BACKGROUND OF THE INVENTION 
     The present invention relates to an oscillator for a semiconductor integrated circuit. 
     A multivibrator circuit is known in the prior art as an oscillator circuit that alternately activates and inactivates two transistors. Further, for an oscillator circuit incorporating an emitter-coupled multivibrator, an oscillation control circuit has been proposed to output a stable oscillation frequency by reducing the influence of temperature dependency and variation between lots on the base-emitter potential difference (for example, refer to Japanese Laid-Open Patent Publication No. 2005-167752, FIG. 1). In the oscillator circuit described in Japanese Laid-Open Patent Publication No. 2005-167752, common current flows through transistors that determine the oscillation frequency. This suppresses the influence of the base-emitter voltage difference. Further, as for current determined by a resistor that controls the dynamic range or the like, an emitter follower is used to suppress the base-emitter voltage difference of transistors determined by this current. 
     Further, an emitter-coupled multivibrator oscillation circuit having a relatively small circuit scale and eliminating the need for an external amplification circuit, which consumes a large amount of power, has been proposed (for example, refer to Japanese Laid-Open Patent Publication No. 6-6187, FIG. 1). In the oscillator circuit described in Japanese Laid-Open Patent Publication No. 6-6187, an AC-coupling capacitor is connected to the output side of the oscillator circuit, and one or more amplification circuits, each formed by a self-biased CMOS inverter, are connected to the AC-coupled capacitor. The self-biased CMOS inverter includes two conductive MOSFETS, which are connected in series between a power supply and ground. The MOSFETs have gates connected to each other. A bias resistor is connected to a node between the MOSFETs. Further, the self-biased CMOS inverter has a gate area determined so that through current, which is generated when the two conductive MOSFETs are switched, is regulated to a predetermined value. 
     However, the swing of an oscillator circuit formed by a CMOS transistor is small. Particularly, a high frequency would affect the operation speed and parasitic capacitance of the transistor. As a result, the oscillation amplitude decreases in inverse proportion to the frequency. For example, in the oscillator circuit described in Japanese Laid-Open Patent Publication No. 2005-167752, the amplitude voltage is expressed by “R3*I2.” Thus, full swing cannot be performed and amplification is necessary. Accordingly, an amplifier for realizing full swing is necessary for the multivibrator. This results in the necessity for bias current. The oscillator circuit described in Japanese Laid-Open Patent Publication No. 6-6187 includes an amplification circuit and is thus capable of full swing. However, the amplification circuit increases power consumption. In this case, when the amplitude is small, the amplification gain and accuracy must be increased. This requires a larger amount of current. 
     SUMMARY OF THE INVENTION 
     The present invention provides an oscillation circuit having a relatively simple circuit structure while enabling full swing with low power consumption. 
     One aspect of the present invention is an oscillation circuit including an oscillation unit for outputting a first oscillation signal. A voltage control unit includes first and second control elements having source terminals that are connected to each other. An output unit includes first and second output control elements, arranged between first and second output current sources, and a first output terminal arranged at a connection node between the first and second output control elements. The first control element has a drain terminal connected to a connection node between the first output current source and the first output control element. The second control element has a drain terminal connected to a connection node between the second output control element and the second output current source. A connection node between the source terminal of the first control element and the source terminal of the second control element is connected so as to receive the first oscillation signal from the oscillation unit. The first control element has a gate terminal for receiving a predetermined bias, and the second control element has a gate terminal for receiving a reference voltage for determining an amplitude of an oscillation signal. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a diagram showing the entire structure of an oscillation circuit according to a preferred embodiment of the present invention; 
         FIG. 2  is a diagram showing the operation of an oscillation core block and a voltage restriction block in the oscillation circuit of  FIG. 1 ; 
         FIG. 3  is a diagram showing the operation of the voltage restriction block and a differential output block in the oscillation circuit of  FIG. 1 ; 
         FIG. 4  is a diagram showing the oscillation of the oscillation circuit of  FIG. 1 ; 
         FIG. 5  is a diagram showing the structure of an oscillation circuit according to another embodiment of the present invention; and 
         FIG. 6  is a diagram showing the structure of an oscillation circuit according to a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An oscillation circuit  10  according to a preferred embodiment of the present invention will now be discussed with reference to  FIGS. 1 to 4 . As shown in  FIG. 1 , the oscillation circuit  10  includes an oscillation core block  100  functioning as an oscillation unit, a voltage restriction block  120  functioning as a voltage control unit, and a differential output block  140  functioning as an output unit. The oscillation core block  100  is a functional block for outputting AN oscillation signal. The voltage restriction block  120  is a functional block for maintaining the oscillation signal of the oscillation core block  100  at a constant output voltage (amplitude) and outputting current that is generated when the voltage is restricted. The differential output block  140  is a functional block for oscillating the current output from the voltage restriction block  120  with an amplitude between the power supply voltage and ground voltage (full swing). 
     The oscillation circuit  10  is supplied with drive voltage VCC via a power supply line and ground voltage GND via a ground line. 
     The structure of the oscillation core block  100  will now be discussed. 
     Referring to  FIG. 1 , the oscillation core block  100  includes p-channel MOS transistors M 1  and M 2  respectively functioning as first and second oscillation control elements. The transistor M 1  has a source terminal connected to the power supply line via a constant current source CS 1 , which functions as a first oscillation current source, and a drain terminal connected to the ground line via a constant current source CS 2 , which functions as a second oscillation current source. The transistor M 2  has a source terminal connected to the power supply line via a constant current source CS 3 , which functions as a third oscillation current source, and a drain terminal connected to the ground line via a constant current source CS 4 , which functions as a fourth oscillation current source. 
     The gate terminal of the transistor M 1  is connected to the drain terminal of the transistor M 2 . The gate terminal of the transistor M 2  is connected to the drain terminal of the transistor M 1 . 
     A capacitor is arranged between the source terminals of the transistors M 1  and M 2 . 
     The drain terminals of the transistors M 1  and M 2  are each connected to the voltage restriction block  120 . The transistor M 1  outputs a first oscillation signal, and the transistor M 2  outputs a second oscillation signal. 
     The structure of the voltage restriction block  120  will now be discussed. 
     The voltage restriction block  120  includes transistors M 3 , M 5 , and M 7  of a predetermined conduction type (n-channel) and transistors M 4 , M 6 , and M 8  of an opposite conduction type (p-channel). The transistors M 7  and M 8 , which are provided with the first oscillation signal, respectively function as first and second control elements. The transistors M 5  and M 6 , which are provided with the second oscillation signal, respectively function as third and fourth control elements. 
     The drain terminal of the transistor M 3  is connected to the power supply line via a constant current source CS 5 . The drain terminal and gate terminal of the transistor M 3  are connected to each other. The source terminal of the transistor M 3  is connected to the source terminal of the transistor M 4 . The drain terminal and gate terminal of the transistor M 4  is connected to the ground line. The transistors M 3  and M 4  generate a predetermined bias voltage relative to the ground voltage GND at the gate terminal of the transistor M 3 . 
     The gate terminal of the transistor M 3  is connected to the gate terminals of the transistors M 5  and M 7 . The drain terminals of the transistors M 5  and M 7  are connected to the differential output block  140 . 
     The source terminals of the transistors M 5  and M 7  are respectively connected to the source terminals of the transistors M 6  and M 8 . A connection node between the transistors M 5  and M 6  is connected to the drain terminal of the transistor M 2  in the oscillation core block  100 . 
     A connection node between the transistors M 7  and M 8  is connected to the drain terminal of the transistor M 1  in the oscillation core block  100 . 
     Further, the gate terminals of the transistors M 6  and M 8  are supplied with a reference voltage Vref. The reference voltage Vref determines the amplitude of the oscillation signal in the voltage restriction block  120 . 
     The structure of the differential output block  140  will now be discussed. 
     The differential output block  140  includes transistors M 9  and M 11  of a predetermined conduction type (p-channel) and transistors M 10  and M 12  of an opposite conduction type (n-channel). The source terminals of the transistors M 9  and M 11 , which are respectively connected to the power supply line via constant current sources CS 6  and CS 8 , are supplied with the drive voltage VCC. Further, the source terminals of the transistors M 9  and M 11  are respectively connected to the drain terminals of the transistors M 5  and M 7  in the voltage restriction block  120 . 
     The gate terminals of the transistors M 9  and M 11  are supplied with voltage VP. The voltage VP is set within the operable range of the constant current sources CS 6  and CS 8  at a value less than the drive voltage VCC by an amount corresponding to the transistor threshold voltage. 
     The drain terminals of the transistors M 9  and M 11  are respectively connected to the drain terminals of the transistors M 10  and M 12 . 
     The gate terminals of the transistors M 10  and M 12  are supplied with voltage VN. The voltage VN is set within the operable range of the constant current sources CS 6  and CS 8  at a value greater than the ground voltage GND by an amount corresponding to the transistor threshold voltage. 
     The source terminals of the transistors M 10  and M 12  are respectively connected to the ground line via constant current sources CS 7  and CS 9 . Further, the source terminals of the transistors M 10  and M 12  are respectively connected to the drain terminals of the transistors M 6  and M 8  in the voltage restriction block  120 . 
     In the preferred embodiment, the constant current sources CS 8  and CS 9  respectively function as first and second output current sources, and the constant current sources CS 6  and CS 7  respectively function as third and fourth output current sources. Further, the transistors M 11  and M 12  respectively function as first and second output control elements, and the transistors M 9  and M 10  respectively function as third and fourth output control elements. 
     A connection node between the transistors M 9  and M 10  and a connection node between the transistors M 11  and M 12  respectively function as an output terminal N 1 , which serves as a second output terminal, and an output terminal N 2 , which serves as a first output terminal. 
     The constant current sources CS 1  to CS 9  respectively generate currents I 1  to I 9 . In the preferred embodiment, the values of the currents I 1  to I 9  are all fixed (iref). 
     The operation of the oscillation core block  100  and the voltage restriction block  120  will now be discussed with reference to  FIG. 2 . 
     When the transistor M 1  goes on and the transistor M 2  goes off in the oscillation core block  100 , current I  1 +I 3  from the constant current sources CS 1  and CS 3  flow to the drain terminal of the transistor M 1 . The constant current source CS 2  cancels some of the current. Thus, the voltage restriction block  120  is supplied with current I 1 +I 3 −I 2 . The current I 1 +I 3 −I 2  is supplied to the transistor M 8 . Thus, the transistor M 8  goes on. 
     The transistor M 2  is off. Thus, the transistor M 5  of the voltage restriction block  120  goes on and supplies the constant current source CS 4  with current I 4 . 
     As a result, voltage Vc 1  of the capacitor C at the side of the transistor M 1  is expressed as shown below.
 
 Vc 1= Vd 2+ Vt 1= Vt 4+ Vt 3− Vt 5+ Vt 1
 
     Voltage Vd 2  refers to the voltage at a connection node between the drain terminal of the transistor M 2  and the constant current source CS 4 . Further, when “i” is a variable, voltage Vti expresses the gate-source voltage when transistor Mi is on. 
     Voltage Vc 2  of the capacitor C at the side of the transistor M 2  is expressed as shown below when the time for reaching voltage Vt 2 +Vref+Vt 8  is a variable.
 
 Vc 2= Vc 1+ I 3/ C *(time)
 
     When the transistor M 2  goes on and the transistor M 1  goes off in the oscillation core block  100 , current I1+I3 flows to the transistor M 2  from the constant current sources CS 1  and CS 3 . The constant current source CS 4  cancels some of the current. Thus, the voltage restriction block  120  is supplied with current I 1 +I 3 −I 4 . The current I 1 +I 3 −I 4  is supplied to the transistor M 6 . Thus, the transistor M 6  goes on. 
     The transistor M 1  is off. Thus, the transistor M 7  of the voltage restriction block  120  goes on and supplies the constant current source CS 2  with current I 2 . 
     As a result, voltage Vc 2  of the capacitor C at the side of the transistor M 1  is expressed as shown below.
 
 Vc 2= Vd 1+ Vt 2= Vt 4+ Vt 3− Vt 7+ Vt 2
 
     Voltage Vd 1  refers to the voltage at a connection node between the drain terminal of the transistor M 1  and the constant current source CS 2 . 
     Voltage Vc 1  of the capacitor C at the side of the transistor M 1  is expressed as shown below when the time for reaching voltage Vt 1 +Vref+Vt 6  is a variable.
 
 Vc 1= Vc 2+ I 1/ C *(time)
 
     In the transistors M 1  to M 8 , if voltage Vt=Vt 1 , . . . , =Vt 8 , and current Iref= 1 , . . . , =I 8  are satisfied, as shown in  FIG. 4 , voltage vc and voltage Vc 2  alternately swing between voltage 2*Vt+Vref and 2*Vt−Vref so that the oscillation amplitude has the same voltage as the reference voltage Vref. In other words, oscillation is performed with the amplitude of the reference voltage Vref regardless of how big or small the swing of the oscillation core block  100  is. 
     The rising gradient of each of voltages Vc 1  and Vd 2  is expressed as Iref/C. The cycle T and frequency F of the oscillation is expressed as described below.
 
 Iref*T/ 2= C*Vref* 2
 
 T= 4 C*Vref/Iref  
 
 F= 1 /T=Iref/ (4 C*Vref )
 
     The operation of the voltage restriction block  120  and the differential output block  140  will now be discussed with reference to  FIG. 3 . 
     When current Iref is supplied from a connection node (voltage Vd 2 ) between the drain terminal of the transistor M 2  and the constant current source CS 4  to a connection node (voltage Vd 1 ) between the drain terminal of the transistor M 1  and the constant current source CS 2 , the transistor M 5  goes off and the current Iref flows to the transistor M 6 . Further, in this case, the transistor M 8  goes off, and current Iref flows to the transistor M 7 . 
     Accordingly, current I 6  flows to the transistor M 9 , and current I 7 -Iref flows to the transistor M 10 . If current I 6  and current I 7  have the same values, the current flowing to the transistor M 9  would be greater than the current flowing to the transistor M 10 , and the differential current (more specifically, current Iref) would be supplied to the output terminal N 1 . This current would increase the voltage at the output terminal N 1  to the drive voltage VCC. 
     If current I 8  and current I 9  have the same values, the current flowing to the transistor M 12  would be greater than the current flowing to the transistor M 11 , and the differential current would be canceled at the output terminal N 2 . This current would decrease the voltage at the output terminal N 2  to the ground voltage GND. 
     Once the voltage at the output terminal N 1  increases to the drive voltage VCC, current would not flow to the output terminal N 1 . Thus, the current flowing to the transistor M 9  and the current flowing to the transistor M 10  would have the same value and be current I 7 -Iref. The output terminal N 2  reaches the ground voltage GND. In the same manner, the current flowing to the transistor M 12  and the current flowing to the transistor M 11  would have the same value and be current I 8 -Iref. If the currents I 6  to I 9  are the same as the current Iref, current would not be consumed in the differential output block  140 . 
     When the oscillation signal is inverted, the voltage at the output terminal N 1  decreases to the ground voltage GND, and the voltage at the output terminal N 2  increases to the drive voltage VCC. This enables full swing without supplying additional current. 
     The oscillation circuit of the preferred embodiment has the advantages described below. 
     In the preferred embodiment, current for restricting voltage is supplied to the differential output block  140 . The differential output block  140  uses the current to perform full swing between the ground voltage GND and the drive voltage VCC. In the prior art, the used current is disposed of by a voltage limiter (in the preferred embodiment, voltage restriction block  120 ). In the oscillation circuit of the present invention, the differential output block  140  effectively uses this current without the supply of additional current and enables full swing. Accordingly, full swing can be realized while reducing power consumption with a simple circuit structure. 
     In the preferred embodiment, the frequency F is a function of the reference voltage Vref. Accordingly, the frequency F may be controlled by changing the reference voltage Vref. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
     In the preferred embodiment, it is preferable that the constant current sources CS 1  to CS 9  all be the same. However, even when the constant current sources CS 1  to CS 9  differ from one another, power may be saved for a common current value. 
     In the preferred embodiment, a multivibrator oscillation circuit is used as the oscillation core block  100 . However, the structure of the oscillation core block  100  is not limited as long as the oscillation signal can be provided. 
     In the preferred embodiment, two complementary signals are output by using the transistors M 11  and M 12 . Instead, an oscillation circuit  11  show in  FIG. 5  may be used to supply a single oscillation signal. Particularly, when there is no need for complementary signals, the oscillation circuit may be realized with a simpler structure. 
     In the preferred embodiment, the reference voltage Vref is supplied from an external device. Instead, as shown in  FIG. 6 , an oscillation circuit  12  may generate the reference voltage vref with a constant current source CS 10 , which is connected to the drive voltage VCC and which functions as a reference current source, and a resistor R. In this case, as described above, the frequency F is expressed as Iref/(4C*Vref). When substituting Vref with Iref*R, F=1/4CR is satisfied. In this manner, when Iref is used so as to supply current that is a constant times greater than another current in an oscillation circuit, the frequency would not change even if the current changes. 
     In the preferred embodiment, the voltage of each of the transistors M 1  to M 8  is such that Vt=Vt 1 = . . . =Vt 8  is satisfied. However, in the present invention, it is only required that Vt 1 =Vt 2 , Vt 3 =Vt 5 =Vt 7 , and Vt 4 =Vt 6 =Vt 8  be satisfied. In this case, swinging occurs between voltage Vt 4 +Vt 3 −Vt 7 +Vt 2 +Vref and voltage Vt 4 +Vt 3 −Vt 7 +Vt 2 −Vref. 
     In the preferred embodiment, the frequency F is controlled by changing the reference voltage Vref using the constant current sources CS 1  to CS 9  as current sources. Instead, the frequency F may be controlled by changing the current Iref for each current source. The frequency F would become the function of current Iref and be proportional to the current Iref. Thus, each current source would be realized by a current mirror circuit, and the frequency may be easily controlled. 
     The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.
An oscillator circuit having a relatively simple circuit structure while enabling full swing with low power consumption includes an oscillation core block, a voltage restriction block, and a differential output block. Drain terminals of first and second transistors are each connected to the voltage restriction block. The voltage restriction block restricts the amplitude of an oscillation signal to a reference voltage. Source terminals of third and fourth transistors are connected to drain terminals of fifth and sixth transistors, and source terminals of seventh and eighth transistors are connected to drain terminals of ninth and tenth transistors. This supplies the differential output block with current generated by the amplitude restriction. The differential output block converts the current into drive voltage to ground voltage to perform full swing.