Ring oscillator using current mirror inverter stages

Abstract

A ring oscillator having an odd number of single ended stages, each stage including two transistors connected as a current mirror. The stage provides for low-voltage performance and improved process tolerance characteristics.

We claim:  
     
       1. A ring oscillator comprising: 
       a plurality of oscillator stages, each stage comprising first and second transistors, wherein the first transistor has a controllable path connected between an output node and a reference voltage and a control node acting as an input node to the stage and wherein the second transistor has a controllable path connected between the output node and the reference voltage and a control node connected to the output node, the gain of each stage being selectively determined by the ratio of the widths of the first and second transistors to produce an output signal having a sawtooth or trapezoidal waveform and each stage further comprising a respective current source which controls the speed of the stage and which is connected to said output node, wherein the input node of one stage is connected to the output node of a preceding stage to form said ring oscillator and wherein the number of stages is selected so that there is a total phase shift of 360° around the ring at the frequency of operation.  
     
     
       2. A ring oscillator according to claim  1 , wherein the first and second transistors are n-channel field effect devices having a gate as the control node and a source/drain path as the controllable path. 
     
     
       3. A ring oscillator according to claim  1 , wherein the first and second transistors are bipolar transistors in which the base is the control node and the controllable path extends between a collector and emitter. 
     
     
       4. A ring oscillator according to claim  1 , wherein the current source comprises a p-channel MOS field effect transistor gated by a control voltage. 
     
     
       5. A ring oscillator according to claim  2  wherein the current source comprises a p-channel MOS field effect transistor gated by a control voltage. 
     
     
       6. A ring oscillator according to claim  3  wherein the current source comprises a p-channel MOS field effect transistor gated by a control voltage. 
     
     
       7. A ring oscillator having improved process tolerance characteristics, said ring oscillator comprising: 
       a plurality of oscillator stages, each stage having a gain, a speed, and an operation frequency wherein an input node of one stage is coupled to an output node of a preceding stage to form a ring, and wherein the number of stages is selected so there is a total phase shift of 360° around the ring at the operation frequency, each stage including:  
       a first transistor having a control node, and a path controlled by the control node, the path coupling a reference voltage to the output node of said stage, wherein the control node is coupled to the input node of said stage;  
       a second transistor having a control node coupled to the output node of said stage and a controllable path which couples the reference voltage to the output node,  
       wherein the gain of said stage is selectively determined by the ratio of widths of said first transistor and said second transistor, and wherein an output signal of the stage is at least one of a sawtooth waveform and a trapezoidal waveform; and  
       a current source, which controls the speed of the stage, coupled to the output node.  
     
     
       8. An oscillator for producing a periodic waveform, the oscillator comprising: 
       
         a first, a middle, and a last serially coupled stage, each stage having an input terminal and an output terminal and the output terminal of the last stage coupled to the input terminal of the first stage, at least one stage including:  
       
       
         an input transistor coupled between the output terminal and a reference voltage, and having a control terminal coupled to the input terminal,  
       
       
         a second transistor coupled between the output terminal and the reference voltage, and having a control terminal coupled to the output terminal,  
       
       
         a current source coupled to the output terminal, and  
       
       
         wherein a ratio of the gain of the input transistor to the second transistor is greater than  2 . 
       
     
     
       9. The oscillator of claim  8  wherein the input and second transistors comprise respective MOS transistors and wherein a drain of the input transistor is coupled to the output terminal, a gate of the input transistor is coupled to the input terminal, and wherein both a drain and gate of the second transistor are coupled to the output terminal. 
     
     
       10. The oscillator of claim  9  wherein the input and second transistors have respective first and second widths, wherein the ratio of the first width to the second width is greater than  2 . 
     
     
       11. The oscillator of claim  8  wherein the input and second transistors comprise respective bipolar transistors and wherein a collector of the input transistor is coupled to the output terminal, a base of the input transistor is coupled to the input terminal, and wherein both a collector and base of the second transistor are coupled to the output terminal. 
     
     
       12. The oscillator of claim  11  wherein the ratio of the gain of the input transistor to the second transistor is a ratio of an area of the input transistor to the area of the second transistor. 
     
     
       13. The oscillator of claim  8  wherein the ratio is at least  2 . 5 . 
     
     
       14. The oscillator of claim  8  wherein the ratio is selected such that at least one of the stages produces a sawtooth waveform at its output terminal. 
     
     
       15. The oscillator of claim  8  wherein the ratio is selected such that at least one of the stages produces a trapezoidal waveform at its output terminal. 
     
     
       16. The oscillator of claim  8  wherein the current source comprises a PMOS transistor having a drain coupled to the output terminal and having a gate coupled to a control voltage. 
     
     
       17. The oscillator of claim  8  wherein the current source comprises a bipolar transistor having a base coupled to a control voltage. 
     
     
       18. The oscillator of claim  8  wherein the speed of the at least one stage is controlled by the current source. 
     
     
       19. An oscillator for producing a periodic waveform, the oscillator comprising: 
       
         at least three stages each having an input terminal and an output terminal, wherein the input terminal of each stage is coupled to the output terminal of another stage so as to constitute a ring, at least one stage including:  
       
       
         an input transistor coupled between the output terminal and a supply voltage, and having a control terminal coupled to the input terminal,  
       
       
         a second transistor coupled between the output terminal and the supply voltage, and having a control terminal coupled to the output terminal,  
       
       
         a current source coupled to the output terminal, and  
       
       
         wherein a gain of the at least one stage is approximately  2 . 
       
     
     
       20. The oscillator of claim  19  wherein the input and second transistors comprise respective MOS transistors and wherein a drain of the input transistor is coupled to the output terminal, a gate of the input transistor is coupled to the input terminal, and wherein both a drain and gate of the second transistor are coupled to the output terminal. 
     
     
       21. The oscillator of claim  20  wherein the input and second transistors have respective first and second widths, wherein the ratio of the first width to the second width is greater than  2 . 
     
     
       22. The oscillator of claim  19  wherein the input and second transistors comprise respective bipolar transistors and wherein a collector of the input transistor is coupled to the output terminal, a base of the input transistor is coupled to the input terminal, and wherein both a collector and base of the second transistor are coupled to the output terminal. 
     
     
       23. The oscillator of claim  22  wherein the ratio of area of the input transistor to the second transistor is greater than  2 . 
     
     
       24. The oscillator of claim  19  wherein the gain is selected such that at least one of the stages produces a sawtooth waveform at its output terminal. 
     
     
       25. The oscillator of claim  19  wherein the gain is selected such that at least one of the stages produces a trapezoidal waveform at its output terminal.