Abstract:
This invention discloses a crystal oscillator, in which by appropriately designing the gain of an amplifier to achieve high trans-conductance and low power consumption. This crystal oscillator includes a first pad, coupled to a first node of a crystal, for receiving a crystal oscillating signal outputted from the crystal; an amplifier, coupled to the first pad, for amplifying the crystal oscillating signal to generate an amplifying signal; an inverter, coupled to the amplifier, for inverting the amplifying signal; and a second pad, coupled to a second node of the crystal, for outputting an oscillating signal to the crystal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to an electronic circuit, particularly to a crystal oscillator. 
         [0003]    2. Description of the Related Art 
         [0004]    The traditional crystal oscillator, as shown in  FIG. 1 , usually uses a inverter  102  to generate an oscillating signal. The trans-conductance Gm of the inverter is 
         [0000]        Gm=Kn ×( Wn/Ln )×(0.5 Vdd−Vtn )+ Kp ×( Wp/Lp )×(0.5 Vdd−Vtp )  equation 1
 
         [0005]    In order to ensure that with all kinds of crystal Xtal, and a variety of circuit board load conditions, the crystal oscillator still be able to maintain a stable output. The trans-conductance Gm of the inverter is usually a large value. For example, trans-conductance Gm is greater than 5 mA/V. But in order to increase the inverter trans-conductance Gm in the traditional crystal oscillator, the aspect ratio (Wn/Ln, Wp/Lp) of the two NMOS and PMOS transistors of the inverter are increased respectively. And this makes the inverter to increase the power consumption. The power consumption of the inverter is: 
         [0000]        I= 0.5 ×Kn ×( Wn/Ln )×( Vdd−Vtn ) 2 ; when input= Vdd  
 
         [0000]        I= 0.5 ×Kp ×( Wp/Lp )×( Vdd−Vtn ) 2 ; when input= Vss   equation 2
 
         [0006]    Therefore, it can&#39;t both increase trans-conductance Gm and reduce power consumption in the traditional crystal oscillator. 
         [0007]    Furthermore, with the inverter consume greater power, it is easier to interference other circuits, specifically for those noise-sensitive circuits, such as analog circuits, radio frequency circuits. And, with the technology develops continuously to the high frequency, high speed, low operating voltage, and low power consumption. For example, digital-to-analog converter (DAC); another example, Ethernet develops from 10M through 100M, 1 G, 10 G, 40 G, and other more high speed development; another example, integrated circuit process develops from 0.5 process through 0.35, 0.25, 0.18, 0.15, and 0.09 processes, that shows the important and necessary of the components with low power consumption, the less interference with other circuits. 
       SUMMARY OF THE INVENTION 
       [0008]    One object of the present invention is to provide a crystal oscillator with reducing the power consumption and the less interference with other circuits. 
         [0009]    The present invention discloses a crystal oscillator, in which by a crystal to generate an output oscillating signal to an IC. The crystal oscillator comprises a first pad, coupled to a first node of a crystal, for receiving a crystal oscillating signal outputted from the crystal; an amplifier, coupled to the first pad, for amplifying the crystal oscillating signal to generate an amplifying signal; an inverter, coupled to the amplifier, for inverting the amplifying signal; and a second pad, coupled to a second node of the crystal, for outputting an oscillating signal to the crystal. Wherein, the amplifier has a first power consumption, the inverter has a second power consumption, the first power consumption is smaller than the second power consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a schematic diagram illustrating a crystal oscillator according to the prior art; 
           [0011]      FIG. 2  shows a schematic diagram illustrating a crystal oscillator to the first embodiment of the present invention; 
           [0012]      FIG. 3  shows a schematic diagram illustrating a crystal oscillator according to the second embodiment of the present invention; 
           [0013]      FIG. 4  shows a schematic diagram illustrating a crystal oscillator according to the third embodiment of the present invention. 
           [0014]      FIG. 5  shows a schematic diagram illustrating a crystal oscillator according to the forth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Please refer to  FIG. 2 , which shows a schematic diagram illustrating a crystal oscillator of the present invention. The crystal oscillator  200  comprises pads  202 ,  204 , an amplifier  206 , and an inverter  208 . The pad  202  couples to a first node of a crystal Xtal to receive a crystal oscillating signal S 1  outputted by the crystal Xtal. The amplifier  206  amplified the crystal oscillating signal S 1  and generated an amplifying signal S 2  to the inverter  208 . Then, the inverter  208  inverts the amplifying signal S 2  and generates an output oscillating signal S 3  to the pad  204 , so that the output oscillating signal S 3  feedbacks to a second node of the crystal Xtal to form a completed feedback system. 
         [0016]    As the circuit structure shown in  FIG. 2 , the invention adds an amplifier  206  between the pad  202  and the inverter  208 , and the gain of the amplifier  206  is A. Therefore, after adding the amplifier  206 , the trans-conductance Gm of the crystal oscillator  200  is: 
         [0000]        Gm=A×Kn ×( Wn/Ln )×(0.5 Vdd−Vtn )+ A×Kp ×( Wp/Lp )×(0.5 Vdd−Vtp )  equation 3
 
         [0017]    Wherein, Kn, Kp are trans-conductance coefficients, Wn/Ln is aspect ratio of the transistor  210 , Wp/Lp is aspect ratio of the transistor  212 , Vtn is threshold voltage of the transistor  212 , Vtp is threshold voltage of the transistor  210 , and Vdd is an operating voltage. 
         [0018]    However, after adding the amplifier  206 , the power consumption of the inverter  208  as shown in following: 
         [0000]        I= 0.5 ×Kn ×( Wn/Ln )×( Vdd−Vtn ) 2 ; when input= Vdd   equation 4
 
         [0000]        I= 0.5 ×Kp ×( Wp/Lp )×( Vdd−Vtn ) 2 ; when input= Vss   equation 5
 
         [0000]    Where, Vss is an operating voltage. 
         [0019]    According to the above equation, it is understood that the trans-conductance Gm of the crystal oscillator  200  is associated with gain A of the amplifier  206 , the amplifier  206  is associated with a multiplication of aspect ratio of the transistors  210  and  212  of the inverter  208 . Therefore, the present invention increases the trans-conductance Gm to ensure starting oscillation without increasing the overall power consumption by designing larger gain A. 
         [0020]    In other words, conventional crystal oscillator need to design a very large aspect ratio of the transistors  110  and  112  for getting a larger trans-conductance Gm to ensure oscillating. However, when the aspect ratio of the transistors  110  and  112  are designed larger, the overall power consumption of the transistors  110  and  112  will increase, and the bouncing noise of the operating voltage Vdd and Vss will increase as well. Therefore, this invention designs an amplifier  206  in front of the inverter  208  and amplifies the crystal oscillating signal S 1  beforehand. Thus, the aspect ratio of the transistors  210  and  212  can be smaller, the power consumption will decrease and have enough trans-conductance Gm to ensure that the circuit can oscillate. 
         [0021]    Moreover, in order to get better power utility rate and regard to the trans-conductance Gm, the power consumption of the amplifier  206  and the inverter  208  can be properly designed. In other words, the power consumption of the amplifier  206  and the inverter  208  are different. For example, when the power consumption of the amplifier  206  is designed to 1/10˜ 1/100 of the inverter  208 , the crystal oscillator  200  can get larger trans-conductance Gm and lower power consumption. But it is not limited to the present invention. 
         [0022]    Furthermore, according to an embodiment of the present invention, a feedback resistance Rf can be set between the input and output terminals of the inverter  208  to make the crystal oscillator  200  easier to start oscillating. Please refer to  FIG. 3 , which shows the approach of coupling the feedback resistance Rf. 
         [0023]    Next, please refer to  FIGS. 4 and 5 ,  FIG. 4  shows an embodiment of the present invention of the crystal oscillator. The crystal oscillator  300  comprises pads  202 ,  204 , an amplifier  306 , an inverter  208 , and feedback resistances Rf 2 , Rf 3 . The difference between this embodiment and foregoing embodiment is that the amplifier  306  is a single-to-double-ended amplifier, the details of the circuit as shown in  FIG. 5 . The amplifier  306  comprises a NMOS transistors  308 ,  314  and PMOS transistors  310 ,  312 . Wherein, the gate of the NMOS transistors  308  receives the crystal oscillating signal S 1 , the source of the NMOS transistor  308  couples to the operating voltage Vss. The drain of PMOS transistor  310  couples to the drain of the NMOS transistor  308 , the source of PMOS transistor  310  couples to the operating voltage Vdd. The gate of the PMOS transistor  312  couples to the gate of the PMOS transistor  310 , the source of the PMOS transistor  312  couples to the operating voltage Vdd. The drain of the NMOS transistor  314  couples to the drain of the PMOS transistor  312 , the source of the NMOS transistor  314  couples to the operating voltage Vss. Wherein, a current mirror circuit is formed by the PMOS transistors  310 ,  312 . 
         [0024]    In the present embodiment, the NMOS transistor  308  of the amplifier  306  amplifies the crystal oscillating signal S 1  at first, and outputs the amplified signal S 21 , S 22  to the gates of transistors  210 , 212  through current mirror as a load. Finally, the transistors  210 ,  212  generate the output oscillating signal S 3  to pad  204  so that form a feedback system. Similar to the previous embodiment, circuit designers can appropriately design the aspect ratio of the NMOS transistors  308 ,  314  and the PMOS transistors  310 ,  312  to determine that the gain A and power consumption of the amplifier  306 . For example, gain A can be designed as 20 to 100, the power consumption can be designed as less than inverter  208  as well, and then the crystal oscillator  300  can be made as a high trans-conductance Gm with low power consumption characteristic. Moreover, in the present embodiment, setting feedback resistances Rf 2 , Rf 3  between input and output terminals of inverter  208  can start oscillating of crystal oscillator  200  more easier. 
         [0025]    Otherwise, the crystal oscillator of the present invention can be applied to various products, for example: Wired network, wireless network, monitor, TV . . . etc., the present invention should not be limited to the specific construction and arrangement. The present invention can also be applied to other related electronic products. 
         [0026]    In view of foregoing, it is clearly understood that the crystal oscillator of the present invention sets an amplifier in front of an inverter. The amplifier provides a gain A, and the trans-conductance Gm of the crystal oscillator is associated with the gain A. Furthermore, since the amplifier provides a gain of A which multiply with the aspect ratio of the transistors. Therefore, the present invention increases the trans-conductance Gm by designing a larger gain A to initiate oscillating without consuming overall power so as to solve some problems of conventional crystal oscillator.