Abstract:
The oscillator of the present invention generally comprises a resonating device that is directly coupled to ground and a circuit portion. The circuit portion is operably coupled to the resonating device and includes a first transistor and a second transistor. The first transistor provides DC feedback to the second transistor and enables a temperature independent bias current through said second transistor.

Description:
RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Application No. 60/120,641, filed Feb. 18, 1999, incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to oscillators, and more particularly, to oscillators that maintain a stabilized frequency over a large temperature range. 
     BACKGROUND OF THE INVENTION 
     Oscillators are used in numerous applications where it is desired to have an alternating current with a stable frequency. However, most oscillation circuits are subject to deviation from their nominal output frequency. These frequency deviations can be due to many sources including temperature variations, which can cause frequency drift, and load variations, which can cause frequency pulling. 
     In attempts to stabilize these frequency deviations, circuits have been developed which use biasing techniques to eliminate frequency drift and/or additional stages to reduce pulling. However, the biasing techniques tend to load down the resonator of the circuit and de-que its performance. Additional stages add to the overall cost and complexity of the oscillator circuit. 
     U.S. Pat. No. 5,126,699 describes the use of a temperature sensor whose reading is used by a temperature compensation algorithm within a microprocessor to determine what DC value should be added to modulation to maintain a desired oscillator frequency. Clearly, a temperature sensor and a microprocessor adds significant cost. 
     In view of the above, there is a need for an oscillator circuit that compensates for frequency drift and frequency pulling due to temperature variations and load variations, respectively, without adding significant cost or complexity to the circuit. 
     SUMMARY OF THE INVENTION 
     The needs described above are in large measure met by a temperature compensated high performance oscillator of the present invention. The oscillator of the present invention generally comprises a resonating device that is directly coupled to ground and a circuit portion. The circuit portion is operably coupled to the resonating device and includes a first transistor and a second transistor. The first transistor provides DC feedback to the second transistor and enables a temperature independent bias current through said second transistor. 
     The temperature independent bias current is achieved by utilizing substantially equivalent resistance on the collector and emitter legs of the first transistor. Note that the first transistor also operates as a high-gain output buffer to external loads. The output buffer enables the frequency of oscillation of the oscillator to be substantially independent of the external load. The resonating device may comprise a resonator or a high Q inductor. The direct connection of the resonating device to ground substantially maximizes the loaded Q of the oscillator. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of an oscillator of the present invention 
     FIG. 2 is a schematic of a current bias circuit that is operable with the oscillator of the present invention. 
     FIG. 3 is a simplified schematic of the current bias circuit of FIG.  2 . 
     FIG. 4 is a small signal approximation of the current bias circuit of FIG.  2 . 
     FIG. 5 is a schematic of an alternative embodiment of the oscillator of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an oscillator  10  of the present invention is depicted. Oscillator  10  is a high-performance oscillator having a high loaded Q, a substantially constant output power, minimal frequency pulling and minimal temperature drift. 
     As shown in FIG. 1, oscillator  10  includes a resonator  12 , e.g., a high Q inductor or coaxial resonator, which is coupled directly to ground  16  and node  18 . Node  18  is tied to the collector of a first PNP transistor  20 . Connected between node  18  and a node  22  is a capacitor  24 . A capacitor  26  is connected between node  22  and ground  16 . Node  22  is connected to the emitter of transistor  20  and to the base of a second PNP transistor  28 . A resistor  30  is connected between the base of transistor  28  and a positive voltage supply  31 , V CC , e.g., +5V. The base of transistor  20  is connected to node  32 . The parallel combination of an inductor  38  and a capacitor  40  are connected between node  32  and a node  42 . The parallel combination of a capacitor  36  and a resistor  34  are connected between node  36  and ground  16 . The output of oscillator  10  is provided at node  42 . Node  42  is connected to the collector of transistor  28 . The parallel combination of a resistor  44  and a capacitor  46  are connected between the emitter of transistor  28  and positive voltage supply  31 , V CC . The preferred component values for oscillator  10  are provided below in Table 1, of course, other component values may be used without departing from the spirit or scope of the invention. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Capacitor 24 
                 5.6 picoFarads 
               
               
                   
                 Capacitor 26 
                 82 picoFarads 
               
               
                   
                 Capacitor 36, 46 
                 1000 picoFarads 
               
               
                   
                 Capacitor 40 
                 7.5 picoFarads 
               
               
                   
                 Resistor 30 
                 500 Ohms 
               
               
                   
                 Resistor 34, 44 
                 350 Ohms 
               
               
                   
                 Inductor 38 
                 15 nanoHenries 
               
               
                   
                   
               
             
          
         
       
     
     Upon application of a positive supply voltage, oscillator  10  operates as follows. Internal noise generated at the desired frequency of operation is amplified by transistor  20  and the output of transistor  20  is selectively fed back to the input of the same amplifier, i.e., transistor  20 , exactly in phase (in order to satisfy the Barkhausen criteria of positive feedback with 0° of phase shift around a closed loop to sustain oscillations). The frequency selective network in the feedback path is composed of resonator  12  (an inductor may be used as well), and capacitors  24  and  26 . Resistor  30  establishes the bias current amplitude over temperature and process variations when DC feedback is employed using transistor  28  as shown in FIG.  1 . The PNP topology allows resonator  12  to be placed directly to ground without any biasing resistor in parallel which might de-que the performance of the ensuing resonant circuit. The remaining components comprise the DC feedback to the oscillator amplifier and provide an output buffer stage. The DC feedback and output buffer stage are described in further detail below. 
     The loaded Q of oscillator  10  is maximized by avoiding any bias resistance across resonator  12 , i.e., resonator  12  is coupled directly to ground without an interceding bias resistor. The ability to avoid bias resistance is achieved by using a current bias circuit  50 , shown in FIG.  2 . The components comprising current bias circuit  50  are easily cross-referenced with oscillator  10  of FIG. 1, as like components use like item numbers. As such, current bias circuit  50 , generally comprises the components of resistors  30 ,  34 , and  44  and transistor  20  and  28 . By using DC feedback with transistor  28 , current bias circuit  50  can form a temperature independent bias current through transistor  20 . This can be shown by the following analysis of current bias circuit  50 . It should be noted that I C20  is the current through the collector of transistor  20 , as indicated, and that I C28  is the current through the collector of transistor  28 , as indicated. 
     Using standard circuit analysis techniques, I C28  may approximately be defined as follows (assuming β&gt;&gt;10):                I   C28     =         V   CC     -       I   C28          R   44       -     V   be     -     V   be         R   34               Eq   .                (   1   )                                  
     where: V be  is the base-emitter voltage for each transistor. 
     Combining common terms and solving for I C28  yields: 
      I C28 R 34 =V CC −I 28 R 44 −2V be   Eq. (2) 
     
       
         I C28 R 34 +I C28 R 44 =V CC −2V be   Eq. (3) 
       
     
     
       
         I C28 (R 34 +R 44 )=V CC −2V be   Eq. (4) 
       
     
     Finally, I C28  may be defined as:                I   C28     =         V   CC     -     2        V   be             R   34     +     R   44                 Eq   .                (   5   )                                  
     Next, using standard circuit analysis techniques, the following equation may be written for I C20 :                I   C20     =           I   C28          R   44       +     V   be         R   30               Eq   .                (   6   )                                  
     Substituting for I C28  yields:                I   C20     =           (         V   CC     -     2        V   be             R   34     +     R   44         )          R   44       +     V   be         R   30               Eq   .                (   7   )                                  
     Multiplying through yields:                      I   C20     =             R   44          V   CC           R   34     +     R   44         -       2        V   be          R   44           R   34     +     R   44         +     V   be         R   30                   =           R   44          V   CC       -     2        V   be          R   44       +       V   be          (       R   34     +     R   44       )             (       R   34     +     R   44       )          R   30                       Eq   .                (   8   )                                  
     Temperature compensation Of V be  for transistor  28  occurs when R 34 =R 44 , as such, Equation 8 may be rewritten as follows:                      I   C20     =             R   44          V   CC         2        R   44         -       2        V   be          R   44         2        R   44         +     V   be         R   30                   =           V   CC     2     -     V   be     +     V   be         R   30                     Eq   .                (   9   )                                  
     Then, eliminating terms and combining common terms, I C20  may be defined as follows:                I   C20     =       V   CC       2        R   30                 Eq   .                (   10   )                                  
     In view of equation 10, and referring once again to FIG. 1, it can be seen that the bias current through transistor  20  of oscillator  10  is set by the voltage, V CC , provided by voltage source  31  and by the value of resistor  30 ; any variance in the bias current that might have been caused by the base-emitter voltage of transistor  28  has been virtually eliminated by setting resistors  34  and  44  equal to each other and by presuming, as indicated above, that the common-emitter gain, β, is high. If β is not high, then I C20  is defined by the following equation:                      I   C20     =                  β       (     β   +   1     )     2       ·                              [                 V   CC          (         R   44          (     1   +   β     )       +     R   30       )       +                 V   be          [         (       R   34     -     R   44       )          (     β   +   1     )       -     2        R   30         ]                 R   30          (       R   44     +     R   34       )         ]                   Eq   .                (10a)                                  
     Additional benefit of using DC feedback with transistor  28  is that it forces the output impedance at the collector of transistor  20  to be extremely high, thus, loading resonator  12  even less. The output impedance at the collector of transistor  20  may be determined by referring to FIGS. 3 and 4. The circuit of FIG. 3 describes the constant current source of FIG.  2  and provides a feedback factor, F, where F is a voltage controlled current source, I 2 . Thus, F is defined as follows:              F   =         I   2       V   2       =       g   m2           R   1          g   m2       +   1                 Eq   .                (   11   )                                  
     A small signal approximation of the current source of FIG. 2 is provided by FIG.  4 . With reference to FIG. 4, equations for V 1 , V 2 , and V X , can be written as follows: 
     
       
         V 2 =I X R e   Eq. (12) 
       
     
     To determine V 1 : 
     
       
         V 1 +V 2 =−R 2 I 2   Eq. (13) 
       
     
     Substituting FV 2  for I 2  yields: 
     
       
         V 1 +V 2 =−R 2 FV 2   Eq. (14) 
       
     
     And, solving for V 1 : 
     
       
         V 1 =−R 2 FV 2 −V 2 =−V 2 (R 2 F+1)  Eq. (15) 
       
     
     Then, substituting for V 2 , V 1  is defined as follows: 
     
       
         V 1 =−I X R e (R 2 F+1)  Eq. (16) 
       
     
     V X  may be defined as follows: 
     
       
         V X =R O (I X −g m V 1 )+R e I X   Eq. (17) 
       
     
     Multiplying through by R O  yields: 
     
       
         V X =R O I X −g m V 1 R O +R e I X   Eq. (18) 
       
     
     Then, substituting for V 1 , V X  is defined as: 
     
       
         V X =R O I X +g m R O I X R e (R 2 F+1)+R e I X   Eq. (19) 
       
     
     Finally, the output impedance, Z O , may be defined as V X /I X , or: 
     
       
         Z O =R O +R e +g m R O R e (R 2 F+1)  Eq. (20) 
       
     
     Substituting for F, equation 20 may be rewritten as:                Z   O     =       R   O     +     R   e     +         g   m          R   O          R   e          R   2          g   m2             R   1          g   m2       +   1                 Eq   .                (   21   )                                  
     Thus, an approximation of output impedance may be written as:                Z   O     ≈       R   O     +     R   e     +         g   m1          R   O          R   e          R   2         R   1                 Eq   .                (   22   )                                  
     With respect to the item numbers of FIG. 2, the equation for output impedance is written as:                Z   O     ≈       R   CE20     +     R   30     +         g   m20          R   CE20          R   30          R   34         R   44                 Eq   .                (   23   )                                  
     Thus, in view of equation 23, it can be seen that the impedance at the collector of transistor  20  is very high. 
     Still another benefit of using DC feedback with transistor  28 , is that transistor  28  may now be used as a common-emitter output buffer. The emitter of transistor  28  is AC shorted to ground via capacitor  46  and the collector of transistor  28  has a tuned output due to inductor  38  and capacitor  40 , which still allows DC feedback to occur to transistor  20 . The top of the tuned output is then bypassed to AC ground via capacitor  36 . This network thus allows transistor  28  to have two functions in the oscillator circuit: (1) providing DC feedback to transistor  20  to allow constant output voltage, as described above; and, (2) providing a high gain output buffer to external loads. 
     The high gain output buffer provides isolation from resonator  12  since this stage samples the RF signal at a low impedance point (emitter of transistor  20 ). More specifically, the output buffer samples the oscillating signal by tapping into the feedback path of the oscillator at the emitter of transistor  20 . The base of transistor  28  has a high impedance input and will not load down the signal at this feedback point since this impedance is order of magnitudes larger than the low impedance presented by the emitter of transistor  20 . The combination of high gain and high input impedance provided by the buffer stage (realized with transistor  28 ) gives very high isolation to the oscillator stage from outside influences. This isolation substantially ensures that the frequency of oscillation is not dependent on external loads. Independence from external loads allows for minimal frequency pulling and constant output power since the biasing is virtually temperature independent. 
     An alternative embodiment of the present invention is depicted in FIG.  5 . Oscillator  100  of FIG. 5 is the dual of FIG. 1 where the component values are preferably the same as in FIG.  1  and the operation is substantially identical, however, NPN transistors are used in place of the PNP transistors of FIG.  1 . NPN transistors may be desirable for higher frequency performance and/or as a matter of topological preference for the location of resonator  12  (referenced to supply or ground). 
     The present invention may be embodied in other specific forms without departing from the essential attributes thereof; therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.