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.

Description:
FIELD OF THE INVENTION 
     This invention relates to an oscillator and more particularly to a ring oscillator. 
     BACKGROUND OF THE INVENTION 
     New manufacturing processes and new applications are forcing power supplies to lower voltages (3.3 v now, with 2.4 v and 1.5 v being expected soon). Advanced Phase-Locked Loops require stable oscillators which may be varied in frequency by a control signal. 
     To help achieve frequency stability, oscillators integrated into a noisy VLSI environment often use a regulator to generate a quiet power supply. This usually has to be at an even lower voltage than the normal power supply. 
     There is thus a desire to provide oscillators which can work at these very low supply voltages and still produce high quality, high frequency output signals. 
     Reference is made to IBM Technical Disclosure Bulletin, Vol. 31, No. 2, July 1988, pages 154 to 156 “CMOS Ring Oscillator with controlled frequency” which describes a ring oscillator using CMOS transistors and is designed to give an almost sinusoidal output. This design suffers from stability problems outside a narrow range of frequencies. In particular, as the frequency increases, the amplitude decreases and it becomes difficult to convert the signal to CMOS levels. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a ring oscillator comprising a plurality of oscillator stages, each stage comprising a first and second transistors. 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. 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 is 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. Each stage further comprise a respective current source which controls the speed of the stage and which is connected to the output node. The input node of one stage is connected to the output node of a preceding stage to form a ring and the number of stages is selected so that there is a total phase shift of 360° around the ring at the frequency of operation. 
     For transistors of the same length, the width of the first transistor can be set to m times the width of the second transistor where m&gt;1 to determine the d.c. gain of the stage. This ratio m determines the shape of the waveform output by the oscillator. The higher the value of m, the more the waveform moves away from a sinusoid. For a three stage oscillator, a ratio of m close to 2 produces a substantially sinusoidal output. The present invention uses a ratio higher than 2 and preferably with a minimum value of 2.5. In practice the smallest value that can be selected to provide an appropriately shaped waveform will be selected. The maximum value of m is limited by practical considerations and particularly layout considerations. A practical maximum value for m is likely to be about 10. 
     The first and second transistors can be n-channel field effect devices having a gate as the control node and the source-drain path as the controllable path. As the transistors are of the same type, process variations affect the transistors in the same manner. The maximum frequency of operation is limited only by the ratio of gain to gate capacitance. 
     The current source can comprise a p-channel transistor gated by a control voltage. 
     The first transistor is preferably operated in its saturation region. 
     The current sources of each stage can either be controlled by a common control signal or by respective different control signals. 
     The present oscillator can operate at voltages down to a level just above the threshold voltages of the transistors. 
     For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a low-voltage inverting gain stage in MOS technology; 
     FIG. 1a is a circuit diagram of an implementation of a current source; 
     FIG. 2 is a circuit diagram of a low-voltage inverting gain stage in bipolar technology; 
     FIG. 3 is a diagram showing the transistor structure of a ring oscillator; 
     FIG. 4 is an equivalent logical schematic for FIG. 3; and 
     FIG. 5 shows typical waveforms for the 3-stage ring oscillator of FIGS.  3  and  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a low-voltage inverting gain stage in MOS technology. The stage comprises first and second transistors T 1 , T 2  which have their drains connected together and their sources connected to ground. The gate of the first transistor T 1  acts as the input S in  for the stage and the gate of the second transistor T 2  acts as the output S out . The gate of the second transistor T 2  is connected to its drain. Each stage is controlled by a control current I which is generated by a current source  2 . The current source  2  is connected between a supply voltage Vcc and the drains of the first and second transistors T 1 ,T 2 . The common node between the current source  2  and the drains of the transistors T 1  and T 2  is denoted  4 . As shown in FIG. 1a, the current source  2  can comprise a p-channel MOS field effect transistor T 3  with its source/drain path connected between the supply voltage Vcc and the node  4  and its gate connected to receive a control signal V which is taken with respect to the supply voltage Vcc. In the following discussion, it will readily be apparent that where reference is made to the control current I, this can be taken in practice as being derived from the control voltage V. The stage also has capacitance C, the largest component of which is the gate capacitance of the transistors connected to the output S out . 
     The ratio of gains of the transistors T 1 ,T 2  is indicated as “m”. The value of m controls the relative charge and discharge rates of the output mode S out , and thus determines the gain of the stage. The speed of the stage (and thus the phaseshift at the frequency of operation) is readily controlled by varying the current I supplied by the current source  2 . 
     FIG. 2 shows the low-voltage inverting gain stage in bipolar technology. This also has excellent low-voltage operation characteristics and the speed can be controlled using a current source  2  in precisely the same way. Although the rest of this specification refers to MOS circuits, it should be understood that the same idea can easily be applied to bipolar technology. 
     In FIG. 2, the first and second transistors are denoted Ti′ and T 2 ′ and are connected in the same way as for FIG. 1, where gates correspond to bases, drains correspond to collectors and sources correspond to emitters. 
     FIG. 3 illustrates a 3-stage ring oscillator, the three stages being denoted S 1 ,S 2 ,S 3 . Each stage S 1 ,S 2 ,S 3  is as illustrated in FIG.  1 . Of course, a similar ring oscillator could be produced using the stages of FIG.  2 . FIG. 4 shows the ring oscillator in an equivalent logical schematic. Each stage is a so-called single-ended stage, that is with a single input and a single output and is inverting. As is well known in the design of ring oscillators, for oscillation to occur it can be shown that there must be: 
     (i) an odd number n of stages 
     (ii) minimum of three stages 
     (iii) if all stages are identical and have a gain ratio of “m”, then 
     
       
         m&gt;1/cos(pi/n)  
       
     
     where 
     pi=3.14 . . . 
     n=number of stages 
     and 
     m=gain of each stage 
     For a 3-stage ring, the formula above gives m&gt;2. 
     Where the transistors are of the same length, the gain m=W(T 1 )/W(T 2 ), where W is the width of a transistor. 
     Thus, by use of an appropriate layout, the parameter m can be made substantially independent of manufacturing process variables which would tend to affect the width of both transistors by corresponding amounts. 
     The required value for m, and hence the transistor sizes, is selected to satisfy small signal and large signal design requirements to provide a sawtooth or trapezoidal waveform. A system designed to produce these waveforms produces a more stable output amplitude from the oscillator across all operating frequencies. A more stable amplitude over a wide range of operating frequencies provides a signal which can be more reliably and easily converted to CMOS levels over a wide range of frequencies. 
     FIG. 5 shows the waveforms for the 3-stage oscillator of FIG. 4 when m=3. Node  1 , node  2  and node  3  are denoted N 1 , N 2  and N 3  in FIG.  4 . 
     The frequency of oscillation of the ring can be controlled by the control current I. In a symmetrical arrangement, each stage has the same phase shift at the frequency of operation (equal to 180°/n for inverting stages) and receives a common control signal so that the control currents I are the same. However, the phase shift can differ for each stage provided that the complete phase shift in the loop is 360° at the frequency of oscillation. In this case, the control currents I for the individual stages can be independently varied.