Abstract:
A differential voltage controlled oscillator (VCO) employed in a frequency synthesizer used as a local oscillator of a wireless communication on-chip transmitter/receiver is provided. More particularly, a differential current negative feedback VCO equipped with a current-current negative feedback circuit that suppresses low- and high-frequency noise is provided. 
     A differential current negative feedback VCO includes a resonator determining oscillation frequency, and an oscillator generating negative resistance. In the oscillator of the differential current negative feedback VCO, transistors Q 1  and Q 2  form a cross-coupled pair, and negative resistance is generated by positive feedback of the cross-coupled pair. And, transistors Q 1  and Q 3  together with an emitter resistor and a capacitor form a current negative feedback part, and transistors Q 2  and Q 4  together with an emitter resistor and a capacitor form another current negative feedback part which is disposed opposite to a resonator. Thus, the VCO operates differentially. 
     In the oscillator of the differential current negative feedback VCO, emitter noise currents generated by base noise voltages of Q 1  and Q 2  induced by low- and high-frequency noise sources in the bases of Q 1  and Q 2  are sampled by emitter resistors, amplified through bases of Q 3  and Q 4 , and thus return to the bases of the Q 1  and Q 2  and suppress the base noise voltages. Measurement of the phase noise of the differential current negative feedback VCO reveals a phase noise reduction of approximately 25 dB compared to a conventional differential VCO.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 2005-119425, filed Dec. 8, 2005, and 2006-71106, filed Jul. 28, 2006, the disclosure of which is incorporated herein by reference in its entirety. 
   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a low phase noise differential LC tank voltage controlled oscillator (VCO) with current negative feedback, in which a phase noise characteristic is improved by mounting a current negative feedback circuit on a typical differential emitter-degeneration VCO. 
   2. Discussion of Related Art 
   A conventional differential LC tank cross-coupled pair VCO when combined with a phase-locked loop constitutes a frequency synthesizer, and is widely employed in a wireless transceiver integrated circuit. The differential LC tank cross-coupled pair VCO has an excellent noise characteristic compared with other oscillators due to a filtering function of an LC tank  100 , however further reduction of noise is required. 
   Accordingly, the differential LC tank VCO increases a Q factor of an inductor in the LC tank  100  to reduce phase noise, or uses an LC filter to eliminate noise from a current source. Since phase noise in the differential LC tank VCO is caused largely by up-conversion of 1/f noise and thermal noise of a cross-coupled pair, efforts have been made to thoroughly reduce the influence of 1/f noise and thermal noise using a low-frequency negative feedback circuit. Recently, a differential emitter-degeneration cross-coupled pair VCO has been developed in order to upwardly adjust an oscillation frequency of the conventional differential LC tank cross-coupled pair VCO, reduce phase noise, and expand a frequency modulation range. 
     FIG. 1  illustrates a conventional differential capacitive-degeneration LC tank VCO  10 . As illustrated in the VCO circuit  10  of  FIG. 1 , when a resistor  121  or a capacitor C P    125  is connected to an emitter of an oscillation transistor  111 , a negative resistance range moves to a higher frequency. Accordingly, at the higher frequency, the VCO oscillates with larger amplitude and has an expanded frequency modulation range and reduced phase noise. Input impedance in a cross-coupled pair of the capacitive-degeneration LC tank VCO  10  is given by Equation 1. 
   
     
       
         
           
             
               
                 
                   Z 
                   in 
                 
                 = 
                 
                   
                     - 
                     2 
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         Z 
                         E 
                       
                       + 
                       
                         1 
                         
                           g 
                           m 
                         
                       
                       + 
                       
                         s 
                         ⁢ 
                         
                           
                             
                               C 
                               π 
                             
                             ⁢ 
                             
                               r 
                               b 
                             
                           
                           
                             g 
                             m 
                           
                         
                       
                     
                     ) 
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ] 
               
             
           
         
       
     
   
   In Equation 1, Z E =R E //(1/sC P ), C P  denotes base-emitter capacitance, r b  denotes base resistance, and g m  denotes transconductance. As shown in Equation 1, negative resistance of the input impedance seen from collector nodes of oscillation transistors  111  and  112  is increased by parallel connection of emitter resistors  121  and  122  and capacitors  125  and  126 . And, the third term in Equation 1 has an inductance character, which shows that the frequency modulation range is also widened. 
   In the capacitive-degeneration LC tank VCO  10  of  FIG. 1 , V nf  denotes a low-frequency noise source corresponding to 1/f noise and thermal noise in a base of each transistor, V nbe  denotes base-emitter voltage of a cross-coupled pair  111  and  112  induced by the low-frequency noise source, I ntail  denotes tail current noise, I E  denotes emitter current, ΔI E  denotes a range of fluctuation in the emitter current due to low-frequency noise, V am  denotes a low-frequency amplitude modulation (AM) signal of oscillation frequency, that is, carrier frequency, due to low-frequency noise, V cm  denotes a common-mode level, and ΔV cm  denotes a range of fluctuation in the common-mode level due to low-frequency noise. 
   Elements affecting phase noise in the differential LC tank VCO  10  of  FIG. 1  are the Q-factor of an inductor, low-frequency noise of a cross-coupled pair transistor, and low- and high-frequency noise of current source transistors  127  and  128 . In the differential LC tank VCO  10  of  FIG. 1 , the base-emitter voltage V nbe  is induced by the low-frequency noise source V nf  at bases of the oscillation transistors  111  and  112 , and the emitter current changes by as much as ΔI E  due to the base-emitter voltage V nbe . After the low-frequency amplitude modulation of a carrier signal by the change in emitter current, a mean capacitance value of a varactor changes due to the amplitude modulation signal V am , and the carrier frequency fluctuates, thereby generating jitter, which contributes to phase noise. 
   Also, the I E  is modulated by low-frequency noise of tail current, thereby causing V cm  to fluctuate by as much as ΔV cm . This fluctuation in turn causes variation in varactor bias voltage, which causes the carrier signal to fluctuate, thereby generating jitter, which contributes to phase noise. 
   Also, in the differential LC tank VCO  10  of  FIG. 1 , the base-emitter voltage V nbe  of the oscillation transistors  111  and  112  induced by the low-frequency noise source and the carrier signal get mixed due to nonlinear characteristics of the oscillation transistors  111  and  112 , and thus the mixed noise contributes to noise around the carrier signal. 
   In the differential LC tank VCO  10  of  FIG. 1 , low- and high-frequency noise of the tail current induced by low-frequency noise sources and the carrier signal become mixed due to nonlinear characteristics of the oscillation transistors  111  and  112 , and the mixed noise contributes to noise around the carrier signal. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to providing a differential current negative feedback voltage controlled oscillator (VCO) that has an excellent phase noise characteristic. 
   The present invention is also directed to providing a differential current negative feedback VCO enabling low- and high-frequency noise of an LC tank VCO to be basically cut off. Accordingly, phase noise of the VCO due to low- and high-frequency noise from oscillation and current source transistors of a differential capacitive-degeneration LC tank VCO can be substantially eliminated. 
   An aspect of the present invention provides a differential current negative feedback VCO comprising: an LC tank resonator for providing inductance and capacitance between a power voltage terminal and a first node, and between the power voltage terminal and a second node; first and second oscillation transistors connected to the first and second nodes, respectively, and cross-coupled between a collector and a base; first and second emitter driving parts functioning as current sources for driving each of the oscillation transistors; and a current negative feedback part including first and second negative feedback transistors receiving as input at bases thereof output from emitters of the oscillation transistors, and offsetting effects of noise components applied to bases of the oscillation transistors and amplified. 
   The present invention suggests a differential LC tank VCO equipped with a low-frequency negative feedback circuit in order to reduce 1/f noise and thermal noise from a transistor that increase phase noise in the conventional differential LC tank VCO. That is, to solve the problem of phase noise in the conventional differential LC tank VCO, a low-frequency current negative feedback circuit is mounted in a conventional differential capacitive-degeneration LC tank VCO. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent to the one of ordinary skill in the art by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a circuit diagram illustrating a circuit of a conventional differential capacitor feedback or capacitive degeneration cross-coupled pair VCO, low- and high-frequency noise sources, and effects of noise on the voltage controlled oscillator (VCO); 
       FIG. 2  is a circuit diagram illustrating a differential current negative feedback VCO according to an exemplary embodiment of the present invention; 
       FIG. 3A  is a circuit diagram illustrating a low-frequency equivalent circuit of the current negative feedback circuit of  FIG. 2 ; 
       FIG. 3B  is a circuit diagram representing noise entering into emitters of oscillation transistors Q 1  and Q 2  in the current negative feedback circuit of  FIG. 2 , for analyzing impedance seen from outside looking into emitters of the oscillation transistors Q 1  and Q 2 ; 
       FIG. 3C  is a circuit diagram representing a low-frequency equivalent circuit for inducing input impedance of emitters of the current negative feedback circuit of  FIG. 2 ; 
       FIG. 4  is a circuit diagram illustrating a differential current negative feedback VCO according to another embodiment of the present invention; 
       FIG. 5  is a circuit diagram illustrating a differential current negative feedback VCO according to still another embodiment of the present invention; 
       FIG. 6  is a circuit diagram illustrating a differential current negative feedback VCO according to yet another embodiment of the present invention; 
       FIG. 7  is a circuit diagram illustrating a differential current negative feedback VCO according to yet another embodiment of the present invention; and 
       FIG. 8  is a graph comparing phase noise measured from a conventional differential VCO and a differential current negative feedback VCO of the present invention. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Hereinafter, a differential current negative feedback voltage controlled oscillator (VCO) according to exemplary embodiments of the present invention will be described in detail with reference to the attached drawings. 
     FIG. 2  illustrates a differential capacitive-degeneration LC tank voltage controlled oscillator (VCO)  20  equipped with a low-frequency current negative feedback circuit according to an exemplary embodiment of the present invention. In an oscillator  200  of the differential current negative feedback LC tank VCO  20 , emitter currents of oscillation transistors (amplifiers) Q 1   211  and Q 2   212  are sampled by emitter-degeneration resistors R f    321  and  322 , and input into bases of negative feedback transistors (amplifiers) Q 3   342  and Q 4   341  for low frequency, respectively. Because of this procedure, this circuit is called a current-current negative feedback circuit. 
   The differential current negative feedback LC tank VCO  20  of  FIG. 2  comprises an LC tank resonator  200  determining oscillation frequency and filtering noise, and an oscillator  300  connected with the LC tank resonator  200  in parallel so as to generate negative resistance for inducing oscillation. The transistors Q 1   211  and Q 2   212  form a cross-coupled pair which induces positive feedback together with the emitter-degeneration resistors R f    321  and  322  and capacitors C P    325  and  326 , thereby generating the negative resistance. 
   In the oscillator  300  in the differential current negative feedback LC tank VCO  20  of  FIG. 2 , the oscillation transistor Q 1   211 , a low-frequency negative feedback transistor Q 3   342 , the emitter-degeneration resistor R f    321 , and the emitter-degeneration capacitor C P    325  are formed in a loop, thereby forming current-current negative feedback. And, on the opposite side of the LC tank resonator  200 , the oscillation transistor Q 2   212 , a low-frequency negative feedback transistor Q 4   341 , the emitter-degeneration resistor R f    322 , and the emitter-degeneration capacitor C P    326  form a current-current negative feedback part, thereby operating differentially. 
   Input impedance of the oscillator  300  of the differential current negative feedback LC tank VCO  20 , that is, input impedance of the oscillator  300  seen from collector nodes of the oscillation transistors Q 1   211  and Q 2   212  (node  1  and node  2 ), may, with reference to Equation 1, be simply given by the following Equation 2. 
   
     
       
         
           
             
               
                 
                   Z 
                   in 
                 
                 = 
                 
                   
                     - 
                     2 
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         
                           ( 
                           
                             
                               Z 
                               E 
                             
                             // 
                             
                               Z 
                               FIN 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           1 
                           
                             g 
                             m 
                           
                         
                       
                       + 
                       
                         s 
                         ⁢ 
                         
                           
                             
                               C 
                               π 
                             
                             ⁢ 
                             
                               r 
                               b 
                             
                           
                           
                             g 
                             m 
                           
                         
                       
                     
                     ) 
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
                 ] 
               
             
           
         
       
     
   
   In Equation 2, Z E =R E //(1/sC P ), Z FIN  denotes input impedance of a negative feedback transistor, C P  denotes base-emitter capacitance, r b  denotes base resistance, and g m  denotes transconductance. 
   In the differential current negative feedback LC tank VCO  20 , the resonator  200  of the differential current negative feedback LC tank VCO  20  comprises resonant inductors L r    201  and  202 , a resonant capacitor C r    203 , and varactors C v    204  and  205 , and oscillation frequency is approximately determined by the resonator  200 . The oscillation frequency of the differential current negative feedback LC tank VCO  20  is precisely determined by the inductance and capacitance of the LC tank resonator  200 , the emitter-degeneration capacitance C P , and the input capacitances of the low-frequency negative feedback transistors Q 3   342  and Q 4   341  of the oscillator  300 . And, the oscillation frequency f O  may be approximately given by Equation 3. 
   
     
       
         
           
             
               
                 
                   f 
                   o 
                 
                 = 
                 
                   1 
                   
                     2 
                     ⁢ 
                     π 
                     ⁢ 
                     
                       
                         
                           L 
                           r 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               C 
                               r 
                             
                             + 
                             
                               C 
                               v 
                             
                             + 
                             
                               C 
                               p 
                             
                             + 
                             
                               C 
                               FIN 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 ] 
               
             
           
         
       
     
   
   In Equation 3, L r  denotes inductance of each of the resonant inductors  201  and  202 , C r  denotes capacitance of the resonance capacitor  203 , C V  denotes capacitance of each of the varactors  204  and  205  of the LC resonator, C P  denotes capacitance of the emitter-degeneration capacitor  325  or  326 , and C FIN  denotes input capacitance of a base input of each of the low-frequency negative feedback transistors Q 3   342  and Q 4   341  seen from each emitter of Q 1  and Q 2 . 
   The resonant inductor L r    201  of the LC tank resonator  200  almost functions as a short circuit at low frequency, so that the oscillation transistors Q 1   211  and Q 2   212  of the oscillator  300  in the differential current negative feedback LC tank VCO  20  function as emitter-follower amplifiers. Meanwhile, since the resonant inductor L r    201  of the LC tank resonator  200  functions as high impedance at high frequency, the oscillation transistors Q 1   221  and Q 2   212  function as emitter-degeneration amplifiers. The low-frequency negative feedback transistors Q 3   342  and Q 4   341  of the oscillator  300  in the differential current negative feedback LC tank VCO  20  function as common-emitter amplifiers amplifying low frequency, and a high-frequency signal is filtered out by the emitter-degeneration capacitances C P    325  and  326 . 
   A method for high-frequency oscillation and a low-frequency feedback characteristic of the differential current negative feedback LC tank VCO  20  are described above. A noise suppression characteristic of the differential current negative feedback LC tank VCO  20  will be described below. 
   In the oscillator  300  of the differential current negative feedback LC tank VCO  20 , each of emitter currents of the oscillation transistors Q 1   211  and Q 2   212  is sampled by each of the emitter-degeneration resistors R f    321  and  322  and then input to the base of each of the low-frequency negative feedback transistors Q 3   342  and Q 4   341 , respectively. In the oscillator  300  of the differential current negative feedback LC tank VCO  20 , emitter noise currents generated by base noise voltages of the oscillation transistors Q 1   211  and Q 2   212  induced by the low- and high-frequency noise sources V nf  existing at the bases of the oscillation transistors Q 1   211  and Q 2   212  are sampled by each of the emitter-degeneration resistors R f    321  and  322 , amplified through bases of the high-frequency negative feedback transistors Q 3   342  and Q 4   341 , and returned to the bases of the high-frequency oscillation transistors Q 1   211  and Q 2   212 . Thus, base-emitter noise voltages V nbe  of the high-frequency oscillation transistors Q 1   211  and Q 2   212  are suppressed by as much as the returned voltages. 
   The base-emitter voltage V nbe  may be represented as Equations 4 and 5 from a low-frequency equivalent circuit model of  FIG. 3A . In the low-frequency equivalent circuit  21  of  FIG. 3A , R tank    221  denotes a resistance component of the LC tank resonator  200 , and a block  340 ′ denotes a low-frequency equivalent circuit for the negative feedback transistor Q 3   342 . 
   
     
       
         
           
             
               
                 
                   V 
                   nbe 
                 
                 = 
                 
                   
                     V 
                     nf 
                   
                   
                     1 
                     + 
                     
                       A 
                       
                         v 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
                 ] 
               
             
           
           
             
               
                 
                   V 
                   nbe 
                 
                 = 
                 
                   
                     V 
                     nf 
                   
                   
                     1 
                     + 
                     
                       A 
                       
                         v 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     + 
                     
                       
                         A 
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       ⁢ 
                       
                         A 
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       ⁢ 
                       
                         
                           r 
                           
                             
                                 
                             
                             ⁢ 
                             π2 
                           
                         
                         
                           
                             r 
                             
                               b 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                           
                           + 
                           
                             r 
                             π2 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
                 ] 
               
             
           
         
       
     
   
   In Equations 4 and 5, A v1  denotes voltage gain of the oscillation transistors Q 1   221  and Q 2   212 , A v2  denotes voltage gain of the negative feedback transistors Q 3   342  and Q 4   341 , r b2  denotes base distributed resistance of the negative feedback transistors Q 3   342  and Q 4   341 , and r p2  denotes forward bias resistance between base-emitters of the negative feedback transistors Q 3   342  and Q 4   341 . 
   Equation 4 represents reduction of low-frequency noise by negative feedback of the emitter-degeneration resistor R f    321 , and Equation 5 represents reduction of low-frequency noise by a current negative feedback part composed of the emitter-degeneration resistor R f    321 , the oscillation transistor Q  1211  and the negative feedback transistor Q 3   342 . In Equation 5, the oscillation transistor Q 1   211  is an emitter-follower, so the voltage gain Av is almost 1, and the negative feedback transistor Q 3   342  functions as a common-emitter amplifier, so the voltage gain A v2  depends on a method of constituting a load resistor and a load resistor value. According to Equation 5, a degree of low-frequency noise suppression is determined by the voltage gain A v2 . If the voltage gain A v2  is infinite, the low-frequency noise voltage V nbe  may be completely suppressed. 
     FIG. 3B  illustrates how to suppress flicker noise (1/f noise), thermal noise of the tail current source  323 , and silicon substrate noise from ground, by the current-current negative feedback part  22  in the oscillator  300  of the differential current negative feedback LC tank VCO  20 , and  FIG. 3C  illustrates a low-frequency equivalent circuit  23  for the current-current negative feedback part  22  of  FIG. 3B . In the current-current negative feedback part  22  of  FIG. 3B , the flicker noise, thermal noise of the tail current source  323 , silicon substrate noise from ground, enter into emitters of the oscillation transistor Q 1   211  and the negative feedback transistor Q 3   342 , and is injected into the LC tank resonator  200 , thereby contributing to phase noise or being converted into phase noise centered around a carrier frequency due to a nonlinear mixing effect of the oscillation transistor Q 1   211 . However, since impedance Z out  of the emitter of the oscillation transistor Q 1   211  seen from the emitter node of Q 1  is increased as much as loop gain T of the current-current negative feedback part  22 , the flicker noise, thermal noise, silicon substrate noise, etc. entering into the emitter of the oscillation transistor Q 1   211  in the current-current negative feedback part  22  are attenuated. This principle may be given by the following Equations 6 and 7 derived from the low-frequency equivalent circuit  23  of the current-current negative feedback part  22  of  FIG. 3C . 
   
     
       
         
           
             
               
                 
                   Z 
                   out 
                 
                 = 
                 
                   
                     ( 
                     
                       
                         
                           β 
                           o 
                         
                         ⁢ 
                         
                           r 
                           
                             b 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       
                         
                           r 
                           
                             b 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         + 
                         
                           r 
                           π1 
                         
                         + 
                         
                           R 
                           f 
                         
                       
                     
                     ) 
                   
                   ⁢ 
                   
                     ( 
                     
                       1 
                       + 
                       T 
                     
                     ) 
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
                 ] 
               
             
           
           
             
               
                 
                   Z 
                   
                     i 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     n 
                   
                 
                 = 
                 
                   
                     r 
                     π2 
                   
                   
                     1 
                     + 
                     T 
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
                 ] 
               
             
           
         
       
     
   
   In Equations 6 and 7, T=(β O R f /r b2 ) denotes loop gain, and β O    15  denotes current gain. 
   In Equation 6, the impedance Z out  looking into the emitter of the oscillation transistor Q 1   211  is increased as much as the loop gain T, and thus all kinds of noise entering into the emitter of the oscillation transistor Q 1   211  are attenuated. 
     FIGS. 4 to 7  illustrate various modified exemplary embodiments in which at least one of the elements of the VCO illustrated in  FIG. 2  is replaced with a different element while keeping with the sprit of the present invention. 
   As described above, the VCO of  FIG. 2  comprises: the LC tank resonator  200  connected between a power voltage terminal and the first node and between the power voltage terminal and the second node; the first and second oscillation transistors  211  and  212  connected to the respective first and second nodes; the first and second emitter driving parts  320 - 1  and  320 - 2  for driving the respective oscillation transistors; and the first and second negative feedback transistors  341  and  342  inputting the emitter outputs of the oscillation transistors to respective bases. 
   First, possible substitutions for elements of  FIG. 2  will be described, and some among many possible VCO embodiments obtained by substituting at least one of the elements of  FIG. 2  will be selected and described with reference to the drawings. 
   The first and second emitter driving parts  320 - 1  and  320 - 2  illustrated in  FIG. 2  comprise: the emitter-degeneration resistors  321  and  322  connected at one end to emitters of the oscillation transistors  211  and  212 ; the emitter current sources  323  and  324  connected between the other end of the emitter-degeneration resistors  321  and  322  and ground; and the emitter-degeneration capacitors  325  and  326  connected between emitters of the oscillation transistors  211  and  212  and ground. 
   According to another embodiment, illustrated in  FIG. 4 , the first and second emitter driving parts may be modified to comprise: emitter-degeneration resistors  421  and  422  connected at one end to emitters of the oscillation transistors  211  and  212 ; emitter current sources  423  and  424  connected between the other end of the emitter-degeneration resistors  421  and  422  and ground; and current source capacitors  425  and  426  connected with the emitter current sources  423  and  424  in parallel. The modified first and second emitter driving parts may be included in structures illustrated in  FIGS. 5 and 6 . 
   According to still another embodiment, illustrated in  FIG. 7 , the first and second emitter driving parts may be modified to comprise: emitter current sources  723  and  724  connected between emitters of the oscillation transistors  211  and  212  and ground; and current source capacitors  725  and  726  connected with the emitter current sources  723  and  724  in parallel. 
   The current negative feedback part  320 - 1  illustrated in  FIG. 2  comprises: the first negative feedback transistor  341  connected at its collector to the first node Node 1 , and at its base to the emitter of the second cross-coupled transistor  212 ; the second negative feedback transistor  342  connected at its collector to the second node Node 2 , and at its base to the emitter of the first cross-coupled transistor  211 ; and the negative feedback current source  359 , connected between emitters of the first and second negative feedback transistors  341  and  342  and ground, for driving the first and second negative feedback transistors  341  and  342 . 
   According to another embodiment, illustrated in  FIG. 4 , the current negative feedback part may be modified to comprise: a first load resistor  445  connected at one end to the first node node  1 ; a second load resistor  446  connected at one end to the second node Node 2 ; a first negative feedback transistor  441  whose collector is connected to the other end of the first load resistor  445  and whose base is connected with the emitter of second cross-coupled transistors  212 ; a second negative feedback transistor  442  whose collector is connected to the other end of the second load resistor  446  and whose base is connected to the emitter of a first cross-coupled transistor  211 ; and a negative feedback current source  459 , connected between emitters of the first and second negative feedback transistors  441  and  442  and ground, for driving the first and second negative feedback transistors  441  and  442 . The substituted current negative feedback part is included in structures illustrated in  FIGS. 5 to 7 . 
   Also, the current negative feedback part illustrated in  FIG. 4  may further include a parallel capacitor  449  between the bases of the first and second negative feedback transistors  441  and  442 , and still be applied to the structures of the following  FIGS. 5 to 7 . 
   The current negative feedback part illustrated in  FIG. 5  may further include: a first low-frequency gain boosting resistor  543  connected between the power voltage terminal and a collector of a first negative feedback transistor  541 ; and a second low-frequency gain boosting resistor  544  connected between the power voltage terminal and a collector of a second negative feedback transistor  542 . 
   The current negative feedback part illustrated in  FIG. 6  may further include: a first low-frequency gain boosting resistor  643  connected between the power voltage terminal and a first node Node 1 ; and a second low-frequency gain boosting resistor  644  connected between the power voltage terminal and a second node Node 2 . 
   The current negative feedback part may be substituted with one comprising: first and second negative feedback transistors  741  and  742  having the structure illustrated in  FIG. 2 ; a first boosting inductor  757  connected at one end to an emitter of the first negative feedback transistor  741 , and at the other end to ground; and a second boosting inductor  758  connected at one end to an emitter of the second negative feedback transistor  742 , and at the other end to ground. The substitute current negative feedback part is illustrated in  FIG. 7 . 
   The LC tank resonator  200  illustrated in  FIG. 2  comprises: the first resonant inductor  201  connected between the power voltage terminal and the first node Node 1 , and between the power voltage terminal and the second node Node 2 ; the second resonant inductor  202  connected between the power voltage terminal and the second node node  2 ; and the resonant capacitor  203  connected between the first and second nodes Node  1  and Node 2 . 
   According to another embodiment, the LC tank resonator may have a first resonant capacitor connected with the first resonant inductor  201  in parallel and a second resonant capacitor connected with the second resonant inductor  202  in parallel instead of the resonant capacitor  203  in  FIG. 2 , which is the same configuration as the conventional LC tank resonator of  FIG. 1 . Also, the LC tank resonator  200  may further include varactors  204  and  205  as in  FIG. 2  for fine tuning of the resonant capacitor. 
     FIG. 4  illustrates a modified structure of a differential current negative feedback LC tank VCO, the structure having the same operation characteristics as the differential current negative feedback LC tank VCO of  FIG. 2  and increasing low-frequency gain of the negative feedback transistors Q 3   442  and Q 4   441 . The operation characteristic of the modified differential current negative feedback LC tank VCO  40  of  FIG. 4  will now be described. 
   In the modified differential current negative feedback LC tank VCO  40 , oscillation transistors Q 1   221  and Q 2   212  of an oscillator  400  function as an emitter follower amplifier at low frequency because the inductor L r    201  of the LC tank resonator  200  acts almost as a short circuit. But, they function as emitter-degeneration amplifiers at high frequency because the inductor L r    201  of the LC tank resonator  200  acts as high impedance. 
   The transistors Q 1   221  and Q 2   212  in the oscillator  400  of the modified differential current negative feedback LC tank VCO  40  form a cross-coupled pair, and positive feedback is induced by the cross-coupled pair Q 1  and Q 2 , the emitter-degeneration resistor R r    421  and the capacitor C P    425 , and thus negative resistance is generated. 
   In the oscillator  400  of the modified differential current negative feedback LC tank VCO  40  of  FIG. 4 , a loop is formed with the oscillation transistor Q 1   211 , the low-frequency negative feedback transistor Q 3   442 , the emitter-degeneration resistor R r    421  and the emitter-degeneration capacitor C p    449  so as to carry out current-current negative feedback. And, on the opposite side of the LC tank resonator  220 , the oscillation transistor Q 2   212 , the low-frequency negative feedback transistor Q 4   441 , the emitter-degeneration resistor R f    422 , and the emitter-degeneration capacitor C p    449  form the current-current negative feedback so as to operate differentially. 
   In the oscillator  400  of the modified differential current negative feedback LC tank VCO  40 , the emitter-degeneration capacitor C p    449  has the same function in the circuit as the emitter-degeneration capacitors C p    216  and  217  of the differential current negative feedback LC tank VCO  20  of  FIG. 2 . In the modified differential current negative feedback LC tank VCO  40  of  FIG. 4 , the resistors R L    445  and  446  are for increasing gains A v2  of the negative feedback transistors Q 3   442  and Q 4   441  and repressing high-frequency negative feedback, and the capacitors C r    425  and  426  filter low- and high-frequency noise of current sources  426  and  424  connected to the emitter-degeneration resistors R f    421  and  422  of the oscillation transistors Q 1   221  and Q 2   212 , respectively. 
     FIGS. 5 and 6  illustrate another modified structure of the differential current negative feedback LC tank VCO, which has the same operation characteristics as the differential current negative feedback LC tank VCO of  FIG. 2  and increases low-frequency gain of the negative feedback transistors Q 3   213  and Q 4   214 . In the modified differential current negative feedback LC tank VCOs  50  and  60 , resistors R o    543 ,  544 ,  643  and  644  increase gains A v2  of negative feedback transistors Q 3  and Q 4   542 ,  642 ,  541  and  641  together with resistors R L    545 ,  546 ,  645  and  646 , and suppress high-frequency negative feedback. 
     FIG. 7  illustrates still another modified structure of the differential current negative feedback LC tank VCO, which has the same operation characteristics as the differential current negative feedback LC tank VCO of  FIG. 2  and increases low-frequency gain of the negative feedback transistors Q 3   742  and Q 4   741  and high-frequency oscillation amplitude. In the modified differential current negative feedback LC tank VCO  70 , resistors R o    743  and  744  increase gains A v2  of the negative feedback transistors Q 3   742  and Q 4   741  together with resistors R L    745  and  746 , and suppress high-frequency negative feedback. 
   In the modified differential current negative feedback LC tank VCO  70 , boosting inductors L B    757  and  758  are connected to emitters of the negative feedback transistors Q 3   742  and Q 4   741 , respectively, and thereby a current negative feedback part causes negative feedback at low frequency and positive feedback at high frequency. In the modified differential current negative feedback LC tank VCO  70  of  FIG. 7 , voltage at each of the boosting inductors L B    757  and  758  forms almost the same phase as base and collector voltages of each of the negative feedback transistors Q 3   742  and Q 4   741  at high frequency, and thus rapid transient response and large oscillation amplitude are obtained. In the modified differential current negative feedback LC tank VCO  70  of  FIG. 7 , the emitter-degeneration resistor R f  of each of oscillation transistors Q 1   221  and Q 2   212  is eliminated and, instead, an output resistor (not illustrated) of a tail current source performs a similar function. Also, even if the modified differential current negative feedback LC tank VCO  70  uses resistors instead of the boosting inductors L B    757  and  758 , the resistors function similar to the boosting inductors. 
     FIG. 8  is a graph comparing phase noise measured from the conventional differential emitter-degeneration VCO  10  of  FIG. 1  and the differential current negative feedback LC tank VCO  20  of  FIG. 2  according to the present invention. As shown in  FIG. 8 , phase noise of the differential current negative feedback LC tank VCO  20  in  FIG. 2  was about 25 dB lower than the phase noise  61  measured in the conventional differential emitter-degeneration VCO  10  at offset frequency of 10 kHz. Both VCOs produced an oscillation frequency of about 5.5 GHz. While the phase noise  61  of the conventional differential emitter-degeneration VCO  10  was −85 dBc/Hz, the phase noise of the differential current negative feedback LC tank VCO  20  in the present invention was −110 dBc/Hz. 
   A low phase noise differential LC tank VCO with current negative feedback according to the present invention has an improved phase noise characteristic. 
   That is, a current negative feedback circuit is mounted in the conventional differential emitter-degeneration VCO so as to suppress low- and high-frequency noise generated from a cross-coupled pair and a tail current source, and thus phase noise of the differential VCO is reduced. 
   And, in the differential current negative feedback LC tank VCO, a boosting inductor is connected to an emitter of the negative feedback transistor, thereby offsetting negative feedback at high frequency and inducing positive feedback. 
   While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.