capacitor circuit becomes more fully effective compensating for the general speed increase of the overall circuit resulting from the increase in the supply voltage Vcc. 
     It can be understood that with proper adjustment of the bias voltage of the second MOS device, the improved signal-delaying capacitive circuit can compensate for undesirable speed variation due to changes in the supply voltage. 
     FIG. 3 illustrates the implementation of the improved signal-delaying circuit in a ring oscillator circuit 13 designed for use as part of an integrated micro-circuit device. The capacitive element is constituted by a series of polysilicon capacitors C1-C5, each in series with a MOS semiconductor M1-M5 having drains connected to a sequence of nodes N1-N5. The nodes are separated by invertors V1-V4 . The gates of the semiconductors M1-M5 are connected to a bias network 14 that provides a regulating signal 15 designed to compensate for any variation of the voltage supply or of any other factor affecting the frequency of the oscillator circuit 13. The regulating signal may also be used to adjust the frequency of the oscillator circuit independently of any undesirable frequency-affecting factor. It should be understood that the capacitive elements and the variable resistance elements in each of the above-described applications may consist of various types of electronic components inherently capable of acting as such elements, as may be available now or later developed. 
     While the preferred embodiments of the invention have been described, modifications can be made and other embodiments may be devised without departing from the spirit of the invention and the scope of the appended claims.