Abstract:
Disclosed is a delay circuit for delaying at least the timing of a rising edge or the timing of a falling edge of an input signal alternating between first and second levels. The delay circuit includes (1) a charge pump in which first and second field-effect transistors of different channels are serially connected; (2) a capacitor connected in parallel with the first field-effect transistor; (3) a charging current control circuit for passing a charging current into the capacitor via the second field-effect transistor of the charge pump when the input signal is at the first level; (4) a discharge current control circuit for releasing a discharge current from the capacitor via the first field-effect transistor when the input signal is at the second level; and (5) a discrimination circuit for outputting a signal of a prescribed logic level based upon a terminal voltage of the capacitor. The values of the charging current and discharge current of the capacitor are controlled to control the slope of the input voltage to the discrimination circuit, thereby adjusting the delay time.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a delay circuit and to an oscillator circuit using the same. More particularly, the invention relates to a CMOS-implemented delay circuit with a variable delay time used in phase adjustment or the like, as well as to an oscillator circuit that uses this delay circuit. 
     CMOS delay circuits have long been in use as delay circuits for subjecting an input signal pulse to a predetermined time delay. FIG. 22 is a diagram illustrating such a CMOS delay circuit according to the prior art. The delay circuit includes a P-MOS transistor (a P-channel field-effect transistor)  101  for supplying a charging current and having a gate terminal to which a charging control voltage  109  is applied; an N-MOS transistor (an N-channel field-effect transistor)  104  for releasing a discharge current and having a gate terminal to which a discharge control voltage  110  is applied; P-MOS and N-MOS transistors  102  and  103  constructing a CMOS inverter and having their gates connected together to provide an input terminal and their drains connected together to provide an output terminal; a charge/discharge capacitor  105  inserted between the CMOS inverter output terminal and ground potential (GND); and a discrimination circuit  106  for discriminating and comparing the capacitor terminal voltage and a predetermined threshold level and outputting a signal having a prescribed logic level in dependence upon the result of the comparison. The discrimination circuit  106  is constituted by a CMOS inverter comprising P-MOS and N-MOS transistors  107  and  108 , for example, and outputs a signal that is the result of inverting the terminal voltage level of the capacitor  105 . 
     The CMOS delay circuit of FIG. 22 smoothens an input signal by the inverter circuitry, which comprises the P-MOS transistors  101 ,  102  and N-MOS transistors  103 ,  104 , and the capacitor  105 , and inverts the smoothened waveform by the discrimination circuit (inverter circuit)  106  comprising the P-MOS transistor  107  and N-MOS transistor  108 , thereby providing a delay. 
     Consider an ideal case in which the P-MOS transistor  101  and N-MOS transistor  104  are completely devoid of parasitic capacitance. FIG. 23A is a diagram useful in describing such an ideal case in which there is no parasitic capacitance. When the input signal is at ground level, the capacitor  105  is charged up to the power-supply level, the input to the discrimination circuit  106  is the power-supply level and the output thereof is the ground level. If the input signal rises to the power-supply level from the ground level under these conditions, the P-MOS transistor  102  turns off, the N-MOS transistor  103  turns on and the electric charge that has accumulated in the capacitor  105  is discharged through the N-MOS transistor  104 . Since the N-MOS transistor  104  acts as a current source controlled by the discharge control voltage  110 , the discharge current is rendered constant. Accordingly, the input voltage of the discrimination circuit  106  falls from the power-supply level to the ground level at a fixed slope controlled by the discharge control voltage  110 . The discrimination circuit  106  discriminates and outputs the input voltage at the threshold voltage value Vth. When the input voltage of the discrimination circuit  106  becomes Vth, therefore, the output signal level rises from the ground level to the power-supply level. 
     As a result of the foregoing, a delay time τ 1  from the rising edge of the input signal to the rising edge of the output signal can be controlled through the slope of the input voltage of discrimination circuit  106  by the discharge control voltage  110 . Similarly, a delay time τ 2  when the input signal decays can be controlled by the charging control voltage  109 . 
     In order for the delay circuit to operate stably, it is required that the input voltage to the discrimination circuit  106  makes a complete swing from the power-supply level to the ground level or from the ground level to the power-supply level in one period. In order for the input voltage to swing completely, the range over which delay time can be varied is limited because the slope of the input voltage to the discrimination circuit  106  is limited. In a case where the threshold voltage value Vth is one-half the power-supply voltage, the range over which the delay time can be varied is half the period T. FIGS. 24A,  24 B are diagrams useful in describing the reason why the input to the discrimination circuit  106  must make a complete swing. FIG. 24A illustrates a case where the input swings completely from the power-supply level to the ground level, and FIG. 24B illustrates a case where the input does not swing completely from the power-supply level to the ground level. In the case of the full swing shown in FIG. 24A, a delay time τ is constant, i.e., is independent of the pattern of the input signal, and operation is stable. When the input to the discrimination circuit  106  does not make a full swing, however, as shown in FIG. 24B, the voltage at the beginning of the slope differs depending upon the input signal pattern and therefore the delay time τ fluctuates (see τ′, τ″) depending upon the input signal and operation becomes unstable. 
     The foregoing relates to the ideal case in which the MOS transistors  101 ,  104  are devoid of parasitic capacitance. In actuality, however, parasitic capacitances  111 ,  114  exist and cause the circuit to behave in a manner different from that of the ideal case. FIG. 23B is a diagram useful in describing operation when the parasitic capacitances  111 ,  114  exist. 
     When the input signal is at ground level, the capacitor  105  is charged up to the power-supply level and the input to the discrimination circuit  106  is the power-supply level. Further, the N-MOS transistor  104  is ON at all times. As a consequence, the parasitic capacitance  114  is discharged to the ground level. If the input signal rises under these conditions, the P-MOS transistor  102  turns off and the N-MOS transistor  103  turns on. As a result, the capacitor  105  and the parasitic capacitance  114  are connected in series and, hence, the input to the discrimination circuit  106  falls instantaneously to a voltage V 1  intermediate the power-supply level and the ground level until the parasitic capacitance  114  is charged. Accordingly, the input voltage to the discrimination circuit  106  subsequently starts falling from the intermediate voltage V 1  at a fixed slope controlled by the discharge control voltage  110 . Since the voltage V 1  is near the threshold voltage value Vth of the discrimination circuit  106 , the range over which the delay time can be varied cannot be enlarged. 
     Similarly, in a case where the input signal decays, the input voltage to the discrimination circuit  106  rises instantaneously to an intermediate voltage V 2  and then rises from the intermediate voltage V 2  at a fixed slope. As a consequence, the range over which the delay time is variable cannot be enlarged. 
     Thus, owing to the influence of the parasitic capacitances  111 ,  114 , the input voltage to the discrimination circuit  106  changes instantaneously to the intermediate voltages V 1 , V 2  when the level of the input signal changes over, as a result of which it is not possible to enlarge the range over which delay time can be varied. 
     Further, since the intermediate voltages V 1 , V 2  vary depending upon the parasitic capacitances  111 ,  114 , there is a great variance in delay time as caused by conditions at the time of manufacture, environmental temperature, etc. 
     With the conventional delay circuit, therefore, it is not possible to enlarge the range over which delay time can be varied because of the influence of the parasitic capacitance of field-effect transistors. 
     Further, with the conventional delay circuit, there is a great variance in delay time as caused by conditions at the time of manufacture, environmental temperature, etc. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a delay circuit that is free of the influence of parasitic capacitance in transistors, thereby making it possible to enlarge the range of delay time variability and reduce variance in delay time. 
     Another object of the present invention is to provide a variety of delay circuits such as a delay circuit in which the pulse width of the output signal is not different from the pulse width of the input signal, a delay circuit for delaying only the rising edge of the input signal, a delay circuit for delaying only the falling edge of the input signal, a delay circuit in which input and output levels are the inverse of each other and a delay circuit in which the input and output levels are not the inverse of each other. 
     Another object of the present invention is to provide a VCO (Voltage-Controlled Oscillator) circuit that uses delay circuits having good controllability and a large range over which frequency can be varied. 
     In accordance with the present invention, the foregoing objects are attained by providing a delay circuit having (1) an inverter in which field-effect transistors of different channels are serially connected, a connection between the transistors is adopted as an output terminal of the inverter and input signals are applied to gates of the field-effect transistors; (2) a capacitor connected to the output terminal of the inverter; (3) a discrimination circuit for discriminating a signal at the output terminal of the inverter; (4) a charging current control circuit connected to the gate of one of the field-effect transistors for controlling, on the basis of an input signal and a control signal, a charging current that flows into the capacitor; and (5) a discharge current control circuit connected to the gate of the other of the field-effect transistors for releasing a discharge current from the capacitor on the basis of the input signal and a control signal. 
     Further, in accordance with the present invention, the foregoing objects are attained by providing a delay circuit for delaying at least one of a timing of a rising edge and a timing of a falling edge of an input signal alternating between first and second levels, comprising (1) a charge pump in which two field-effect transistors of different channels are serially connected; (2) a capacitor connected in parallel with one (i.e., a first) of the field-effect transistors; (3) a charging current control circuit for passing a charging current into the capacitor via the other (i.e., a second) of the field-effect transistors of the charge pump when the input signal is at the first level; (4) a discharge current control circuit for releasing a discharge current from the capacitor via the first field-effect transistor when the input signal is at the second level; and (5) a discrimination circuit for outputting a signal of a prescribed logic level based upon a terminal voltage of the capacitor. 
     Further, in accordance with the present invention, the foregoing objects are attained by providing an oscillator circuit constituted by delay circuits, in which levels of input and output signals are in inverse relation to each other, connected serially into a ring of an odd number of stages, each delay circuit having (1) a charge pump, (2) a capacitor, (3) a charging current control circuit for passing a current into the capacitor, (4) a discharge current control circuit for releasing a discharge current from the capacitor, and (5) a discrimination circuit for outputting a signal of a prescribed logic level based upon a terminal voltage of the capacitor. 
     Further, in accordance with the present invention, the foregoing objects are attained by providing an oscillator circuit constituted by an even number of delay circuits in which levels of input and output signals are not in inverse relation to each other, the delay circuits being connected serially into a ring with a single signal-level inverting circuit and each delay circuit having (1) a charge pump, (2) a capacitor, (3) a charging current control circuit for passing a current into the capacitor, (4) a discharge current control circuit for releasing a discharge current from the capacitor, and (5) a discrimination circuit for outputting a signal of a prescribed logic level based upon a terminal voltage of the capacitor. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram useful in describing an overview of the present invention; 
     FIG. 2 is a diagram showing a first embodiment of a CMOS delay circuit according to the present invention; 
     FIG. 3 is a diagram useful in describing the operation of the first embodiment; 
     FIG. 4 is a diagram showing an embodiment of a current control circuit according to the present invention; 
     FIG. 5 is a diagram showing a second embodiment of a CMOS delay circuit according to the present invention; 
     FIG. 6 is a diagram useful in describing the operation of the second embodiment; 
     FIG. 7 is a diagram showing a third embodiment of a CMOS delay circuit according to the present invention; 
     FIG. 8 is a diagram useful in describing the operation of the third embodiment; 
     FIG. 9 is a diagram showing a fourth embodiment of a CMOS delay circuit according to the present invention; 
     FIG. 10 is a diagram useful in describing the operation of the fourth embodiment; 
     FIG. 11 is a diagram showing a fifth embodiment of a CMOS delay circuit according to the present invention; 
     FIG. 12 is a diagram useful in describing the operation of the fifth embodiment; 
     FIG. 13 is a diagram showing a sixth embodiment of a CMOS delay circuit according to the present invention; 
     FIG. 14 is a diagram useful in describing the operation of the sixth embodiment; 
     FIG. 15 is a diagram showing a seventh embodiment of a CMOS delay circuit according to the present invention; 
     FIG. 16 is a diagram useful in describing the operation of the seventh embodiment; 
     FIG. 17 is a diagram showing an eighth embodiment of a CMOS delay circuit according to the present invention; 
     FIG. 18 is a diagram showing a first embodiment of an oscillator circuit according to the present invention; 
     FIG. 19 is a diagram useful in describing the operation of a CMOS delay circuit of the first embodiment (oscillator circuit); 
     FIG. 20 is a waveform diagram useful in describing the operation of the first embodiment (oscillator circuit); 
     FIG. 21 is a diagram showing a second embodiment of an oscillator circuit according to the present invention; 
     FIG. 22 is a diagram showing a CMOS pulse delay circuit according to the prior art; 
     FIGS. 23A,  23 B are diagrams useful in describing the operation of a CMOS pulse delay circuit according to the prior art; and 
     FIGS. 24A,  24 B are diagrams useful in describing the reason why the input to a discrimination circuit must make a complete swing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (A) Overview of the Present Invention 
     FIG. 1 is a diagram useful in describing an overview of the present invention. 
     In FIG. 1, a charging current control circuit  1  performs control in such a manner that a charging current flows into a capacitor  5  when an input signal to the circuit is at a first level (e.g., the level of the power supply). A discharge current control circuit  2  performs control in such a manner that a discharge current is released from the capacitor  5  when the input signal is at a second level (e.g., the level of ground). A P-channel field-effect transistor  3  and an N-channel field-effect transistor  4  (e.g., P-MOS and N-MOS transistors, respectively) have their drains connected in series to construct a CMOS charge pump. 
     The source of the P-MOS transistor  3  is connected to a power-supply line and its gate terminal is connected to the output of the charging current control circuit  1 . The source of the N-MOS transistor  4  is connected to a ground line and its gate terminal is connected to the output of the discharge current control circuit  2 . The capacitor  5 , which is for charge and discharge, is connected in parallel with the N-MOS transistor  4  across the drain terminal of the charge pump and the ground line. A discrimination circuit  6  determines whether the terminal voltage of the capacitor  5  is equal to or greater than a predetermined threshold level and outputs a signal of a prescribed logic level in dependence upon the determination made. 
     The P-MOS transistor  3  acts as a charging current source, which is controlled by the charging current control circuit  1 , when the input signal is at the power-supply level, and turns off when the input signal is at the ground level. The N-MOS transistor  4 , on the other hand, turns off when the input signal is at power-supply level and acts as a discharge current source, which is controlled by the discharge current control circuit  2 , when the input signal is at the ground level. The charging current control circuit  1  performs control in such a manner that a charging current proportional to a delay control current  7  flows through the P-MOS transistor  3 , and the discharge current control circuit  2  performs control in such a manner that a discharge current proportional to a delay control current  8  flows through the N-MOS transistor  4 . Accordingly, the charge and discharge current of the capacitor  5  can be controlled on the basis of the input signal level, the magnitudes of the charging current and discharge current can be controlled, so can the slope of the input voltage of the discrimination circuit  6 , and so can the delay time. 
     In accordance with this arrangement, there is no parasitic capacitance that is connected to the capacitor  5  in series when the input signal changes over between the rising and falling states. When the input signal changes over, therefore, the input voltage to the discrimination circuit  6  does not change instantaneously to an intermediate potential and the input voltage of the discrimination circuit  6  can be controlled at a fixed slope from the ground level to the power-supply level or from the power-supply level to the ground level. Accordingly, the range over which the delay time is variable can be enlarged and a variance in delay time can be reduced. 
     The discharge current control circuit  2  has a third field-effect transistor (a MOS transistor, which is not shown) for passing a delay control current  8  that conforms to signal delay time. This third MOS transistor and the N-MOS transistor  4  construct a current mirror circuit. When the input signal is at the ground level, the delay control current  8  flows into the third MOS transistor and a discharge current proportional to the delay control current  8  flows into the N-MOS transistor  4  owing to the current mirror function. When the input signal level is at the power-supply level, a delay control current does not flow into the third MOS transistor and the discharge current is made zero. The charging current control circuit  1  has a fourth field-effect transistor (a MOS transistor, which is not shown) for passing a delay control current  7  that conforms to signal delay time. This fourth MOS transistor and the P-MOS transistor  3  construct a current mirror circuit. When the input signal is at the power-supply level, the delay control current  7  flows into the fourth MOS transistor and a charging current proportional to the delay control current  7  flows into the P-MOS transistor  3  owing to the current mirror function. When the input signal level is at the ground level, a delay control current does not flow into the fourth MOS transistor and the charging current is made zero. 
     The foregoing relates to a case where the circuit outputs a signal obtained by delaying the input signal at both its rising and falling edges. However, it is possible to adopt an arrangement in which the input signal is delayed at only one edge. Further, it is possible to implement a delay circuit in which there is no change in the pulse width of the input and output signals, a delay circuit in which input and output levels inversely related and a delay circuit in which input and output levels are not inversely related. 
     Further, an oscillator circuit can be constructed by serially connecting, into a ring, an odd number of delay circuits in which the input and output signals are the inverse of each other, or by serially connecting, into a ring, an even number of delay circuits, in which input and output levels are not the inverse of each other, and a single signal-level inverting circuit. Moreover, by controlling the delay time of the delay circuit through control of voltage, oscillation frequency can be controlled and a voltage-controlled oscillator circuit can be constructed. 
     (B) First Embodiment of CMOS Delay Circuit 
     FIG. 2 is a diagram showing the construction of a first embodiment of a CMOS delay circuit according to the present invention. 
     In FIG. 2, the charging current control circuit  1  performs control in such a manner that a charging current flows into the capacitor  5  when an input signal to the circuit is at the level of the power supply. The discharge current control circuit  2  performs control in such a manner that a discharge current is released from the capacitor  5  when the input signal is at the level of ground. The P-MOS transistor  3  and the N-MOS transistor  4  have their drains connected in series to construct a CMOS charge pump. The source of the P-MOS transistor  3  is connected to the power-supply line and its gate terminal is connected to the charging current control circuit  1 . The source of the N-MOS transistor  4  is connected to the ground line and its gate terminal is connected to the discharge current control circuit  2 . The capacitor  5 , which is for charge and discharge, is connected in parallel with the N-MOS transistor  4  across the drain terminal of the charge pump and the ground line. The discrimination circuit  6  determines whether the terminal voltage of the capacitor  5  is equal to or greater than a predetermined threshold level and outputs a signal of a prescribed logic level in dependence upon the determination made. 
     A current control circuit  15  performs control in such a manner that a delay control current  7 , which conforms to a rising-edge delay time τ H , flows into the charging current control circuit  1 , and in such a manner that a delay control current  8 , which conforms to a falling-edge delay time τ T , flows into the discharge current control circuit  2 . 
     The charging current control circuit  1  is composed of P-MOS transistors  10 ,  11 , and the discharge current control circuit  2  is composed of N-MOS transistors  12 ,  13 . The discrimination circuit  6  is constituted by an inverter circuit comprising a P-MOS transistor  21  and an N-MOS transistor  22 , and the input capacitance of the discrimination circuit  6  serves as the capacitor  5 . 
     The P-MOS transistor  10  of the charging current control circuit  1  acts as a switch and is turned off if the input signal is at the power-supply level and is turned on if the input signal is at the ground level. The P-MOS transistor  11  and the P-MOS transistor  3  construct a current mirror circuit. Accordingly, if the input signal is at the power-supply level, the delay control current  7  flows through the P-MOS transistor  11  and not the P-MOS transistor  10 . As a result, a current proportional to the delay control current  7  flows into the P-MOS transistor  3  owing to the current mirror function and the P-MOS transistor  3  acts as a current source that introduces charging current. If the input signal is at the ground level, the delay control current  7  flows through the P-MOS transistor  10  and not the PMOS transistor  11 . Owing to the current mirror function, therefore, the P-MOS transistor  3  turns off and no charging current flows. In this case a charging current that prevails when the input signal rises from the ground level to the power-supply level can be controlled by the delay control current  7 . Consequently, the delay time of the rising edge is controlled by controlling the value of this delay control current. 
     Similarly, the delay time of the falling edge of the input signal can be controlled by the delay control current  8 . That is, an N-MOS transistor  12  of the discharge current control circuit  2  acts as a switch and is turned off if the input signal is at the ground level and turned on if the input signal is at the power-supply level. An N-MOS transistor  13  and the N-MOS transistor  4  construct a current mirror circuit. Accordingly, if the input signal is at the ground level, the delay control current  8  flows through the N-MOS transistor  13  and not the N-MOS transistor  12 . As a result, a current proportional to the delay control current  8  flows into the N-MOS transistor  4  owing to the current mirror function and the N-MOS transistor  4  acts as a current source that introduces a discharge current. If the input signal is at the power-supply level, the delay control current  8  flows through the N-MOS transistor  12  and not the M-MOS transistor  13 . Owing to the current mirror function, therefore, the N-MOS transistor  4  turns off and no discharge current flows. In this case a discharge current that prevails when the input signal falls from the power-supply level to the ground level can be controlled by the delay control current  8 . Consequently, the delay time of the falling edge is controlled by controlling the value of this delay control current. 
     As a result of the operation described above, the input voltage of the discrimination circuit  6  can be controlled at a fixed slope from the ground level to the power-supply level and from the power-supply level to the ground level, as shown in FIG.  3 . This means that it is possible to obtain a CMOS delay circuit having a broad range τ over which delay time can be varied and a small variance in terms of delay time. 
     Further, delay control currents  7 ,  8  can be varied by the current control circuit  15  while a fixed ratio is maintained between them. As a result, delay time can be controlled by equalizing the delay times τ H , τ T  of the rising and falling edges, respectively. If this arrangement is adopted, it is possible to obtain a CMOS delay circuit in which input and output pulse width does not change. 
     FIG. 4 shows an embodiment of the current control circuit  15 . The current control circuit  15  includes N-MOS transistors  221 - 223  and P-MOS transistor  224 ,  225 . Three current mirror circuits are constructed by the N-MOS transistor  221  and N-MOS transistor  222 , by the N-MOS transistor  221  and N-MOS transistor  223 , and by the P-MOS transistor  224  and P-MOS transistor  225 . Accordingly, by passing a delay control current  14  into the N-MOS transistor  221 , the delay control currents  7 ,  8 , which are proportional to the delay control current  14 , can be made to flow into the charging current control circuit  1  and discharge current control circuit  2 , respectively, by the current mirror function. 
     (C) Second Embodiment of CMOS Delay Circuit 
     FIG. 5 is a diagram showing the construction of a second embodiment obtained by modifying the first embodiment so as to delay only the rising-edge timing of an input signal, and FIG. 6 is a waveform diagram useful in describing the operation of the second embodiment. 
     In FIG. 5, the charging current control circuit  1  performs control in such a manner that a charging current flows into the capacitor  5  when an input signal to the circuit is at the level of the power supply. The discharge current control circuit  2  performs control in such a manner that a discharge current is released from the capacitor  5  when the input signal is at the level of ground. The P-MOS transistor  3  and the N-MOS transistor  4  have their drains connected in series to construct a CMOS charge pump. The source of the P-MOS transistor  3  is connected to the power-supply line and its gate terminal is connected to the charging current control circuit  1 . The source of the N-MOS transistor  4  is connected to the ground line and its gate terminal is connected to the discharge current control circuit  2 . The capacitor  5 , which is for charge and discharge, is connected in parallel with the N-MOS transistor  4  across the drain terminal of the charge pump and the ground line. The discrimination circuit  6  determines whether the terminal voltage of the capacitor  5  is equal to or greater than a predetermined threshold level and outputs a signal of a prescribed logic level in dependence upon the determination made. 
     The charging current control circuit  1  is composed of P-MOS transistors  10 ,  11 , and the discharge current control circuit  2  is constituted by an inverter circuit comprising a P-MOS transistor  201  and an N-MOS transistor  202 . The discrimination circuit  6  comprises a comparator  205  and a voltage source  206 . 
     The P-MOS transistor  10  of the charging current control circuit  1  acts as a switch and is turned off if the input signal is at the power-supply level and on if the input signal is at the ground level. The P-MOS transistor  11  and the P-MOS transistor  3  construct a current mirror circuit. Accordingly, if the input signal is at the power-supply level, the delay control current  7  flows through the P-MOS transistor  11  and not the P-MOS transistor  10 . As a result, a current proportional to the delay control current  7  flows into the P-MOS transistor  3  owing to the current mirror function and the P-MOS transistor  3  acts as a current source that introduces charging current. If the input signal is at the ground level, the delay control current  7  flows through the P-MOS transistor  10  and not the P-MOS transistor  11 . Owing to the current mirror function, therefore, the P-MOS transistor  3  turns off and no charging current flows. Accordingly, a charging current that prevails when the input signal rises from the ground level to the power-supply level can be controlled by the delay control current  7 , and the delay time of the rising edge can be controlled. 
     The output of the inverter circuit constituting the discharge current control circuit  2  is the ground level if the input signal is at the power-supply level and is at the power-supply level if the input signal is at the ground level. The N-MOS transistor  4 , therefore, acts as a switch and is turned off if the input signal is at the power-supply level. If the input signal is at the ground level, the N-MOS transistor  4  turns on, the capacitor  5  conducts to the ground line to discharge the accumulated charge, and the input voltage (capacitor terminal voltage) of the discrimination circuit  6  is made the ground level instantaneously. The discrimination circuit  6  outputs the power-supply level if the input voltage is equal to or greater than the threshold level Vth and outputs the ground level if the input signal is less than the threshold level Vth. 
     By virtue of the foregoing operation, the second embodiment is such that the falling-edge delay time is fixed (zero), as shown in FIG.  6 . It is possible to control only the rising-edge delay time τ H  of the input signal by the delay control current  7 , and the input and output are not inversely related. 
     (D) Third Embodiment of CMOS Delay Circuit 
     FIG. 7 is a diagram showing the construction of a third embodiment obtained by modifying the first embodiment so as to delay only the falling-edge timing of an input signal, and FIG. 8 is a waveform diagram useful in describing the operation of the third embodiment. 
     In FIG. 7, the charging current control circuit  1  performs control in such a manner that a charging current flows into the capacitor  5  when an input signal to the circuit is at the level of the power supply. The discharge current control circuit  2  performs control in such a manner that a discharge current is released from the capacitor  5  when the input signal is at the ground level. The P-MOS transistor  3  and the N-MOS transistor  4  have their drains connected in series to construct a CMOS charge pump. The source of the P-MOS transistor  3  is connected to the power-supply line and its gate terminal is connected to the charging current control circuit  1 . The source of the N-MOS transistor  4  is connected to the ground line and its gate terminal is connected to the discharge current control circuit  2 . The capacitor  5 , which is for charge and discharge, is connected in parallel with the N-MOS transistor  4  across the drain terminal of the charge pump and the ground line. The discrimination circuit  6  determines whether the terminal voltage of the capacitor  5  is equal to or greater than a predetermined threshold level and outputs a signal of a prescribed logic level in dependence upon the determination made. 
     The charging current control circuit  1  is constituted by an inverter circuit comprising a P-MOS transistor  203  and an N-MOS transistor  204 , and the discharge current control circuit  2  is composed of the N-MOS transistors  12 ,  13 . The discrimination circuit  6  is constituted by an inverter circuit comprising the P-MOS transistor  21  and the N-MOS transistor  22 , and the input capacitance of the discrimination circuit  6  serves as the capacitor  5 . 
     The output of the inverter circuit constituting the charging current control circuit  1  is the ground level if the input signal is at the power-supply level and is at the power-supply level if the input signal is at the ground level. The P-MOS transistor  3 , therefore, acts as a switch and is turned off if the input signal is at the ground level. If the input signal is at the power-supply level, the P-MOS transistor  3  turns on, the capacitor  5  becomes connected to the power-supply line and is charged, and the input voltage (capacitor terminal voltage) of the discrimination circuit  6  is made the power-supply level instantaneously. The discrimination circuit  6  outputs the power-supply level if the input voltage is the ground level and outputs the ground level if the input signal is the power-supply level. When the input signal rises from the ground level to the power-supply level, therefore, the output signal becomes the ground level instantaneously. 
     The N-MOS transistor  12  of the discharge current control circuit  2  acts as a switch and is turned off if the input signal is at the ground level and turned on if the input signal is at the power-supply level. The N-MOS transistor  13  and the N-MOS transistor  4  construct a current mirror circuit. Accordingly, if the input signal is at the ground level, the delay control current  8  flows through the N-MOS transistor  13  and not the N-MOS transistor  12 . As a result, a current proportional to the delay control current  8  flows into the N-MOS transistor  4  owing to the current mirror function and the N-MOS transistor  4  acts as a current source that passes a discharge current. If the input signal is at the power-supply level, the delay control current  8  flows through the N-MOS transistor  12  and not the M-MOS transistor  13 . Owing to the current mirror function, therefore, the N-MOS transistor  4  turns off and no discharge current flows. Accordingly, a discharge current that prevails when the input signal falls from the power-supply level to the ground level can be controlled by the delay control current  8 , and the delay time of the falling edge can be controlled. 
     By virtue of the foregoing operation, the third embodiment is such that the rising-edge delay time is fixed (zero), as shown in FIG.  8 . It is possible to control only the falling-edge delay time τ T  of the input signal by the delay control current  8 , and the input and output are not inversely related. It should be noted that the CMOS delay circuit of the third embodiment is capable of being used as a pulse-width control circuit as well. 
     (E) Fourth Embodiment of CMOS Delay Circuit 
     FIG. 9 is a diagram showing the construction of a fourth embodiment, and FIG. 10 is a waveform diagram useful in describing the operation of the fourth embodiment. 
     In FIG. 9, the charging current control circuit  1  performs control in such a manner that a charging current flows into the capacitor  5  when an input signal to the circuit is at the ground level. The discharge current control circuit  2  performs control in such a manner that a discharge current is released from the capacitor  5  when the input signal is at the power-supply level. The P-MOS transistor  3  and the N-MOS transistor  4  have their drains connected in series to construct a CMOS charge pump. The source of the P-MOS transistor  3  is connected to the power-supply line and its gate terminal is connected to the charging current control circuit  1 . The source of the N-MOS transistor  4  is connected to the ground line and its gate terminal is connected to the discharge current control circuit  2 . The capacitor  5 , which is for charge and discharge, is connected in parallel with the N-MOS transistor  4  across the drain terminal of the charge pump and the ground line. The discrimination circuit  6  determines whether the terminal voltage of the capacitor  5  is equal to or greater than a predetermined threshold level and outputs a signal of a prescribed logic level in dependence upon the determination made. The delay control currents  7  and  8  flow through the charging current control circuit  1  and discharge current control circuit  2 , respectively, under the control of a current control circuit (see FIG.  4 ), which is not shown in FIG.  9 . 
     The charging current control circuit  1  is composed of P-MOS transistors  211 ,  212  and an N-MOS transistor  213  and is so adapted that the delay control current  7  flows into the P-MOS transistor  211  at all times. The discharge current control circuit  2  is composed of N-MOS transistors  214 ,  215  and a P-MOS transistor  216  and is so adapted that the delay control current  8  flows into the N-MOS transistor  214  at all times. 
     When the input signal to the charging current control circuit  1  is at the power-supply level, the P-MOS transistor  212  turns off and the N-MOS transistor  213  turns on. As a result, the output of the charging current control circuit I attains the power-supply level, the P-MOS transistor  3  turns off and no charging current flows. When the input signal is at the ground level, on the other hand, the P-MOS transistor  212  turns on and the N-MOS transistor  213  turns off. Consequently, the P-MOS transistor  211  and the P-MOS transistor  3  construct a current mirror circuit and the P-MOS transistor  3  acts as a charging current source for passing a current that is proportional to the delay control current  7 . 
     Similarly, when the input signal is at the power-supply level, the N-MOS transistor  4  acts as a discharge current source for passing a current that is proportional to the delay control current  8  under the control of the discharge current control circuit  2 . More specifically, when the input signal to the discharge current control circuit  2  is at the ground level, the N-MOS transistor  215  turns off and the P-MOS transistor  216  turns on. As a result, the output of the discharge current control circuit  2  attains the ground level, the N-MOS transistor  4  turns off and no discharge current flows. When the input signal is at the power-supply level, on the other hand, the N-MOS transistor  215  turns on and the P-MOS transistor  216  turns off. Consequently, the N-MOS transistor  214  and the N-MOS transistor  4  construct a current mirror circuit and the N-MOS transistor  4  acts as a discharge current source for passing a current that is proportional to the delay control current  8 . 
     By virtue of the foregoing operation, the input voltage of the discrimination circuit  6  can be controlled so as to vary at a constant slope from the power-supply level to the ground level and from the ground level to the power-supply level, as shown in FIG.  10 . This means that it is possible to obtain a CMOS delay circuit having a broad range τ over which delay time can be varied and a small variance in terms of delay time. Further, it is possible to obtain a CMOS delay circuit in which delay time can be controlled while the rising-edge and falling-edge delay times τ H , τ T  kept equal, as a result of which there is no change in the input and output pulse widths. 
     (F) Fifth Embodiment of CMOS Delay Circuit 
     FIG. 11 is a diagram showing the construction of a fifth embodiment obtained by modifying the fourth embodiment so as to delay only the rising-edge timing of an input signal, and FIG. 12 is a waveform diagram useful in describing the operation of the fifth embodiment. 
     In the fifth embodiment, the discharge current control circuit  2  of the fourth embodiment is eliminated and the signal input terminal is connected directly to the N-MOS transistor  4 . The N-MOS transistor  4  acts as a switch for turning a discharge current on and off. The N-MOS transistor  4  is turned on when the input signal is at the ground level and is turned off when the input signal is at the power-supply level, thereby connecting the capacitor  5  to the ground line to discharge the accumulated electric charge and send the input voltage (the terminal voltage of capacitor  5 ) of the discrimination circuit  6  to the ground level instantaneously. The discrimination circuit  6  outputs the ground level if the input voltage is equal to or greater than the threshold level Vth and outputs the power-supply level if the input signal is less than the threshold level Vth. 
     By virtue of the foregoing operation, the fifth embodiment is such that the falling-edge delay time is fixed (zero), as shown in FIG.  12 . It is possible to control only the rising-edge delay time τ T  of the input signal by the delay control current  7 , and the input and output are not inversely related. 
     It should be noted that the charging current control circuit  1  of the fourth embodiment (FIG. 9) may be eliminated and the signal input terminal connected directly to the P-MOS transistor  3  in a manner similar to that described above, thereby making it possible to construct a CMOS delay circuit in which only the rising edge of the input signal is delayed. 
     (G) Sixth Embodiment of CMOS Delay Circuit 
     FIG. 13 is a diagram showing a sixth embodiment of a CMOS delay circuit for making the pulse width of the input signal and the pulse width of the output signal equal, and FIG. 14 is a waveform diagram useful in describing the operation of the sixth embodiment. 
     CMOS delay circuits (see FIG. 7)  30 ,  31  of the third embodiment are serially connected and, as shown in FIG. 8, each delays only the falling edge of its input signal and has an input and an output that are in a mutually inverse relationship. A current control circuit  32  is adapted to apply a delay control voltage  233  to the gates of P-MOS transistors  231 ,  232  of identical characteristics so that equal currents flow into these transistors. The currents that flow through these P-MOS transistors  231 ,  232  flow into the CMOS delay circuits  30 ,  31  as delay control currents  8 . Since the delay control currents  8  that flow into the CMOS delay circuits  30 ,  31  are equal, the discharge currents also are equal and so are the falling-edge delay times τ T  of the two delay circuits. 
     As shown in FIG. 14, the rising edge of the input signal is not delayed in the CMOS delay circuit  30  and is delayed by τ T  in the CMOS delay circuit  31 . On the other hand, the falling edge of the input signal is delayed by τ T  in the CMOS delay circuit  30  and is not delayed in the CMOS delay circuit  31 . Overall, therefore, the input signal has both its rising and falling edges delayed by τ T . Thus there is obtained a CMOS delay circuit that applies a time delay τ T  without any change in the pulse width of the input and output signals. 
     (H) Seventh Embodiment of CMOS Delay Circuit 
     FIG. 15 is a diagram showing a seventh embodiment of a CMOS delay circuit for making the pulse width of the input signal and the pulse width of the output signal equal, and FIG. 16 is a waveform diagram useful in describing the operation of the seventh embodiment. 
     CMOS delay circuits (see FIG. 5)  33 ,  35  delay only the rising edge of the input signal thereto and have an input and an output that are not in a mutually inverse relationship, as illustrated in FIG.  6 . Inverters  34 ,  36  invert the signal level. The CMOS delay circuits  33 ,  34  and the inverters  34 ,  36  are serially connected as shown. A current control circuit  37  has a construction identical with that of the current control circuit  32  according to the sixth embodiment (FIG. 13) and is so adapted that equal delay control currents  7  flow into the CMOS delay circuits  33 ,  35 . Since the delay control currents  7  that flow into these CMOS delay circuits  33 ,  35  are equal, the charging currents are equal and so are the rising-edge delay times τ H  of the two delay circuits. 
     The CMOS delay circuit  33  and inverter  34  correspond to the CMOS delay circuit  30  of the sixth embodiment, the CMOS delay circuit  35  and inverter  36  correspond to the CMOS delay circuit  31  of the sixth embodiment, and these operate in the same manner as in the sixth embodiment to output a signal of delay time τ H , the output signal having a pulse width no different from that of the input signal. 
     As shown in FIG. 16, the rising edge of the input signal is delayed by τ H  only in the CMOS delay circuit  33 , is inverted by the inverter  34 , is not delayed by the CMOS delay circuit  35  and is inverted by the inverter  36 . On the other hand, the falling edge of the input signal is not delayed by the CMOS delay circuit  33 , is inverted by the inverter  34 , is delayed by τ H  in the CMOS delay circuit  35  and is inverted by the inverter  36 . Overall, therefore, the input signal has both its rising and falling edges delayed by τ H . Consequently, it is possible to output a signal having a time delay τ H , the output signal having a pulse width no different from that of the input signal. 
     (I) Eighth Embodiment of CMOS Delay Circuit 
     FIG. 17 is a diagram showing an eighth embodiment of a CMOS delay circuit for making the pulse width of the input signal and the pulse width of the output signal equal. 
     CMOS delay circuits (see FIG. 5)  39 ,  41  delay only the rising edge of the input signal thereto and have an input and an output that are not in a mutually inverse relationship, as illustrated in FIG.  6 . Inverters  38 ,  39  invert the signal level. The CMOS delay circuits  39 ,  41  and the inverters  38 ,  40  are serially connected as shown. A current control circuit  42  has a construction identical with that of the current control circuit  32  according to the sixth embodiment (FIG. 13) and is so adapted that equal delay control currents  7  flow into the CMOS delay circuits  39 ,  41 . Since the delay control currents  7  that flow into these CMOS delay circuits  39 ,  41  are equal, the charging currents are equal and so are the rising-edge delay times τ H  of the two delay circuits. 
     The seventh and eighth embodiments differ only in terms of whether the inversion is performed before or after the delay in each CMOS delay circuit. Accordingly, in a manner similar to that of the seventh embodiment, the eighth embodiment makes it possible to output a signal having a time delay τ H , the output signal having a pulse width no different from that of the input signal. 
     (J) First Embodiment of Oscillation Circuit Using CMOS Delay Circuits 
     FIG. 18 is a diagram showing the construction of a first embodiment of an oscillator circuit using CMOS delay circuits. The oscillator circuit is constructed by connecting serially, in the form of a ring, an odd number of CMOS delay circuits whose input and output signal levels are the inverse of each other. 
     As shown in FIG. 18, an oscillator circuit  53  is constructed by connecting serially, in the form of a ring, an odd number of CMOS delay circuits  241 - 245  (see the first embodiment in FIG. 2) whose input and output signal levels are the inverse of each other. A current control circuit  55  has P-MOS transistors  251 - 255  for passing delay control currents  14  into the CMOS delay circuits  241 - 245 , respectively. The current control circuit  55  is capable of controlling, by a control voltage  54 , the value of the delay control currents  14  that flow into the CMOS delay circuits  241 - 245 . As a result, oscillation frequency can be changed by changing delay time. 
     FIG. 19 is a diagram useful in describing the operation of each CMOS delay circuit. The characteristics of the CMOS delay circuits are such that the delay time is τ, with the output and input of each CMOS delay circuit being in inverse relationship. It should be noted that the input waveform of the discrimination circuit  6  is the inverse of that shown in FIG.  3 . FIG. 20 shows the input and output waveforms of the CMOS delay circuits  241 - 245 , with the output of the last CMOS delay circuit  245  coinciding with the input of the first CMOS delay circuit  241 . 
     The oscillation frequency of the oscillator circuit thus constructed is as follows: An output current (delay control current) Icnt output by the current control circuit  55  is varied by a control voltage Vg s  in accordance with the following equation: 
     
       
           Icnt= (½)·β·( Vg   s   −Vth ) 2   (1) 
       
     
     where β represents a drain current coefficient and Vth represents a threshold voltage value. 
     In a case where the construction of the first embodiment is adopted for the CMOS delay circuits  241 - 245 , the delay time τ per delay circuit stage is given by the following equation if the threshold value of the discrimination circuit  6  is Vd/2: 
     
       
         τ=( C   0   ·Vd )/(2 ·I   charge )  (2) 
       
     
     where C 0  represents the capacitance of the capacitor  5 , Vd the power-supply voltage and I charge  the value of the charge/discharge current, which is proportional to Icnt. Assuming that I charge =Icnt holds for the sake of simplicity, we have 
      τ=( C   0   ·Vd )/(2 ·Icnt )  (3) 
     If n represents the number of CMOS delay circuit stages, the following equation is established: 
     
       
           T/ 2= n·τ   (4) 
       
     
     where T represents the oscillation period. The frequency f, therefore, is given by 
     
       
           f =1/(2· n·τ )  (5) 
       
     
     Accordingly, we derive the following from Equations (1), (3) and (5): 
     
       
           f =(½)·β·( Vg   s   −Vth ) 2 /( n·C   0   ·Vd )  (6) 
       
     
     Thus, the oscillation frequency f can be controlled by the control voltage Vg s . Further, since the CMOS delay circuits  241 - 245  have a broad range over which delay time can be varied, the range over which the oscillation frequency is variable can be enlarged. 
     (K) Second Embodiment of Oscillation Circuit Using CMOS Delay Circuits 
     FIG. 21 is a diagram showing the construction of a second embodiment of an oscillator circuit using CMOS delay circuits. The oscillator circuit is constructed by connecting serially, in the form of a ring, an even number of CMOS delay circuits whose input and output signal levels are not the inverse of each other, and a single signal-level inverter circuit. 
     As shown in FIG. 21, an oscillator circuit  53  is constructed by connecting serially, in the form of a ring, two CMOS delay circuits  261 ,  262  (see the fourth embodiment in FIG. 9) whose input and output signal levels are not the inverse of each other, and one signal-level inverter circuit  51 . The current control circuit  55  has a construction identical with that of the current control circuit shown in FIG.  18  and passes the delay control currents  14  into the CMOS delay circuits  261 ,  262 . The current control circuit  55  is capable of controlling, by the control voltage  54 , the value of the delay control currents  14  that flow into the CMOS delay circuits  261 ,  262 . As a result, oscillation frequency can be changed by changing the delay time of the CMOS delay circuits. The oscillation frequency f is given by Equation (6) just as in the first embodiment. Since the CMOS delay circuits  261 ,  262  have a broad range over which delay time can be varied, the range over which the oscillation frequency is variable can be enlarged. 
     Though the embodiments have been described in regard to use of MOS transistors, the present invention is not limited to MOS transistors but can be applied to other field-effect transistors such as MIS transistors. 
     Thus, in accordance with the present invention, there is no parasitic capacitance that appears in conjunction with the capacitor of the discrimination circuit when the input signal changes over between the rising and falling states. When the input signal changes over, therefore, the input voltage to the discrimination circuit does not change instantaneously to an intermediate potential and the input voltage of the discrimination circuit can be controlled at a fixed slope from the ground level to the power-supply level or from the power-supply level to the ground level. As a result, according to the present invention, the range over which the delay time is variable can be enlarged and a variance in delay time can be reduced. 
     Further, in accordance with the present invention, it is possible to provide various delay circuits such as a delay circuit in which there is no change in the pulse width of the input and output signals, a delay circuit for delaying only the rising edge of the input signal, a delay circuit for delaying only the falling edge of the input signal, a delay circuit in which input and output levels are the inverse of each other and a delay circuit in which the input and output levels are not the inverse of each other. As a result, the range over which the delay time is variable can be enlarged and a variance in delay time can be reduced. 
     Further, in accordance with the present invention, an oscillator circuit is constructed by connecting serially, in the form of a ring, an odd number of delay circuits whose input and output signal levels are the inverse of each other, or by connecting serially, in the form of a ring, an even number of delay circuits whose input and output signal levels are not the inverse of each other and one signal-level inverting circuit. If such an arrangement is adopted, the oscillation frequency can be controlled by varying the delay times of the delay circuits through control of voltage. This makes it possible to realize a voltage-controlled oscillator (VCO). Further, since the delay times of the delay circuits constituting the oscillator circuit can be varied over a broad range, the range over which the oscillation frequency of the VCO is variable can be enlarged. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.