GB2084421A

GB2084421A - Oscillator Circuit With Low Current Consumption

Info

Publication number: GB2084421A
Application number: GB8128089A
Authority: GB
Original assignee: Ebauches Electroniques SA
Current assignee: Ebauches Electroniques SA
Priority date: 1980-09-19
Filing date: 1981-09-17
Publication date: 1982-04-07
Also published as: DE3136300A1; DE3136300C2; CH641316GA3; JPS6234281B2; CH641316B; JPS5783904A; GB2084421B; FR2490895B1; FR2490895A1

Abstract

The oscillator circuit comprises an active amplifier transistor 1 which is supplied by a current source comprising a second active amplifier transistor 13 whose control electrode 13a is connected to the control electrode 1b of the first transistor by way of capacitive decoupling means 22, 23. The first and second transistors 1, 13 are of opposite conductivity types. Bias circuits 4, 14 apply to the control electrode of the second transistor a control signal 13a which contains the oscillation signal VA which is superimposed on a d.c. voltage which depends on the amplitude of the oscillation signal. This circuit can be used in particular for the time bases of electronic watches. <IMAGE>

Description

SPECIFICATION An Oscillator Circuit with Low Current Consumption The present invention relates to an oscillator circuit of the type wherein the oscillations of a resonator are maintained by the circuit. The resonator may be a quartz crystal and the circuit is of particular utility as the time base of an electronic quartz watch.

The CMOS oscillator circuit which is most widely used in timepieces at the present time is the circuit disclosed in a French Patent published under the number 2 110 109. In that known circuit, the active element comprises an inverter which is supplied by a direct current source. A bias resistor of sufficiently high value (more than 1 0M Ohm) is connected between the output and the input of the inverter, in parallel with the quartz resonator. Two capacitors, one of which is variable to permit adjustment of the oscillation frequency, are connected between one terminal of the supply voltage source and the input and the output respectively of the inverter.

This known oscillator circuit is very simple but it is not satisfactory as regards its consumption of electric current and variability of the frequency of oscillation in dependence on other parameters.

This is because, in the normal operating mode; the resonator and the oscillator circuit itself are over-excited. One must consider the most unfavourable conditions, when deciding the values of the components of the circuit, having regard to the operating parameters such as supply voltage, impedance of the resonator, and charge capacitance. In consequence the slope of the known oscillator circuit is too high in the normal operating mode, resulting in over-excitation. In addition, in order for triggering of the oscillation to occur, the two transistors of the inverter must be conductive at the same time and, in the known circuit, this requires a supply voltage which is higher than the sum of the threshold voltages of the two transistors.

It has already been proposed that the slope of the oscillator circuit can be reduced by using voltage regulators or resistors. However, these means are still unsatisfactory.

In order to arrive at a satisfactory result, it would be necessary to design an oscillator circuit in which the bias current of the active element is automatically regulated so as to produce the lowest possible oscillation amplitude allowing the driving of a circuit such as an amplifier or a frequency divider which may be connected thereto.

An oscillator circuit of this kind which is described for example in Swiss patent No 580 358 is diagrammatically shown in Figure 1.

The known circuit comprises a p-channel MOS transistor 1 which is biased by a resistor 2 connected between its drain 1 a and its gate 1 b so that the mean potential of the gate 1 b is equal to that of the drain 1 a. A current source 3 is connected in series with the drain-source path of the transistor 1, between the terminals of a supply voltage source, as indicated at +V, OV.

The current source 3 which comprises an nchannel MOS transistor whose controlled current path is connected in series with the transistor 1 between the terminals +V and OV applies to the transistor 1 a mean drain current whose value is just greater than the critical value of triggering the oscillation. A regulating circuit 4 is connected between an input terminal 5 of the oscillator and a control terminal 3a of the current source 3; by regulating the current source 3 in dependence on the amplitude of the oscillation signal of the resonator 6, the circuit 4 makes it possible to stabilise the current flowing in the drain-source path of the transistor 1.

When the transistor 1 is operating in a low inversion mode, the slope of the oscillator is given by: lo 9m=- Vc in which lo is the current provided by the source 3 and Vc is a characteristic voltage of the transistor 1 , of a typical value of 50 mV, which characteristic voltage remains approximately constant for a given kind of technology (see Vittoz's paper, IEEF Journal of Solid-State Circuits, Vol SC-12 No 3 June 1977, pp 224231).

As will be seen from Figure 1, the known oscillator circuit naturally also includes a quartz resonator 6 which is connected between the input terminal 5 of the oscillator and an output terminal 7; the bias resistor 2 is connected between the input terminal 5 and the output terminal 7; the input terminal 5 is connected to the gate 1 b of the transistor 1; the output terminal 7 is connected to the drain 1 a of the transistor 1; and oscillator circuit also comprises two capacitors 8a and 8b which are each connected between the terminal OV of the supply voltage source and the input terminal 5 and the output terminal 7 respectively of the oscillator.

Although such a circuit permits a very low level of current consumption, it nonetheless suffers from the disadvantage of operating in class A mode. It is well known that the efficiency of class A amplifier is low.

Swiss patent application which are published under No 15 657/77 describes another known oscillator, which is shown in Figure 2. In Figure 2, the components which are identical or similar to those shown in Figure 1 are denoted by identical references. This known oscillator comprises a p channel MOS transistor 9 and an n-channel MOS transistor 10, the transistors 9 and 10 being arranged as an inverter with a common bias resistor 11. A current source 3 is connected in parallel with a filter capacitor 12 between the source of the transistor 10 and the terminal OV of the supply voltage source. The current 3 fixes the current flowing in the inverter 9 to 11 at a value which is just sufficient to permit triggering of the oscillations. The current source 3 is also controlled by a regulator 4 in dependence on the amplitude of the oscillation signal of the resonator 6.

By virtue of the capacitor 12 which shortcircuits any alternating component in the oscillation voltage, the supply voltage of the inverter may be considered as constant within an oscillation period. However, the supply voltage at the terminals of the inverter 9, 10, 11 is adapted to a value such that the current consumption of the oscillator circuit is identical to the value of the current supplied by the current source 3.

This known circuit consumes even less current than that shown in Figure 1, but it suffers from two serious disadvantages. As in the case of the inverter circuit described hereinbefore, the supply voltage must be greater than the sum of the threshold voltages of the two transistor 9 and 10.

Moreover, the filter capacitor 1 2 takes up a substantial amount of space on the integrated circuit.

The object of the present invention is in particular to provide an oscillator circuit which combines the advantages of the known circuits, as referred to above, without suffering from the disadvantages thereof.

According to the invention, there is provided an oscillator circuit for sustaining the oscillation signal of a resonator, comprising an input terminal, an output terminal, supply terminals, a first active element, means for biasing the first active element, a second active element whose controlled current path is connected in series with that of the first active element between the supply terminals, an input capacitor and an output capacitor which are connected between one supply terminal and the input terminal and the output terminal respectively, and bias regulating means responsive to the signal present at the input terminal and to the amplitude of the oscillation signal of the resonator to apply control signals to the control electrodes of the first and second active elements, each said control signal consisting of the input signal superimposed on a d.c. signal having a voltage which varies in dependence on the amplitude of the oscillation signal.

Thus, the two active elements operate in an amplifier mode and may each operate in a class C mode, which gives a substantial reduction in the current consumption of the oscillator.

In addition, the first and the second active elements can operate in such a way that, when the first element is conductive, the second element is non-conducting, thereby preventing a direct flow of current through the active elements from one terminal of the power source to the other.

In addition, the above-mentioned d.c. voltage signal can be so selected, in respect of each active element, that the current flowing through that active element when it is conductive is of substantially the minimum value for permitting the oscillations of the resonator to be sustained.

The features and advantages of the oscillator circuit according to the present invention will be better appreciated from the following description of an embodiment, given by way of example, with reference to the accompanying drawings in which: Figures 1 and 2, which have already been described, show circuit diagrams of the oscillator circuits in accordance with two prior art specifications, Figure 3 shows the diagram of a particular embodiment of a sustaining circuit according to the invention, and Figure 4 shows the characteristic in respect of drain current/potential difference between gate and source, of the second active element, and two time diagrams in respect of the control signal applied to that active element.

Just like the circuit shown in Figures 1 and 2, the circuit illustrated in Figure 3 comprises a quartz resonator 6, an input terminal 5, an output terminal 7, a first capacitor 8a which is connected between the input terminal 5 and one of the terminals OV of the electrical power supply source, a second capacitor 8b which is connected between the output terminal 7 of the oscillator and the terminal OV, and a p-channel MOStransistor 1 with a drain 1 a connected to the output terminal 7 of the oscillator and a gate 1 b connected to the input terminal 5 of the oscillator.

A second, n-channel MOS-transistor 13 is connected in series with the source-drain path of the first MOS-transistor between the terminals +V and OV of the electrical supply source, and a regulator 4 receives the oscillation signal VA at the input terminal 5 of the oscillator.

However, in the circuit shown in Figure 3, interposed between the gate 1 3a of the second transistor 1 3 and the regulator 4 is an intermediate circuit 14 which responds to the signal supplied by the regulator 4 and to the signal VA which occurs at the terminal 5, to supply a control signal 1 5 which contains the alternating oscillation signal VA which is superimposed on a d.c. voltage VC, the value of which depends on the amplitude A of the oscillation signal.

The drain 1 3b of the transistor 13 is connected to the drain 1 a of the transistor 1, and its source 1 3c is connected to the terminal OV of the supply source. Since the source 1 C of the transistor 1 is connected to the terminal +V of the electrical power source, the controlled current paths of the transistors 1 and 1 3 are connected in series between the terminals +V and OV of the electrical power source.

The intermediate circuit 1 4 comprises a current source transistor 17, a further n-channel MOS-transistor 1 8 whose drain-source path is connected in series with the current source 17, a bias resistor 1 9 connected between the gate 1 8a and the drain 1 8b of the transistor 18, and a filter capacitor 20 connected in parallel with the drainsource path of the transistor 1 8 between the junction point 21 of the resistor 1 9 and the drain 1 8h, and the terminal OV of the electrical power source. The gate 1 8a of the transistor 1 8 is connected to the gate 1 3a of the transistor 1 3 and the source 1 8c of the transistor 18 is connected to the terminal OV.The transistors 13 and 1 8 are thus disposed in a current mirror relationship so that the current iflowing through the transistor 13, in the equilibrium condition, remains proportional to the current supplied by the current source 1 7.

The gates 1 b and 1 3a of the transistors 1 and 1 3 are connected to the input terminal 5 of the oscillator by way of respective capacitors 22 and 23. The quartz resonator 6 is connected between the input terminal 5 of the output terminal 7.

The regulator circuit 4 which receives the input signal VA at the input terminal of the oscillator controls the current supplied by the current source 1 7 in such a way as to stabilise and minimise the current consumed by the oscillator.

The regulating circuit 4 shown in Figure 3 is similar to the amplitude regulator of the oscillator shown in Figure 15 on page 139 of the Conference Report by E. A. Vittoz "Quartz Oscillator for Watches" published in the transactions of the dixieme congres international de chronometrie ("Tenth International Chronometry Congress"), Geneva, September 1979, volume 3, pages 131-140.

In that construction, the regulator 4 comprises a first pair of complementary transistors 24 and 25 having a node common to the drains and being connected by their sources to the corresponding terminals +V and OV of the supply voltage source. The gate of the p-channel transistors 25 is connected to its drain and to the gate 1 7a of the transistor 1 7.

The regulator 4 comprises a second pair of complementary transistors 26 and 27 which also have a node common to the drains, and which are connected by their sources to the corresponding terminals +V and OV of the voltage source. The gate of the n-channel transistor 24 is connected on the one hand to the gate of the n-channel transistor 26 by way of a resistor 28 and on the other hand to the terminal OV by way of a capacitor 29. The source of the transistor 24 is connected to the terminal OV by way of a resistor 30. The gate of the transistor 26 is connected to its drain by way of a resistor 31 and also to the input terminal 5 by way of a capacitor 32. Finally, the drain of the transistor 26 is connected to the terminal OV by way of a capacitor 33.

The mode of operation of the circuit shown in Figure 3 is as follows. In the absence of oscillation, the operating point of the transistor 1 8 is established by the current supplied by the current source 17. The voltages at the drain 1 8b and the gate 1 8a of the transistor are given by the characteristic in respect of gate voltage in dependence on the drain current of the transistor 1 8. Likewise, the operating point of the transistor 1 is determined by the current iflowing in the drain-source path of the transistor 13, that current being proportional to the current supplied by the source 17.

When the oscillation is triggered, an alternating voltage VA is superimposed on a d.c. voltage VC on the gate 1 8a of the transistor 1 8. In proportion as the amplitude A of the signal VA increases, because of the non-linearity of the characteristic of the transistor 1 8, the mean current through the transistor 1 8 tends to become greater than the current supplied by the source 17, which obliges the capacitor 20 to discharge and cause a reduction in the voltage at the terminals of the capacitor 20.The value of the capacitor 20 is advantageously so selected that the voltage at the terminals of the capacitor remains approximately constant for each period of the alternating voltage signal VA, in order to ensure that the transistor 1 8 operates in a saturation condition, the peak-to-peak amplitude of the alternating voltage VA being, for the type of oscillator involved here, less than the threshold voltage of the transistors that it comprises.

As the mean value VC of the voltage at the gate 1 8a of the transistor 1 8 must remain equal to the voltage at the terminals of the capacitor 20, a current flows into the resistor 1 9 until the mean current flowing in the drain-source path of the transistor 1 8 again becomes equal to the current supplied by the source 1 7. The operating point of the transistor 1 8 is therefore shifted in dependence on the amplitude A of the alternating voltage VA at the input terminal 5 of the oscillator, and in dependence on the value of the current supplied by the source 1 7. The result of this is that the mean value of the voltage VC falls in dependence on amplitude A when this amplitude increases.

By suitable dimensioning of the transistor 18, it is arranged that, in the absence of oscillation, the above-mentioned mean voltage has a value VCO which is substantially equal to the threshold voltage VT of the transistor 1 3 (see Figure 4).

It can be seen that the mean value V1 C of the voltage applied to the gate 1 t of the transistor 1 is an increasing function of the amplitude A of the alternating voltage VA. By suitable dimensioning of the transistors 1 and 1 3, it is also arranged that the value V1C has when A=O a value V1 Co which is substantially equal to the threshold voltage V'T of the transistor 1. Therefore, the latter transistor 1 receives at its gate 1 b a control signal 100 formed by the signal VA superimposed on a d.c.

voltage signal V1 C rising from a value V1 CO (for A=O) which is substantially equal to the threshold voltage V'T of the transistor 1.

Thus, when the oscillator starts operating, the transistors 1 and 13 amplify substantially only the negative and positive half-cycles respectively of the signal VA, while, during operation of the oscillator, the transistors 1 and 13 amplify only the negative and positive peaks respectively of the signal VA.

In Figure 4, the hatching illustrates the signal portions 1 5 amplified by the transistor 1 3 whose characteristic ID=f(VgVs) is illustrated, at 34.

The parts of the signal VA, which the transistor 1 will amplify, are substantially similar to but of opposite polarity to the hatched parts shown in Figure 4.

The transistors 1 and 13 therefore form, as it were, a push-pull stage which, under normal conditions of oscillation, operates in class C. Thus, the current does not flow simultaneously through the transistors 1 and 13, thereby avoiding an excessive current consumption.

In operation, current supplied by the source +V, OV passes into the transistor 1 only during a small part of the negative half-cycle of the signal VA, that current serving to charge the output capacitor 8b; during the positive half-cycles of the signal VA, the output capacitor 8b is discharged to the terminal OV through the conductive transistor 13.

The sustaining circuit which is thus provided therefore operates in class C after a short starting phase substantially in class B. By making the transistors 1, 1 3 and 1 8 of such a size that the mean current supplied by the source 17 is approximately equal to 1 0% of the mean current flowing in the transistor 13, the current consumption of the oscillator circuit is barely higher than that of the circuit shown in Figure 2.

Moreover, in comparison with the circuit shown in Figure 2, the additional elements, transistor 18, capacitors 20, 22 and 23 and bias resistor 19, take up much less space than the filter capacitor 12 used in the circuit in Figure 2; in addition, the added elements are totally compatible with the technology used for producing a circuit as shown in Figure 1 or Figure 2.

The bias resistors 2 and 19 associated with the transistors 1 and 1 8 may comprise for example diodes formed by side junctions produced in polycrystalline silicon in the manner described in the above-mentioned Report by E. A. Vittoz. This is appropriate in a technology employing high doped poly-silicon to form the FET gates.

Alternatively, the bias resistors may comprise systems of MOS transistors.

In addition, the circuit according to the invention operates with a supply voltage which is slightly higher than a single MOS-transistor threshold voltage, as there are no transistor in series with gates connected in a continuous mode.

It will be appreciated that the control means formed by the circuits 4 and 14 could be replaced by any other circuit which responds to the amplitude A of the oscillation signal of the resonator 6, and to the signal VA occurring at the input terminal 5, producing a control signal formed by a d.c. voltage signal VC whose value falls in dependence on said amplitude A, being superposed on the signal VA.

In addition, the invention is not limited to CMOS-type sustaining circuits. In particular, the two amplifier MOS-transistors 1 and 13 could be of the same conductivity type. In this case, the circuit uses d.c. voltage signals VC and V1 C which both vary in a rising or falling manner, in dependence on the amplitude A, depending on whether they are p-channel or n-channel transistors respectively.

Claims

1. An oscillator circuit for sustaining the oscillation signal of a resonator, comprising an input terminal, an output terminal, supply terminals, a first active element, means for biasing the first active element, a second active element whose controlled current path is connected in series with that of the first active element between the supply terminals, an input capacitor and an output capacitor which are connected between one supply terminal and the input terminal and the output terminal respectively, and bias regulating means responsive to the signal present at the input terminal and to the amplitude of the oscillation signal of the resonator to apply control signals to the control electrodes of the first and second active elements, each said control signal consisting of the input signal superimposed on a d.c. signal having a voltage which varies in dependence on the amplitude of the oscillation signal.

2. A circuit according to claim 1, wherein the bias regulating means comprise a current source, a third active element with a controlled current path connected in series with the current source, a control electrode connected to the control electrode of the second active element, and a controlled current path electrode opposite to the current source connected to the same terminal of the supply voltage source as the corresponding electrode of the second active element, the third active element being of the same conductivity type as the second active element, means for biasing the third active element, and a capacitor connected in parallel with the controlled current path of the third active element.

3. A circuit according to claim 2, wherein the bias regulating means further comprise a regulator circuit which reacts to the amplitude of the oscillation signal of the resonator to control the current supplied by the current source in dependence on the said amplitude.

4. A circuit according to claim 2 or 3, wherein the first and second active elements are MOStransistors of opposite conductivity types, and wherein the transistors are produced, with the third active element (18), the current source and the capacitors in the form of a CMOS integrated circuit.

5. A circuit according to claim 2, 3 or 4, wherein the active devices are transistors with highly doped polycrystalline silicon gates, and wherein the means for biasing the first and/or the third active elements comprise a resistor connected between the gate and the drain of that element, the resistor comprising diodes formed by side junctions produced in the polycrystalline silicon.

6. A circuit according to claim 2, 3 or 4, wherein the means for biasing the first and/or the third active element comprise systems of transistors.

7. An oscillator circuit substantially as hereinbefore described with reference to and as illustrated in Fig. 3 of the accompanying drawings.