Voltage transformation circuit

A voltage transformation circuit comprising a first input, a second input, a first output, first and second impedances and a current mirror having master and slave terminals, wherein the first impedance is connected between the first input and the master terminal of the current mirror, the second impedance is connected between the second input and the slave terminal of the current mirror, and the first output is connected to the slave terminal of the current mirror.

FIELD OF THE INVENTION

The present invention relates to a voltage transformation circuit. Such a circuit is particularly useful for removing a common mode component from two voltage measurements such that the difference between these measurements can be more easily determined.

BACKGROUND OF THE INVENTION

It is often desirable to be able to make two measurements and to remove the common mode component of those measurements to look at a difference between the measurements. By way of example, consider a relatively high voltage supply line providing power to a load. It may be desirable to measure the current provided to the load, and this can be done by inserting a low value sense resistor, Rsense, in series with the load. The voltage at the supply side of the resistor will be the supply voltage VHTwhereas the voltage on the load side of the sensing resistor will be VHT−(Rsense×IHT) where IHTrepresents the current flowing through the resistor. VHTmay be many tens or hundreds of volts while the voltage dropped across the sensing resistor Rsensemay only be in the region of millivolts.

It is often desirable to amplify the voltage drop across the sensing resistor Rsenseand then provide this to other circuits where this value can be displayed or used. However the amplifiers and subsequent components often work at relatively low voltages, for example five or less volts, referenced to a ground potential. The semiconductor process used to form these components is often unsuitable for high voltages and frequently incapable of withstanding them without damage.

It is known that the voltages occurring across the sense resistor Rsensecould be attenuated by resistor potential dividers such that the voltage at the output of the potential divider is suitable for provision to an amplifier or other signal processing circuitry using relatively low voltage transistors. However the potential divider attenuates the common mode voltage and the differential voltage equally and the resistors used in the potential divider act as noise sources.

SUMMARY OF THE INVENTION

According to the present invention there is provided a voltage transformation circuit comprising a first input, a second input, a first output, first and second impedances and a current mirror having master and slave terminals, wherein the first impedance is connected between the first input and the master terminal of the current mirror, the second impedance is connected between the second input and the slave terminal of the current mirror, and the first output is connected to the slave terminal of the current mirror.

It is thus possible when faced with first and second signals having a common mode component which is common to both the first and second signals and a differential mode component which represents a difference between the first and second signals to use the action of the current mirror to remove the common mode component without attenuating the differential component.

The output may operate in a current mode manner, such that the magnitude and direction of current flow at the output directly represents the difference between the signals supplied to the first and second inputs of the voltage transformation circuit. However a current to voltage conversion circuit may be attached to the first output in order to provide a voltage output representative of the voltage difference between the first and second inputs.

Advantageously the voltage transformation circuit has a second output connected to the master terminal of the current mirror. The voltages at the first and second outputs may then be provided to an operational amplifier which may be configured to simply amplify the voltage difference, or may be configured to perform other functions, such as integration.

The current mirror may be implemented by any suitable technology in which the current flowing at a first terminal thereof which represents a master terminal is used to force the current flowing at a second terminal thereof, which represents a slave terminal, to match that flowing at the master terminal. Current mirror circuits are well known and can be implemented using field effect transistors or bipolar transistors.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

FIG. 1is a circuit diagram of a first embodiment of the present invention used in conjunction with a current sensing resistor Rsensein order to measure the voltage on a supply line VHTwhere the voltage on the supply line is too great to be directly connected to a measuring circuit. It is known that the voltage dropped across the resistor Rsenseis directly proportional to the current IHTflowing through the resistor. In order to measure the voltage dropped across the sensing resistor Rsensea first input2of the voltage transformation circuit, generally designated4, is connected to a first terminal of the resistor Rsensewhereas a second input6of the voltage transformation circuit is connected to a second terminal of the resistor Rsense. A first resistor8extends between the first input2and a master input10of a current mirror12. A second resistor14is connected between the second input6of the voltage transformation circuit and a slave terminal16of the current mirror12. A first output terminal18is also connected to the slave terminal16of the current mirror12.

In use the action of the current mirror is to cause the current flowing at the slave terminal16of the mirror12to track the current flowing at the master terminal10. The current mirror comprises first and second field effect transistors20and22connected in the well known current mirror configuration. Gate terminals of the field effect transistors20and22are connected together and also to the drain of the transistor20which forms the master terminal of the current mirror. The drain of the second transistor22forms the slave terminal of the current mirror and the sources of the transistors20and22are connected together and to a common node24which may be connected to any arbitrary potential, but which is normally connected to a system ground potential, which in the case of devices dealing with power supplies is often an earth.

In use the transistor20maintains the voltage at the master terminal10reasonably close to that at the common node24. Therefore, when VHThas a voltage of several tens or hundreds of volts the voltage drop across the first resistor8is substantially VHT(less a small drop occurring across the transistor20) and the circuit designer can select the value of the resistor8to give rise to whatever current value he deems to be appropriate to flow in the current mirror. Typically this would be expected to be in the several micro-amps to several hundreds of micro-amps. Because the transistors20and22are well matched and their gates are held at the same potential and their sources are held at the same potential then transistor22will seek to conduct the same current as flows through transistor20, assuming that the transistors have identical geometries. If the second resistor14accurately matches the value of the first resistor8then the current flowing through the resistor will depend upon the voltage dropped across that resistor. This voltage drop is modified by the voltage dropped across the current sensing resistor Rsense. Therefore when a load current IHTis supplied to a load30then the current flowing through the second resistor14will be slightly different to that flowing through the first resistor8, however the currents flowing through the transistors20and22are identical. Therefore any current imbalance results in the current flowing out of, or alternatively into, the output node18. The magnitude of this current is directly proportional to the voltage dropped across the sensing resistor Rsenseand the direction of the current is indicative of the direction of current flow through the sensing resistor Rsense. The current occurring at the output18could be supplied directly to a current mode measuring circuit.

Many users prefer to have a voltage output rather than a current output. In order to achieve this, the arrangement shown inFIG. 1may be modified, as shown inFIG. 2to include an operational amplifier30. As shown, an inverting input32of the operational amplifier30is connected to the first output18. Additionally a non-inverting input34of the operational amplifier30is connected to a second output36which is connected to the master terminal10of the current mirror12. A feedback circuit, which in this example comprises a capacitor40with a shorting circuit42is provided. In use, the amplifier30seeks to maintain the voltage at its inverting input32to be equal to that at its non-inverting input34. Therefore the output44of the amplifier30will attain whatever voltage is necessary in order to cause the current flowing through the feedback component40to compensate for the current imbalance between resistors8and14. As the feedback component is a capacitor, then the output of the amplifier44will integrate the voltage difference occurring at the first and second inputs2and6, respectively such that after a suitable period of time a measurement of the voltage difference, and hence the current flowing through the sensor resistor Rsensecan be made.

In order to maintain proper operation of the circuit, at least for measurement purposes, the capacitor40needs to be periodically discharged via the switch42such that the output voltage of the amplifier does not become constrained by its supply rails.

The use of operational amplifier also has the advantage that, as mentioned hereinbefore, the amplifier seeks to keep the voltages occurring at its inputs equal to each other. This means that the drain-source voltage across transistor22is held equal to the drain voltage across transistor20. This further improves the matching of the current flow in each of the transistors20and22.

As noted, the current mirror can be implemented in bipolar, andFIG. 3schematically illustrates an alternative current mirror12′ using bipolar transistors20′ and22′ which match the action of the current mirror12implemented using the field effect transistors20and22.

The resistors8and14may be formed within a monolithically integrated circuit along with the current mirror12and the amplifier30. In which case the components can expect to be well matched, but additional trimming blocks may be provided in association with either one or both the resistors8and14. Such trimming blocks, which generally include a plurality or resistors either in series or parallel with the respective resistor8or14, whereby the trimming resistors can be switched into or out of conduction are well known to the person skilled in the art. Alternatively the resistors8and14may be provided as external components in which case the engineer can include additional trimming components or can pre-measure the value of resistors in order to sort the resistors into matched pairs.

The current mirror may exhibit an offset which may be undesirable when working with very small voltage drops across the sense resistor Rsense. Whilst it can be difficult to eliminate these offsets completely it is possible to enable the offset to be measured or the difference component to be converted from a direct current signal to an alternating current signal by use of a swapping current mirror.FIG. 4illustrates such an arrangement.

Consequently an output voltage resulting from current in the sense resistor will be reversed in polarity whereas an offset voltage is not reversed in polarity. Thus periodically operating a swap circuit50to swap the current flow into the current mirror over enables the desired signal to be separated from DC offsets.

The swap circuit comprises first to fourth transistors60a,62a,62band60b. The drains of the transistors60aand62aare connected in series with the first resistor8. The source of the transistor60ais connected to the master terminal of the current mirror whereas the source of the transistor62ais connected to the salve terminal of the current mirror. The transistor60ais directly driven by a swap control signal “NORMAL/SWAP” whereas the transistor62areceives an inverted version of this signal. Thus current flowing through the resistor8can be steered to either side of the current mirror. A similar arrangement is provided by transistors60band62bto steer the current flowing through the resistor14.

Thus operation of the swap circuit can be used to quantify offsets occurring within the voltage transformation circuit. The transistors60a,62a,62band60ball work at a low voltage. It is advantageous for a delay to be inserted in the swap circuit to ensure that the transistors are never all off at the same time whilst the circuit is in use.

It is thus possible to provide a voltage transformation circuit which suppresses the common mode component of a voltage occurring at first and second inputs to and6, respectively, thereof without attenuating a differential component between the input voltages.