Output stage

An output stage, comprising a first transistor operable to pull a voltage at an output node towards a first voltage, and a rechargeable energy store having a potential difference between first and second terminals wherein the rechargeable energy store is arranged to be controllably connected between the output node and a second voltage supply such that the voltage at the output node can be driven to a voltage outside of a range defined between the first and second voltages.

FIELD OF THE INVENTION

The present invention relates to an output stage and an amplifier including such an output stage.

BACKGROUND OF THE INVENTION

It is known that the class B or class AB output stage using complementary push-pull transistors can be used to drive a load.FIG. 1schematically illustrates such an output stage where a preamplifier, generally designated2, generates drive signals to control first and second transistors4and8in a complementary manner such that the first transistor4is driven to be conducting during positive half cycles so as to pull the voltage at an output node6up towards the positive supply, +V, whereas a second transistor8is driven conducting during the negative half cycles so as to pull the voltage back to node6down towards the negative supply rail V−. In battery powered equipment the potential difference between the positive supply rail, +V, and the negative supply rail, which in practice is a local ground, can be quite small. Thus, in the context of hearing aids, headsets, music players or mobile telephones the voltage range might be as little as 1.5 volts. This restricts the voltage swing that can be obtained at the output node6.

Where space is not an issue, then the peak to peak voltage range at the output transducer can be shifted by the inclusion of a DC blocking capacitor at the output node6. However in order to achieve acceptable lower frequency bandwidth (for example 20 Hz), with audio applications typically having 16 Ohm loads, then the capacitor needs to have a size of 330 micro Farads. Within the context of integrated circuits, this represents a large component with an unacceptably high implementation cost.

In certain applications, it is possible to gain access to the terminals at either side of the output transducer and in such circumstances the effective voltage swing across the transducer can be increased by driving it within a “H bridge” circuit arrangement. However this is not always possible.

A common example of a device where reasonable to loud audio amplitude output is required, but where it is not possible to drive the transducer within an H bridge is the mobile telephone. Here, as illustrated inFIG. 2, the left and right earpiece transducers10and12share a common return line to a headphone jack, generally designated14. Thus within this arrangement the common return must be held at a common voltage in order to reduce cross talk or to comply with the commonly used configuration of the headphone jack, and this is generally the local ground (i.e. negative battery potential) within the mobile device. This therefore reduces the output swing which can be provided to the left and right channels (designated A and B) from a stereo output amplifier16. The common return line may also be shared with input devices, such as a microphone, which precludes use of an H bridge drive arrangement.

Returning toFIG. 1, one way to enhance the output amplitude would be to use a charge pump to generate a negative reference voltage, V−, such that the transistor8could pull the load voltage down from ground to V−. Charge pumps are well known to the person skilled in the art and consequently this is, at least initially, a very tempting possibility. However, within the context of an integrated circuit there are significant cost penalties incurred in building an integrated circuit where the output transistor8can connect to a negative reference voltage which lies below the ground voltage (e.g. as provided by the negative terminal of the battery) connected to the substrate of the integrated circuit and these problems will be discussed shortly. Whilst running the entire integrated circuit between the V+ and V− voltages would overcome the difficulties associated with integrated circuit fabrication, this would be at the cost of increasing the voltage difference across the integrated circuit as a whole, and hence increasing the power dissipation within the integrated circuit and consequently shortening battery life within a mobile device having such a circuit. This, therefore, is clearly undesirable, especially in devices where manufacturers strive to archive long “stand by” times.

It is worthwhile considering the formation of an integrated circuit in order to see where these problems arise. It is also worth bearing in mind that devices such as mobile telephones and Bluetooth headsets require the integration of highly complex digital processing circuitry together with RF and analog circuitry. It is also worth noting that the standard CMOS application process for creating digital circuitry can be used to form analog amplifiers.FIG. 3schematically shows the formation of a P type transistor and an N type transistor within the standard CMOS fabrication process which is well known to the person skilled in the art and which is readily available at a large number of semiconductor fabrication facilities around the world. Within the standard CMOS chip a substrate40is doped with a P type impurity. Then, in order to form an N type transistor first and second regions42and44are doped with an N type impurity in order to form the source and drain of a field effect transistor. A conducting electrode46is then formed within the space between the regions42and44in order to form the gate. The gate46is insulated from the P type substrate40via a thin insulating layer48often of silicon dioxide. The interface between the N doped regions42and44and the P doped substrate40forms a parasitic diode, but this remains reversed biased, and consequently non-conducting, whilst the voltages within the N doped regions42and44remain higher than the voltage at the P type substrate40.

In order to form a PMOS transistor it is necessary to dope a region of the substrate with an N type dopant in order to form an N-well60. Then first and second regions62and64are doped with a P type material in order to form the source and drain of the PMOS field effect transistor. A gate electrode66is formed between the drain and source regions. The interface between the source and drain regions62and64, and the N well60has the potential to form parasitic diodes, but these remain turned off provided that the potential in the N well is not less than the potential of the source and drain regions. In order to achieve this a further connection, known as a back gate70is provided such that the voltage at the N well can be held positive with respect to the potential of the P type substrate and the potentials of the source and drain terminals, or equal to the most positive of all three.

It can be seen from inspection ofFIG. 1in combination withFIG. 3that where one of the first transistors is an NMOS device, trying to run the output node6at a voltage below the voltage of the P type substrate40will switch on parasitic diodes associated with the NMOS transistor.

However, if it is desired to be able to use the PMOS and NMOS transistors within a CMOS output stage capable of operating at a voltage below the potential of the P type substrate, then additional processing steps must be undertaken around the NMOS transistor in order to modify it.FIG. 4shows a CMOS output stage where the NMOS transistor can operate at voltages below the potential of the substrate of the integrated circuit. In order to achieve this the standard NMOS transistor needs to be formed within its separate P well80which itself must be formed within an N well82within the P type substrate40. Individual electrodes are connected to the P well80and the N well82such that the potential of these wells can be controlled in order to form back to back reverse biased diodes in order to prevent charge leakage from the source or drain terminals42and44of the NMOS transistor to the P type substrate. This formation of additional sub wells increases the complexity of fabrication of the semiconductor die.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided an output stage, comprising a first transistor operable to pull a voltage at an output node towards a first voltage supply, and a rechargeable energy store having a potential difference between first and second terminals wherein the rechargeable energy store is arranged to be selectively placed in current flow communication between the output node and a second voltage supply such that the voltage at the output node can be driven to a voltage outside of a range defined between the first and second voltages.

It is also possible to drive the voltage at the output node to a voltage within the range between the first and second voltages.

It is thus possible to provide an output stage which can exhibit the peak to peak voltage swing that can be achieved whilst running the output stage at +V and −V, reference with respect to the substrate, whilst only using a conventional (e.g. single well) CMOS fabrication process where the NMOS transistors within the integrated circuit only operate between +V and the substrate's potential. As a consequence the supply voltages for the integrated circuit can also be limited to +V and ground (the substrate's voltage). This gives, for example, enhanced audio output amplitude within a mobile device, such as a mobile telephone, without needing the use of more complex and consequently more costly fabrication process in order to form the integrated circuits of the device. This is of significant benefit in the manufacture of products, such as mobile telephones, which are manufactured in large numbers.

Advantageously the rechargeable store can be selectively recharged in order to prevent it becoming depleted. In an embodiment of the present invention the rechargeable store is used to energize an output transducer during negative half cycles of the output of an amplifier having an output stage constituting an embodiment of the present invention. The time taken to recharge the rechargeable store is significantly less than the half cycle time of the highest in-band frequency of interest and consequently one or more recharges can be achieved within the negative half cycle. The recharge does not significantly degrade the performance of the output stage.

Advantageously where the amplifier operates in a closed loop mode, output errors resulting during the recharge process are compensated for by the closed loop such that the total amount of energy delivered into the transducer during the half cycle is substantially correct. However open loop operation is also possible.

The rechargeable energy store is advantageously provided as a capacitor. The ability to recharge the capacitor during use means that the capacitor behaves as if it stored more charge than it is physically able to do for a given potential difference across the capacitor. Thus the combination of the capacitor and a recharge circuit can be regarded as an apparatus for synthesising a capacitor of a first size from an electrically smaller capacitor.

The recharge circuit may include switches for connecting the energy store or capacitor to a voltage supply during recharging. Optionally additional switches may be provided for removing the capacitor from the output state during recharging. Alternatively the output stage or a driver or preamplifier for the output stage may be modified such that it acts to shut off current flow through the output stage during recharging of energy store.

A plurality of energy stores may be provided and switches may be provided in association with the energy stores such that different energy stores may be selected for use at different times, either singularly or in combination with other ones of the stores.

According to a second aspect of the present invention there is provided an output stage where a least one device is operable to control a voltage occurring at an internal node, and wherein at least one rechargeable voltage store is arranged to be selectively connected between an output node and the internal node such that the voltage at the output node is the sum of the voltage across the or each rechargeable voltage store connected between the internal node and the output node and the voltage at the internal node.

According to a third aspect of the present invention there is provided a method of modifying the voltage swing available at an output of an output stage which has a first transistor operable to pull a voltage at an output node towards a first supply voltage and a second transistor, wherein the method comprises providing a rechargeable energy store having a voltage of +VFCvolts across its terminals interposed between the second transistor and the output node, and the second transistor is operable to control current flow between the output node and a second supply rail.

According to a fourth aspect of the present invention there is provided a recharge control system comprising a load current monitor for measuring a load current, a recharge current monitor for monitoring a recharge current and a controller responsive to the recharge current monitor and the load current monitor and arranged to compare the load current and the recharge current and to modulate the recharge current as a function of the comparison.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 5shows, in highly conceptualised terms, an output stage constituting an embodiment of the present invention. Conceptually the stage is quite similar to that of the conventional totem pole output stage shown inFIG. 1in that first and second transistors4and8are provided in a series connected arrangement between the positive supply V+ and a local ground “GND” which may be provided by a single battery. An output node6represents the output of the output stage and consequently it can be seen that the first transistor4can be switched into a conducting state by the preamplifier2so as to pull the output node6towards V+. It can also be seen that, in the absence of any further components, the second transistor8could be switched on to pull the voltage at the output node6towards ground/the voltage applied to the substrate of the integrated circuit. However, in the present invention a floating voltage source100, (which, in use, floats with respect to the supply voltages but not with respect to the output voltage—except during recharge as will be described later) is connected between the output node6and the transistor8. Suppose, for simplicity, that the floating voltage source100has a voltage +VFCoccurring across it such that when the first transistor4is not conducting the voltage at the output node6(in the absence of any load) would be −VFCvolts less than the voltage occurring at the interconnection between the voltage source100and the second transistor8. Put another way, if the voltage drop across the second transistor8is V2, then the output voltage at the output node6is V2−VFCmeasured with respect to the local ground.

If we consider the operation of this circuit in simplistic terms, it can be seen that when transistor8is non conducting then the voltage source100simply floats together with the potential at the output node6and hence transistor4can pull the voltage at the output node6up to the voltage V+, less any small voltage dropped across the transistor4due, for example, to resistive losses therein when a current is flowing. It can also be seen that, when transistor4is non-conducting and transistor8is conducting then effectively the lowermost terminal (as shown inFIG. 5) of the floating voltage source can be connected to ground through the transistor8and consequently the voltage at the output node6can be reduced to −VFCvolts with respect to ground. It can also be seen that the transistors4and8need not be driven hard on and hard off but could be driven in a linear manner in order to control the voltage at the node6to assume any desired voltage lying within the range V+ to −VFC, measured with respect to the local ground.

Such an arrangement is relatively well suited to being implemented within CMOS transistors fabricated using the standard CMOS process as will now be explained.

Looking atFIG. 5, it can be seen that the source terminal of transistor4is always held at V+, whereas the drain terminal may be required to go as negative as −VFC. From brief inspection of the configuration of CMOS transistor (FIG. 3), it can be seen that this can be tolerated if transistor4is a PMOS transistor. If the back gate70of the N well60of the PMOS transistor is generally held close V+ then it can be seen that the potential difference between the drain terminal and the N well60is never sufficient to switch the parasitic diode formed therebetween on. Similar considerations apply to the source terminal. Furthermore, with the N well60being held above the potential of the P substrate40again this junction forms a reverse biased diode such that no parasitic leakage occurs. Finally, the drain62of the PMOS transistor may go negative with respect to the ground voltage but the junction between the drain62and the N well60will always form a reverse biased diode so again no parasitic problems occur. Similarly, if transistor8is formed as an NMOS transistor then its drain source voltages are always above ground and consequently this transistor works in its normal operating range. It can therefore be seen that an enhancement of the output range of the output stage is achievable provided that the floating energy store can maintain an energy difference of VFCacross it. It should be noted that VFCneed not be a constant value.

FIG. 6schematically illustrates a first embodiment of the present invention. In order to assist in identifying the transistor types, PMOS transistors will be designated with a P, whereas NMOS transistors are designated with an N. Therefore, as can be seen, a first transistor P1extends between the positive supply rail V+ and the output node6. Similarly, and as shown inFIG. 5, the rechargeable energy store100has a negative terminal114connected to the output node6and a positive terminal112connected to the drain of a first N type MOSFET N1whose source is connected to the local ground/zero volt supply. The transistors P1and N1are driven by the preamplifier stage2. The energy store100can be constituted by a capacitor as capacitors are relatively easy to fabricate within an integrated circuit. The capacitor may also be a small external component. However, it can be seen that when the second transistor N1is conducting that current flow occurs through the capacitor100and that the energy stored therein is discharged to the load. As the current flows through the capacitor the voltage across the capacitor terminals will drop. It is therefore necessary to provide a recharge arrangement, generally designated110, for recharging the capacitor100. The recharge arrangement comprises a third transistor, P2, which is a PMOS transistor which has its source terminal connected to the positive rail V+ and its drain terminal connected to the positive plate112of the capacitor100. A fourth transistor P3which is an PMOS transistor has its drain/source connected to the negative plate114of the capacitor (this corresponds to connecting to the output node6) and its source/drain connected to the ground rail 0V. It should be noted that P3could be replaced by an NMOS device in series with a protection diode.

It can be seen that, if both the first and second transistors P1and N1are switched to a non-conducting state then the capacitor100can be recharged such that its positive plate112is more positive than its negative plate114by switching the third transistor P2on thereby effectively connecting the positive plate112to the positive supply V+ and by switching the fourth transistor P3on thereby connecting the negative plate114to ground. This can be achieved by asserting a “cap refresh” signal120which in this embodiment is active when low and which is provided to the gate of the fourth transistor P3thereby switching it into a conducting state. An level shifter122may optionally be provided in the path to the gate of the third transistor P2. A similar level shifter123may condition the signal provided to P3. When the “cap refresh” signal is reset, i.e. taken high, then both transistors P2and P3switch off and the recharged capacitor can then be used once again in conjunction with the second transistor N1to take the voltage at the output node6negative with respect to the local ground, 0V.

As noted before, the voltage across the capacitor VFCneed not be a constant value. This can be exploited to provide further increase in system efficiency. Referring back toFIG. 6, it can be seen that if

VFC=V+2
and P2is configured such that it can be used to drive the load in a linear fashion, then the current into the load during a positive half cycle can be driven through transistor P2, whereas during the negative half cycle the current is driven through transistor N1. In this regime transistors P1and P3need not be active at all. Thus the current coming from P2during the positive half cycle charges the capacitor100, and this charge is then discharged to the load during the negative half cycle. In this regime a refresh to the capacitor100is not required and the capacitor100effectively halves the voltage supply and doubles the efficiency whilst still providing a level shifting function. However this improved efficiency is only achievable if the charge stored in the capacitor100is sufficient to provide the energy for the next full half cycle. As such, this regime is most likely to be entered when the signal amplitude is low. As the signal amplitude increases then the mode of operation can be changed, for example to a mixed mode, where small amplitudes are dealt with by control of transistors P2and N1, and larger amplitudes are dealt with by switching transistors P1and N1, and then engaging in recharge events using transistors P2and P3. This mixed mode of operation is beneficial because the class AB output stage efficiency is worst at low signal amplitude. Thus we get the benefit of increasing efficiency at low amplitudes, whilst also increasing the peak to peak voltage range that can be output to the load.

Although the arrangement shown inFIG. 6works well, the recharging scheme has some disadvantages. Firstly, the recharge current is to a first approximation, determined solely by the capacitance of the capacitor in combination with the on resistance of the third and fourth transistors and the voltage across the transistors. The standard CMOS process seeks to make these resistances as low as reasonably possible and as a consequence relatively large transient currents may flow during the capacitor recharge. This can give rise to stress in the circuit's power supply. It can also be a cause of electromagnetic interference. This is not necessarily a problem in many systems, but where sensitivity to electromagnetic interference is expected to be a problem then it is desirable to be able to control the charge current. It can also be expected that the inter-charging interval will be proportional to the output current required by the load. This gives the option to have the recharging occur at a irregular spacings in time which has the advantage of smearing the interference generated during recharging out across the frequency spectrum. The smearing effect can be further enhanced by varying the recharge current which in turn modifies the amount of recharging that occurs during each recharge instance and consequently can further randomise the inter-charge interval that is required between succeeding recharges.

FIG. 7schematically illustrates a modification to the arrangement shown inFIG. 6where the third transistor is now replaced by a parallel bank of P type transistors P21, P22, P23. . . P2N. Each of the transistors P21to P2Nhas its gate connected to a switching controller130which has an input responsive to the “cap refresh” signal such that the controller130can switch one or more of the transistors P21to P2Non. A level shifter132may be provided to shift the drive voltage level to P3, or it may be omitted. Each transistor is smaller than would be the case if only a single switching transistor were used and hence each exhibits a higher internal resistance RDS-on when switched fully on. Therefore the charging current can be modulated by controlling the number of transistors which are switched on. The controller130is responsive to a charge current demand signal ICwhich can be derived in an open loop manner by making the assumption that the charging current should be related to the expected load current. It should be noted that in-rush currents to the capacitor could also be limited by deliberately extending the transition time for the transistors P21to P2Nto switch from being non-conducting to being conducting. This may, however, increase the dissipation within the devices and may require use of a larger die or the need for a package with lower thermal resistance.

The controller130can act to switch one or more of the transistors on, thereby modulating the current flow into the capacitor100. Where multiple transistors are to be switched on, then they can either be switched on in one go, or their switch on times can be slightly staggered. Staggering the switch on further smears out radio frequency interference resulting from the recharge process. The selection of the transistors involved may also be randomised.

The refresh current may be estimated based on the load and signal amplitude. However, this is likely to be difficult and instead it may be preferable to sense the load current and the refresh current at a recharge instant and to use this data to control the number of transistors P21to P2Nthat are switched on at the subsequent charging instant.

In order to do this, means need to be provided for measuring the refresh and load currents. It would be possible to interpose measuring devices within the circuit. For example small known resistances could be inserted in the recharge path and the current path through transistor N1, and the voltage drops that occur could be measured so as to indicate the current flow. However this has the disadvantage of perturbing the operation of the amplifier and the refresh circuit.

An alternative approach is to fabricate additional charge transistors and an output stage that can be used to replicate the performance of the output stage and the recharge circuit. Such circuits will be discussed later.

FIGS. 8aandbcompare the voltage at the output of the output stage with respect to a voltage at the input of an inverting amplifier constituting an embodiment of the present invention.FIG. 8arepresents the input, whereasFIG. 8brepresents the output. It can be seen that, during the positive half cycle, generally designated130, the voltage at the output node6follows the input voltage. This is because current flow only occurs via the field effect transistor P1. However, during the negative half cycle current flows through transistor N1and through the floating voltage source100. Therefore the capacitor100becomes discharged by the current flow through it and the voltage across its terminals decreases. It is therefore necessary to operate the recharge circuit to recharge the capacitor during the negative half cycle132. The recharge instance causes the voltage at the output node to briefly tend towards 0 V, as is diagrammatically shown by the vertical lines134inFIG. 8b. It can be seen that the frequency of the recharging increases with increasing output voltage amplitude, and hence increase in current flow through the capacitor100. It should be noted that it is also possible to recharge the capacitor during the positive half cycle, either through the recharge circuit or through N1.

Although the excursions towards 0 V designated134inFIG. 8bare shown as near instantaneous, in fact each excursion has a finite duration as shown in greater detail inFIGS. 9aand9b. It can be seen that the enclosed area134of the excursion shown inFIG. 9acauses energy to be removed from the output signal. That energy can be schematically represented as the area enclosed within the rectangle134. The inventor noted that some of this energy can be returned to the transducer by allowing some overshoot to occur within a second area designated136. Furthermore the inventor also noted that a pre-distortion could be introduced into a third area designated138, as shown inFIG. 9b. Where pre-distortion is provided, it is desirable to try to get the areas136and138to be equal in size, and then sum of their enclosed area to equal the area134.

In order to consider the simpler case of returning some of the energy that was lost into the area designated136inFIG. 9, consider the circuit arrangement shown inFIG. 10. Here the amplifier150represents both the input stage preamplifier2ofFIG. 5together with the output stage comprising transistors4,6and floating voltage source100. The amplifier150is connected to a load, designated152and assumed to comprise both a capacitance and a resistance to ground. The amplifier150receives a signal to be amplified, which can be supplied from either voltage source154connected to the inverting input of the amplifier150, voltage source156connected to the non-inverting input of the amplifier150or both voltage sources154and156where the amplifier is driven in a differential mode. A feedback loop comprising resistor180in parallel with capacitor182and a further resistor184in series between the voltage source154and tie non-inverting input is provided, as is known to the person skilled in the art.

The feedback loop acts to compare the voltage at the output node6with the voltage at the inputs of the amplifier150, to amplify any error therebetween and to provide a correction to the output node6. The feedback loop is maintained operative whilst the amplifier150is driving a positive half cycle into the load152and whilst the amplifier150is driving a negative half cycle but is not engaged in recharging of the floating energy supply.

Returning toFIG. 6, it can be seen that when it is desired to recharge the capacitor100, both transistors P1and N1within the amplifier have to be switched into a non-conducting state. It can be seen that this causes the feedback loop to be broken. During this time an error occurs between the voltage that actually occurs at the output node6and that which should occur at the output node6. In order to account for this error, a further capacitor190is provided which extends between the non-inverting input of the amplifier150and ground, and which in use acts to integrate the error that occurs during the recharging period. In turn, once normal operation of the feedback loop is established, the amplifier must then seek to overcompensate for the error before the voltage at the output node6can return to its proper value. This gives a way of returning some of the “lost” area under the output curve that occurs during recharging, thereby compensating for some of the power that was not dissipated into the load during the recharging moment.

It can be seen that a similar pre-compensation could be provided by causing some overdrive at the output just prior to operation of the recharge circuit. For optimum performance this over voltage should represent a portion, for example half, of the missing energy that is failed to be delivered during the output refresh. The other portion, e.g. half, of the missing energy is then delivered after the refresh cycle. The two halves should be balanced in the time domain around the centre of the refresh so that there is little or no phase error between the missed energy and the compensation. The overdrive can be achieved by driving the amplifier with a temporarily increased signal that is proportional to the signal that would have been driven by the amplifier in the absence of the overdrive condition.

Although the invention as described hereinbefore has been directed to extending the voltage swing at an output stage of an amplifier so as to swing negative with respect to the available supply, it can be seen that the extension can occur either side of the voltage supply rails to the integrated circuit.FIG. 11shows an embodiment of the present invention where a series of voltage sources can be selectively connected between the output node6and an internal node200representing the connection between the transistor P1and the transistor N1. Each of the voltage sources V1to Vn can be selectively switched into or removed from the circuit by associated switches s11, s12, s21, s22, sn1, sn2where associated pairs of switches work in unison. Thus, switches s11and s12work together so as to either connect the first of voltage source V1between the nodes200and6or to bypass the voltage source V1. The other switches work in a similar manner such that the contribution of each floating voltage source can be added to the voltage occurring at the node200. The polarity across each voltage source is freely determinable by the way in which the voltage source is recharged, and hence each one of the voltage sources can either add or subtract a voltage from the voltage at the node200. The voltage sources can be recharged as described hereinbefore by briefly switching them out of the current flow path between node200and node6, and into a recharge path. A closed loop around the amplifier circuit can then compensate for the changes in the voltage across each capacitor as it discharges. Optionally the floating voltage source VFCbetween N1and node200can be included or omitted, at the discretion of the designer.

Although the invention has been described with respect to charging the capacitors by briefly disconnecting them from the output circuit and connecting them between the power supply rails, other techniques for charging the floating energy stores are also possible. Thus, inductors could be used. Transistors would be used to switch an inductor into a current flow path between the voltage supply rails so as to induce current to flow in the inductor and thereby to cause a magnetic field to build up around the inductor. The inductor can then be placed in parallel with the floating energy store and the current flow path between the voltage supply rails broken. The collapsing magnetic field around the inductor causes energy to be transferred into the floating energy store. This technique has the advantage that the energy store does not need to be removed from the output circuit for charging, although it does have the disadvantage of using inductors which are not favored within integrated circuits because of the difficulty it fabricating them and the space that they take up. However, if the inductors are provided as external components then these problems are immediately overcome. An inductor, or other recharge circuit, can be shared between several of the floating capacitors in one or more output channels.

Furthermore, a capacitor can also be used to recharge the energy store in a similar manner.

As noted hereinbefore, it may be advantageous to provide additional circuitry for measuring the load current and the refresh current.

FIG. 12illustrates the circuit for measuring the load current in which a measurement transistor220is fabricated which has its gate connected to the gate connection of the transistor N1, but which has an area considerably smaller than that of N1, for example 1000 times smaller such the current flow through the transistor220is proportionally scaled to that of the transistor N1. However, the ratio is determinable by the circuit designer so is designed X:1.

In order for the current flow through the transistor220to match that occurring through the transistor N1, subject to the scaling provided by the relative transistor sizes, then the drain source voltage occurring across the transistor220must be made to closely match that occurring across the transistor N1. In order to do this an operational amplifier230and field effect transistor235are provided. The transistor235has its gate connected to an output of the operational amplifier230whereas its source is connected to the drain of the transistor220which in turn is also connected to the inverting input of the amplifier230. A non-inverting input of the amplifier230is connected to a sample capacitor240which can be selectively connected to the drain of the transistor N1via a switch242. If switch242is temporarily closed on a periodic basis, then the capacitor240will sample the voltage occurring at the drain of the transistor N1. The action of the operational amplifier and the transistor235will be to try and cause the voltage occurring at the inverting input of the amplifier230to match the voltage occurring at the non-inverting input thereof. Therefore the drain source voltage across the transistor220will be held to be substantially equal to the drain source voltage occurring across the transistor N1at the sampling instant. Given that the drain source voltage across the transistor220is the same as the drain source voltage across the transistor N1, and that the gate source voltage at the gate of transistor220is equal to the gate source voltage of transistor N1, then the current flowing through the transistor220is proportional to that flowing through N1, and has a ratio which is determined by the relative sizes of the transistors. This current also flows through transistor235which in turn is connected to transistors250and252arranged in a current mirror configuration with the drain of transistor250being connected to the gate of the transistors250and252thereby ensuring that the current flowing through transistor252matches that flowing through transistor250, which in turn matches that flowing through transistor220. A resistor260is connected between the drain of the transistor252and ground, and the voltage developed across that resistor is a product of the resistance of that resistor and the current flowing through transistor220. Therefore the voltage across the resistor can be measured in order to determine the current flow to the load connected to the node6.

It is possible to use a similar technique to measure the current flowing to the capacitor100during recharging.FIG. 13shows a modification to the arrangement shown inFIG. 7. Five charging transistors P21to P25are shown, and each has its own individual drive circuit. For each transistor the gate source voltage could be assumed to be substantially constant. Therefore the current flowing through the transistor is reasonably well related to its drain source voltage. As a consequence, measuring the drain source voltage provides a way of estimating the current flowing to the capacitor100. A sampling switch280is provided such that the voltage occurring across the transistors P2, to P25during recharge can be sampled on to a capacitor282. The sampling switch280need therefore only be closed very briefly in order to cause the capacitor282to track the voltage occurring across the transistors P21to P25. The capacitor282is connected to the non-inverting input of an amplifier290. A small transistor300representing a mirror of one of the charging transistors P21to P25has its source connected to the positive supply rail and its gate permanently connected to ground.

The transistor300is much smaller than any of the transistors P21to P25such that the current flowing therein is smaller than the recharge current, but is representative of the recharge current provided that the drain source voltage across a transistor300is the same as that occurring across the charge transistors P21to P25. Under these circumstances the current will then be effected by the relative scaling of the small transistor300to each of the transistors P21to P25and also by the number of those transistors P21to P25which are switched on. The voltage occurring at the drain of the transistor300is supplied to the non-inverting input of the operational amplifier290. An output of the amplifier290drives the gate of a N type transistor302which is in series with the transistor300and who's source is connected to ground. The feedback action around the amplifier290is such that the voltage occurring at the drain of the transistor300is set to be substantially equal to the voltage at the drains of the driving transistors P21to P25when they are recharging the capacitor100and consequently the current flowing the transistor300accurately represents the charging current to the capacitor100. A transistor308forms a load to a further current mirror formed by transistors310and312and transistor308has its gate tied to the gate of transistor302and its source tied to ground such that the gate-source voltage across transistor308is the same as that for transistor302. As a consequence the current flowing through the transistor312is proportional to the current flowing through transistor300, which in turn is proportional to the charge current into the capacitor100. The current flowing through transistor312then flows through five series connected resistors320to324, each of which has a parallel shorting transistor330to334. The voltage occurring at the drain of the transistor312is representative of the current recharging the capacitor100.

However, a scaling problem exists because one, two, three, four or five of the transistors P21to P25may be on and therefore some form of scaling is required. In order to achieve this, each of the transistors330to334is arranged to be non-conducting when the corresponding transistor P21to P25is conducting. Therefore, if all five transistors P21to P25are conducting, then all of the transistors330to334are in a non-conducting state and the voltage occurring at the source of the transistor312is representative of the current into the capacitor100. If, however, only one of the transistors P21to P25was conducting then for the same drain source voltage across that transistor the current in the capacitor100would be five times lower. In order to accurately reflect this, four of the resistors320to324would be shorted out thereby causing the voltage occurring at the drain of the transistor312to accurately represent the current into the capacitor100.

Thus the circuits shown inFIGS. 12 and 13allow both the load current and recharging current to be estimated. The load current can then be compared with the charging current in order to determine how many of the transistors P21to P25should be turned on in the next charging cycle.

In one embodiment of the invention if the charging current at a Kthrecharge event is less than five times the load current in one charging cycle, then the number of transistors P21to P25is incremented for a subsequent (K+1th) charging cycle. However, if the charging current is greater than five times the load current, then the number of transistors switched on is decremented for the next recharging cycle. In any recharge cycle one of the transistors is always switched on thereby ensuring that a minimum level of recharge occurs. Similarly, once the recharge current exceeds a maximum threshold value and then no more transistors can be switched on.

This arrangement reduces the stress on the power supply to the circuit, and also seeks to limit the inrush currents to the capacitor100during charging, whilst ensuring that sufficient charge is placed on to the capacitor during each charging event for it to be able to hold the required voltage across it until the next scheduled recharging time.

As noted before, the interval between recharging events can also be randomized, and in an embodiment of the present invention the recharge rate can be varied between 1.9 and 2.1 MHz. It is thus possible to provide an inexpensive output circuit which allows for an enhanced voltage to be used to drive a load, without requiring a DC blocking capacitor.

FIG. 14shows a timing interval generator for generating the recharge signal, for example the “cap refresh” as illustrated inFIG. 6. A current source350is used to provide a current to charge a capacitor360. A comparator362compares the voltage occurring at the capacitor with a reference voltage generated, for example, by a digital to analog converter364driven by a pseudo random number generator366. When the voltage at the capacitor exceeds the reference voltage the comparator362outputs a signal which causes a capacitor discharge switch368to close so as to discharge the capacitor. An output of the comparator362is also provided to an inverter370which provides an output to a frequency divider372and also to the control terminal of a further switch374which interrupts the current flow path to the capacitor360during discharge. It should be noted that switch374could be omitted if desired. By providing a variable voltage reference signal the time taken for each capacitor recharge cycle varies slightly. This signal in an embodiment is then frequency divided to a signal in the range of 2 MHz±0.1 MHz.

It is thus possible to randomize in time the occurrences of the recharge events, thereby spreading the noise generated by recharge across the frequency domain.

It is possible to provide floating rechargeable energy stores in parallel. Thus, referring toFIG. 7or11the or each capacitor could be implemented by two or more parallel capacitors such that one of the capacitors is being recharged whilst the other is being used. Such an arrangement is being inFIG. 15which represents a variation on the circuit shown inFIG. 5.

If we compareFIG. 15withFIG. 5, we see that an additional circuit400has been substituted in place of the capacitor100. The circuit400comprises first and second capacitors402and404arranged in parallel. If we continue to use the terminology adopted forFIG. 5, then each of the capacitors402and404has a “positive” plate and a “negative” plate. The negative plate of capacitor402is associated with the switch410which is operable in a first switch position to connect the negative plate to the output node6, and in a second switch position to connect the negative plate to a recharge circuit430. A switch412is operable to connect the positive plate of the capacitor402to either the drain of transistor N1or to the recharge circuit430. Therefore it can be seen that the switches410and412can be operated to connect the first capacitor402between the output node6and the transistor N1, or optionally to disconnect the capacitor402and to connect it to the recharge circuit430for recharging.

The second capacitor404is similarly associated with switches420and422such that its negative plate can be optionally connected to the output node6or to terminals of the recharge circuit430, and that its positive plate can be optionally connected to the drain of transistor N1or to the recharge circuit430. If the switches410and412act in unison, and in anti-phase to the switches420and422, then it can be seen that the first capacitor402can be placed in the current flow path between N1and the output node6whilst the second capacitor404is being charged. Once the first capacitor402becomes sufficiently discharged, then the switches are altered such that it is connected to the charging circuit430and the second capacitor404is placed between the output node6and N1. This means that either capacitor is, on average, being charged for the same duration that the other capacitor is being discharged, and hence the charge current into either capacitor can be reduced thereby reducing the strain on the power supply. Additionally the disruption to the output voltage at the output node6is also reduced. However, it can also be seen that a different mode of operation is possible and that the first capacitor402could remain permanently connected between the output node6and N1. The second capacitor404can be periodically recharged by the recharge circuit430. When the voltage across the first capacitor402becomes diminished, second capacitor404can be placed in parallel with it such that charge redistribution between the capacitors causes the first capacitor402to become recharged. The capacitor404can then be removed from parallel connection with the capacitor402and reconnected to the recharge circuit430. In this arrangement the feedback circuit around the amplifier never has to be broken and consequently perturbations in the output voltage due to the recharging can be reduced.