Abstract:
A ring oscillator having a stable output signal without influence of MOS devices is disclosed. The ring-oscillator has a bias circuit to drive a plurality of delay cells. The bias circuit has a first loading unit with a p-n junction, a second loading unit with a p-n junction, and a resistor electrically connected to the p-n junction of the second loading unit. The second loading unit and the resistor are positioned at a current path of a current mirror, and the first loading unit is positioned at another current path of the current mirror. The area of the p-n junction in the second loading unit is not equal to the area of the p-n junction in the first loading unit. The magnitudes of the current passing on the two current paths are only controlled by characteristics of the p-n junctions of the first and second loading units.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to a ring oscillator. In particular, the present invention discloses a ring oscillator whose output signal is not affected by the characteristics of MOS devices. 
     2. Description of the Prior Art 
     In modern information society, a digital system capable of processing digital data needs a clock signal to arbitrate and coordinate timings associated with processing and transmission of the digital data. Therefore, an oscillator used to generate the required clock signal becomes a fundamental component in a modern digital circuit. In addition, a phase lock loop (PLL) of signal processing circuits used in a general communication system, an optical disk drive, and a hard-disk drive commonly applies a voltage-controlled oscillator (VCO). That is, voltages are used to control a voltage-controlled oscillator to make the clock signal have a specific period or a specific frequency. With the improvement of transmission and processing speeds for digital signals, it becomes an important issue to manufacture oscillators that are capable of generating high-frequency (short period) clock signals. 
     Please refer to FIG. 1, which is a diagram of a prior art ring oscillator  10 . The ring oscillator  10  has a plurality of delay cells cascaded to form a closed loop. Please note that only three delay cells  12   a ,  12   b ,  12   c  are shown in FIG. 1 for simplicity. Operation of the delay cells  12   a ,  12   b ,  12   c  are similar to that of an inverter, and functionality of the delay cells  12   a ,  12   b ,  12   c  are to make input ports IP 1 , IP 2 , IP 3  and related output ports OP 1 , OP 2 , OP 3  correspond to opposite voltage levels. For example, when the input port IP 1  corresponds to a high voltage level, the corresponding output port OP 1  corresponds to a low voltage level; on the other hand, when the input port IP 1  corresponds to the low voltage level, the corresponding output port OP 1  corresponds to the high voltage level. 
     Operation of the ring oscillator  10  is described as follows. When the input port IP 1  of the delay cell  12   a  corresponds to the high voltage level, the output port OP 1  of the delay cell  12   a  corresponds to the low voltage level. Because the output port OP 1  of the delay cell  12   a  is electrically connected to input port IP 2  of the following delay cell  12   b , the input port IP 2  of the delay cell  12   b , therefore, corresponds to the low voltage level. The output port OP 2  of the delay cell  12   b  then corresponds to the high voltage level. Similarly, because output port OP 2  of the delay cell  12   b  is electrically connected to input port IP 3  of the following delay cell  12   c , the input port IP 3  of the delay cell  12   c  corresponds to the high voltage level for driving output port OP 3  of the delay cell  12   c  to correspond to the low voltage level. 
     It is noteworthy that output port OP 3  of the delay cell  12   c  is electrically connected to the input port IP 1  of the delay cell  12   a , and the input port IP 1  initially corresponds to the high voltage level. However, the closed loop formed by the delay cells  12   a ,  12   b ,  12   c  forces the input port IP 1  of the delay cell  12   a  to correspond to the low voltage level after the delay cells  12   a ,  12   b ,  12   c  sequentially operate. After input port of each delay cell  12   a ,  12   b ,  12   c  receives an input signal, the corresponding delay cell requires a delay time Td to generate an output signal having a voltage level opposite to that of the input signal. Therefore, voltage level at the output port and input port of each delay cell  12   a ,  12   b ,  12   c  has a level transition every three delay time 3*Td. 
     The level transition means that the voltage level transits from the original high voltage level to the low voltage level or the voltage level transits from the original low voltage level to the high voltage level. In other words, the period of the clock signal F 0  generated from the ring oscillator  10  becomes 6*Td. In addition, the control voltage Vc is used to adjust the delay time of each delay cell  12   a ,  12   b ,  12   c . Therefore, period of the clock signal F 0  is controllable with the adjustment of the voltage value of the control voltage Vc. 
     Please refer to FIG. 2, which is a diagram of the delay cell  12   a  shown in FIG.  1 . The delay cell  12   a  includes a plurality of p-channel metal oxide semiconductor (PMOS) transistors  14   a ,  14   b , and a plurality of n-channel metal oxide semiconductor (NMOS) transistors  16   a ,  16   b . The transistors  14   a ,  14   b ,  16   a ,  16   b  are fabricated according to a CMOS semiconductor process. The transistors  14   a ,  16   a  are matched to correspond to the same transistor characteristics such as an identical doping concentration, an identical channel width/length ratio, etc. In addition, the transistors  14   b ,  16   b  are matched as well. 
     The transistors  14   a ,  16   a  function as current sources. That is, the transistors  14   a ,  16   a  operate in a saturation region. Therefore, when a control voltage V c1  is inputted into a gate of the transistor  14   a , a fixed reference current I 1  flowing from a voltage source Vdd (high voltage level) toward the transistor  14   b  is generated. Similarly, when a control voltage V c2  is inputted into a gate of the transistor  16   a , a fixed reference current I 2  flowing from the transistor  16   b  toward a voltage source Vss (low voltage level) is generated. 
     Operation of the delay cell  12   a  is briefly described as follows. If the input port IP 1  corresponds to a high voltage level, the transistor  16   b  is turned on, and the transistor  14   b  is turned off. Therefore, the reference current I 2  starts discharging the output port OP 1  to make the output port OP 1  correspond to the low voltage level. 
     As mentioned above, when the delay cell  12   c  operates, the level transition occurs at the input port IP 1  of the delay cell  12   a . Therefore, after the voltage level of the input port IP 1  transits from the high voltage level to the low voltage level, the transistor  14   b  is turned on, and the transistor  16   b  is then turned off. The reference current I 1 , therefore, begins charging the output port OP 1  to make the output port OP 1  correspond to the high voltage level. In other words, the transistors  14   b ,  16   b  function as switches used to determine that the output port OP 1  needs to be charged or discharged according to the voltage level of the input port IP 1 . Then, the voltage levels of the output port OP 1  and the input port IP 1  correspond to opposite voltage levels. 
     In addition, the magnitudes of the reference currents I 1 , I 2  affect the delay time Td of the delay cell  12   a . If the reference current I 1  is increased, the reference current I 1  raises the voltage level of the output port OP 1  much quicker. Similarly, if the reference current I 2  is increased, the reference current I 2  decreases voltage level of the output port OP 1  much quicker. Please note that the magnitudes of the reference currents I 1 , I 2  are dominated by the control voltages V c1 , V c2 . As shown in FIG. 1, the control voltage Vc generating the control voltages V c1 , V c2  is then capable of adjusting the delay time Td to alter period of the clock signal F 0 . 
     However, with regard to the transistors  14   a ,  16   a , the transistor characteristics varies with the operating temperature, voltage sources Vdd, Vss, etc. For instance, when the operating temperature increases, mobility of electrons in the transistors  14   a ,  16   a  is suppressed. Therefore, under the same gate-to-source bias, current values of the reference currents I 1 , I 2  decrease owing to an increase of the operating temperature. On the other hand, when the operating temperature decreases, mobility of the electrons in the transistors  14   a ,  16   a  is improved. Therefore, under the same gate-to-source bias, current values of the reference currents I 1 , I 2  increase owing to a decrease of the operating temperature. In other words, if the operating temperature varies randomly, the period of the clock signal F 0  accordingly becomes unstable. That is, the frequency of the clock signal F 0  deviates from an ideal target value. Besides, the unstable voltages provided by the voltage sources Vdd, Vss alter gate-to-source biases of the transistors  14   a ,  16   a  so that the magnitudes of the reference currents I 1 , I 2  varies. Similarly, the period of the clock signal F 0  varies accordingly and deviates from the ideal target value. To sum up, transistor characteristics of the transistors influence the frequency of the clock signal F 0  so that the clock signal F 0  becomes unstable. 
     SUMMARY OF INVENTION 
     It is therefore a primary objective of this invention to provide a ring oscillator whose output signal is not affected by transistor characteristics of MOS devices. 
     Briefly summarized, the preferred embodiment of the claimed invention discloses a bias circuit of a ring oscillator for driving the ring oscillator to output a clock signal with a predetermined period. The ring oscillator has a plurality of delay cells. Each two adjacent delay cells are cascaded in series with an output port of a leading delay cell and an input port of a following delay cell being connected. An input port of a first delay cell within the delay cells is electrically connected to an output port of a last delay cell within the delay cells. 
     Each delay cell includes a first driving transistor for outputting a first bias current to drive a voltage at an output port of the delay cell to correspond to a first logic level, a second driving transistor for outputting a second bias current to drive the voltage at the output port of the delay cell to correspond to a second logic level, and a switch circuit electrically connected to the first driving transistor and the second driving transistor for deciding whether the output port of the delay cell is electrically connected to either the first driving transistor or the second driving transistor according to a voltage at an input port of the delay cell. 
     The bias circuit has a first loading unit including a p-n junction and a second loading unit including a p-n junction where the area of the p-n junction of the second loading unit is not equal to the area of the p-n junction of the first loading unit, a first reference circuit, a resistor electrically connected between the p-n junction of the second loading unit and a second current mirror circuit, and a second reference circuit electrically connected to the first reference circuit for establishing a current mirror connection used to make a current transmitted by the second reference circuit and the currents transmitted by the first current mirror and the second current mirror circuit correspond to a first predetermined ratio. 
     The first reference circuit has the first current mirror circuit electrically connected to the p-n junction of the first loading unit, and the second current mirror circuit symmetric to the first current mirror circuit. The second current mirror is electrically connected to the first current mirror circuit for establishing a current mirror connection used to make the first current mirror circuit and the second current mirror circuit respectively transmit currents with an identical current value to drive the corresponding first loading unit and the second loading unit. The first reference circuit is electrically connected to the first driving transistor of each delay cell for establishing a current mirror used to make the first bias current and the currents transmitted by the first current mirror and the second current mirror circuit correspond to a second predetermined ratio. The second reference circuit is electrically connected to the second driving transistor of each delay cell for establishing a current mirror used to make the second bias current and the currents transmitted by the first current mirror and the second current mirror circuit correspond to a third predetermined ratio. 
     It is an advantage of the present invention that the claimed ring oscillator makes use of the intrinsic band gap associated with a p-n junction to control a bias current. The claimed ring oscillator adopts either two diodes or two BJTs whose p-n junctions correspond to different areas, and then generates the bias current through different voltage difference caused by the unmatched p-n junctions. In addition, a resistor having a positive temperature coefficient is also utilized to compensate variation of the bias current caused by an unsteady temperature. The claimed ring oscillator then utilizes the bias current and established current mirror connection to provide each delay cell with a desired reference current. Therefore, the reference current is fixed even though transistor characteristics of any MOS transistor within the claimed ring oscillator is unsteady. In other words, the claimed ring oscillator is capable of outputting a stable clock signal. 
     These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a diagram of a prior art ring oscillator. 
     FIG. 2 is a diagram of a delay cell shown in FIG.  1 . 
     FIG. 3 is a circuit diagram of a ring oscillator according to the present invention. 
     FIG. 4 is a diagram illustrating a first equivalent circuit of a bias circuit shown in FIG.  3 . 
     FIG. 5 is a diagram illustrating a second equivalent circuit of the bias circuit shown in FIG.  3 . 
     FIG. 6 is a diagram illustrating a third equivalent circuit of the bias circuit shown in FIG.  3 . 
     FIG. 7 is a diagram illustrating a fourth equivalent circuit of the bias circuit shown in FIG.  3 . 
     FIG. 8 is a diagram illustrating a fifth equivalent circuit of the bias circuit shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     Please refer to FIG. 3, which is a circuit diagram of a ring oscillator  30  according to the present invention. The ring oscillator  30  has a bias circuit  32  and a plurality of delay cells  34 . The delay cells  34  are cascaded to form a closed loop. The delay cell  34  includes a plurality of transistors  36   a ,  36   b ,  38   a ,  38   b ,  40 . The transistors  36   a ,  36   b  are PMOS transistors and the transistors  38   a ,  38   b ,  40  are NMOS transistors. A source and drain of the transistor  40  are connected together. Because an oxide layer is positioned between the gate and the source and is positioned between the gate and the drain, the functionality of the transistor  40 , therefore, is equivalent to a capacitor. 
     The bias circuit  32  has a plurality of transistors  42   a ,  42   b ,  44   a ,  44   b ,  46 ,  48 , two bipolar junction transistors (BJTs)  50 ,  52 , and a resistor  54 . In the preferred embodiment, all of the transistors  42   a ,  42   b ,  44   a ,  44   b ,  46 ,  48  operate in the saturation region. The transistors  42   a ,  42   b  are matched to correspond to identical transistor characteristics, and the transistors  44   a ,  44   b  are matched as well to correspond to identical transistors. The gate of the transistor  44   a  is electrically connected to the drain of the transistor  44   a  and the gate of the transistor  44   b . Therefore, a current mirror is established. Similarly, the transistors  42   a ,  42   b ,  46  are connected according to a current mirror connection. In addition, each delay cell  34  and the bias circuit are connected according to the current mirror connections as well. For example, the connection between the transistors  36   a ,  42   b  and the connection between the transistors  38   b ,  48  establish well-known current mirrors. 
     Operation of the bias circuit  32  is described as follows. In the preferred embodiment, BJTs  50 ,  52  are p-n-p BJTs. A p-n junction between an emitter and a base of the BJT  50  and a p-n junction between an emitter and a base of the BJT  52  correspond to different areas. Therefore, a voltage difference V be  between the emitter and the base of the BJT  50  is different from a voltage difference V be  between the emitter and the base of the BJT  52  because the area of the p-n junction associated with the emitter and base of the BJT  50  is less than the area of the p-n junction associated with emitter and base of the BJT  52 . As a result, when both BJTs  50 ,  52  are enabled to conduct currents with the same magnitude, a voltage difference V be  of the BJT  50  is greater than a voltage difference V be  of the BJT  52 . 
     The transistors  44   a ,  44   b  are matched, and are connected according to the current mirror connection. In addition, the transistors  44   a ,  44   b  operate in the saturation region. Therefore, based on an equation (1) represented by 
     
       
           Id=Kn ( Vgs−Vn)   2   Equation (1)  
       
     
     where Id is a conducting current, Kn is a conduction parameter, Vgs is a voltage difference between the gate and the source, and Vt is a threshold voltage, Vgs (44a)  is equal to Vgs (44b)  when the reference current I 0  equals the reference current I 1 . Because the gate of the transistor  44   a  is electrically connected to the gate of the transistor  44   b , Vg (44a)  is equal to Vg (44b) . In other words, voltages associated with the sources of the transistors  44   a ,  44   b  are equal. That is, Vs (44a)  is equal to Vs (44b) . 
     Collectors of the BJTs  50 ,  52  are simultaneously connected to voltage source Vss (low voltage level). Therefore, a voltage difference Vbe (50)  is equal to the summation of the voltage difference Vbe (52)  of the transistor  52  and a voltage difference between two terminals of the resistor  54  whose resistance equals R. Consequently, an equation (2) is represented by the following. 
     
       
           Vbe   (50)   =Vbe   (52)   +I*R   Equation (2)  
       
     
     It is well-known that voltage difference Vbe of the prior art BJT is equal to Vt*ln(I/Is). The Is is saturation current that is an intrinsic characteristic of the BJT, and is proportional to the area of the p-n junction associated with an emitter and a base of the BJT. The Vt is a thermal voltage generally represented by            k   *   T     q     ,                          
     wherein k is a Boltzmann&#39;s constant, T is an absolute temperature, and q is the coulomb of an electron. Therefore, the thermal voltage Vt changes according to variation of the temperature. The equation (2) is fiber represented by an equation (3). 
     
       
           Vt* ln( I/Is   1 )= Vt *ln( I/Is   2 )+ I*R   Equation (3)  
       
     
     The Is 1  and Is 2  respectively are saturation currents of the BJTs  50 ,  52 . The thermal voltage Vt is capable of being represented by            k   *   T     q     ,                          
     and the resistance of the resistor  54  is capable of being represented by R*(1+Tc*T) wherein Tc is a temperature coefficient of the resistor  54 . 
     It is obvious that if Tc is a positive value, the resistor  54  corresponding to a positive temperature coefficient increases its resistance when the operating temperature of the resistor  54  increases. Similarly, if Tc is a negative value, the resistor  54  corresponding to a negative temperature coefficient decreases its resistance when the operating temperature of the resistor  54  increases. The equation (3) is further rearranged to produce an equation (4).              I   =       [       (       k   *   T     q     )     *     ln        (     Is2   Is1     )         ]       R        (     1   +     Tc   *   T       )                 Equation                   (   4   )                                  
     It is obviously shown in equation (4) that a current value I of each reference current I 0 , I 1  is affected by the temperature T. In the preferred embodiment, the resistor  54  with a positive temperature coefficient is adopted, and is capable of compensating deviation of the thermal voltage Vt influenced by temperature variation. For example, when the temperature T increases, the thermal voltage Vt accordingly increases. At the same time, resistance of the resistor  54  is increased owing to an increase in the temperature. An actual variation of the current value I is alleviated because of compensation introduced by the resistor  54 . Similarly, when the temperature T decreases, the thermal voltage Vt accordingly decreases. At the same time, resistance of the resistor  54  is decreased owing to the decrease in the temperature. The actual variation of the current value I is also alleviated because of compensation introduced by the resistor  54 . To sum up, the preferred embodiment is capable of outputting a stable current value I. 
     Because the transistor  46  and the transistor  42   b  is electrically connected according to the current mirror connection, the reference current I 1  and the reference current I 2  correspond to a predetermined ratio. For instance, if the transistors  46 ,  42   b  are matched to have the same transistor characteristics, the magnitude of the reference current I 2  is identical to the magnitude of the reference current I 1 . On the other hand, if the channel width/length ratio of the transistor  46  is a double of the channel width/length ratio of the transistor  42   b , the magnitude of the reference current I 2  is twice as great as the magnitude of the reference current I 1 . Besides, the transistor  38   b  of the delay cell  34  and the transistor  48  of the bias circuit  32  are also electrically connected according to the current mirror connection. As mentioned above, the transistor characteristics of both of the transistors  38   b ,  48  determine a ratio relation between the magnitudes of two reference currents I 2 , I 4 . Similarly, the transistor  36   a  of the delay cell  34  and the transistor  42   b  of the bias circuit  32  are also electrically connected according to the current mirror connection. 
     As mentioned above, transistor characteristics of both of the transistors  36   a ,  42   b  determine a ratio relationship between the magnitudes of the two reference currents I 1 , I 3 . Because the transistors  36   a ,  38   b  of the delay cell  34  operate in the saturation region, the transistors  36   a ,  38   b  are capable of functioning as current sources to respectively provide the reference currents I 3 , I 4 . As described above, the magnitude of the reference current I 3  is controlled by the magnitude of the reference current I 1 . In addition, the magnitude of the reference current I 4  is controlled by the magnitude of the reference current I 2 , and the magnitude of the reference current I 2  is further controlled by the magnitude of the reference current I 1 . 
     In other words, a current value I of each reference current I 1 , I 0  dominates the magnitudes of the reference currents I 3 , I 4 . As shown in the equation (4), the current value I merely changes according to the operating temperature, and the current value I is not influenced by an input/output voltage variation or an intrinsic transistor characteristic of each MOS transistors used in the ring oscillator  30 . The current value I is stabilized owing to compensation caused by the resistor  54 . The reference currents I 3 , I 4  are fixed even though the transistors  36   a ,  38   b  in the delay cell  34  may have variations on the transistor characteristics such as an increase or a decrease of the electron mobility. Besides, the reference currents I 3 , I 4  are also fixed even though voltage levels respectively maintained by the voltage sources Vdd, Vss of the ring oscillator  30  become unstable. 
     When the input port IP 1  corresponds to the high voltage level, the transistor  38   a  is switched on. Therefore, charges accumulated at the gate of the transistor  40  are driven to the voltage source Vss (low voltage level) through the reference current I 4 . The output port OP 1  then corresponds to the low voltage level. On the other hand, when the input port IP 1  corresponds to the low voltage level, the transistor  38   b  is switched on. Therefore, the gate of the transistor  40  is charged by the voltage source Vdd (high voltage level) through the reference current I 3  to accumulate charges. The output port OP 1  then corresponds to the high voltage level. It is noteworthy that magnitude of the reference currents I 3 , I 4  is not influenced by transistor characteristics of the MOS transistors. Therefore, the delay time corresponding to each delay cell  34  is a fixed value so that the clock signal F 0  generated from the ring oscillator  30  steadily corresponds to a predetermined frequency. 
     Please refer to FIG.  4 ˜FIG.  8 . FIG.  4 ˜FIG. 8 are diagrams illustrating equivalent circuits of the bias circuit  32  shown in FIG. 3. A bias circuit  60  shown in FIG. 4 is similar to the bias circuit  32  shown in FIG.  3 . The only difference is that the bias circuit  60  adopts n-p-n BJTs  62 ,  64  to substitute p-n-p BJTs  50 ,  52  originally used in the bias circuit  32 . The area associated with a p-n junction between a base and an emitter of the BJT  62  is not equal to the area associated with a p-n junction between a base and an emitter of the BJT  64 . Therefore, the BJTs  62 ,  64  correspond to different voltage differences between the base and the emitter of each BJT  62 ,  64  when conducting currents with the same magnitude. Operation of the bias circuit  60  is identical to that of the bias circuit  32 , and the lengthy description is not repeated for simplicity. 
     A bias circuit  70  shown in FIG. 5 is similar to the bias circuit shown in FIG.  3 . The only difference is that the bias circuit  70  adopts diodes  72 ,  74  to substitute for the p-n-p BJTs  50 ,  52  originally used in the bias circuit  32 . The area associated with a p-n junction of the diode  72  is not equal to the area associated with a p-n junction of the diode  74 . Therefore, the diodes  72 ,  74  correspond to different voltage differences when conducting currents with the same magnitudes. Operation of the bias circuit  70  is identical to that of the bias circuit  32 , and the lengthy description is not repeated for simplicity. 
     The bias circuits  80 ,  90 ,  100  individually illustrated in FIG. 6, FIG. 7, and FIG. 8 are respectively similar to the bias circuits  32 ,  60 ,  70  individually shown in FIG. 3, FIG. 4, and FIG.  5 . Each of the bias circuits  80 ,  90 ,  100  mainly alters a connection between the voltage sources Vdd, Vss and the transistors. For instance, the voltage source Vdd is electrically connected to the transistors  42   a ,  42   b  within each of the bias circuits  32 ,  60 ,  70 . However, with regard to each of the bias circuits  80 ,  90 ,  100 , the voltage source Vss is electrically connected to the transistors  42   a ,  42   b  instead. Operation of the bias circuits  80 ,  90 ,  100  are identical to that of the bias circuit  32 . Therefore, the lengthy description is not repeated for simplicity. 
     Please note that the resistor  54  with a positive temperature coefficient is adopted in the preferred embodiment to compensate the above-mentioned temperature effect imposed upon the current value. However, the resistor  54  with a negative temperature coefficient can be used to meet different requirements. For example, if a user needs, the ring oscillator  30  is capable of quickly increasing the frequency of the clock signals F 0  when the temperature increases, and is capable of rapidly decreasing frequency of the clock signal F 0  when the temperature decreases. Concerning this case, it is necessary to use the resistor  54  with a negative temperature coefficient for achieving the above-mentioned objective. It is noteworthy that the reference currents I 3 , I 4  are totally controlled and set based on the current value I calculated according to the equation (4). Therefore, the magnitudes of the reference currents I 3 , I 4  has nothing to do with transistor characteristics of MOS transistors and voltage levels outputted from voltage sources. 
     In contrast to the prior art ring oscillator, the claimed ring oscillator makes use of the intrinsic band gap associated with a p-n junction to control a bias current. The claimed ring oscillator adopts two loading units such as diodes or BJTs whose p-n junctions correspond to different areas, then generates the wanted bias current through different voltage differences caused by the unmatched p-n junctions. In addition, a resistor having a positive temperature coefficient is also utilized to compensate variation of the bias current caused by the unsteady temperature. At the same time, the claimed ring oscillator utilizes the bias current and current mirror connections to provide each delay cell with a desired reference current. Therefore, the reference current is not affected even though transistor characteristics of any MOS transistor within the claimed ring oscillator are unsteady. To sum up, the claimed ring oscillator is capable of outputting a stable clock signal. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.