Low-Voltage Comparator With Wide Input Voltage Swing

The invention relates to low-voltage operational amplifier ("op amp") circuits, and more particularly to low-voltage op amps in a differential pair configuration using MOSFET transistors.

Operational amplifiers are widely used in electronic circuits. The operational amplifiers are popular because they have characteristics that are desirable for voltage amplification. The operational amplifier circuits are characterized by high input impedance and low output impedance that allow extremely small input currents, and hence low power, to drive the circuit and relatively stable output for large current flow.

A pair of complimentary metal-oxide semiconductor (CMOS) transistors, including a p-channel MOS and an n-channel MOS, are used to further reduce the input power consumption. The CMOS transistors in field-effect configuration are called MOS field-effect transistors (MOSFET). The MOSFET transistors draw negligible current relative to bipolar transistors because the transistors are controlled at the gate terminal by an electric field rather than by current flow. The electrical current flows from the source terminal to the drain terminal of an n-channel MOSFET transistor when a voltage applied to the gate terminal has a higher value than the voltage applied to the source terminal. In a p-channel MOSFET transistor, the voltage applied to the gate terminal must be lower by a certain value than the voltage applied to the source terminal. This certain value is referred to as a threshold voltage. A threshold voltage of approximately 0.7 to 0.9 volts is required to form a conducting channel between the source and the drain terminals.

A comparator is a circuit whose output indicates which of its two input voltages is larger. The comparator senses small differences between the two input signal voltages and generates a digital output level indicating which input voltage corresponds to the differences.

FIG. 1 shows a comparator circuit 100 with two n-channel MOSFET transistors 106 and 108 configured as a differential amplifier. The p-channel MOSFET transistors 102 and 104 are configured in a cross-coupled pair with positive feedback.

A series of switches S1 through S2, implemented in practice with MOSFET transistors, are used to transition between modes. When the S1 switches are closed, the comparator circuit is in preset mode. The source and the drain terminals of the MOSFET transistors are set to supply voltage level (V.sub.dd) and the transistors are turned off. When the S1 switches are opened and the S2 switch is closed, the circuit enters a differential amplifier comparator mode. Typical V.sub.dd is between 3.0 volts and 5.0 volts.

The circuit current, and thus the power, can be further reduced by lowering the reference voltage (V.sub.dd) as much as possible. Lowering this voltage to 1.0 volt or lower would also allow the amplifier to operate from a single standard battery. However, the reference voltage cannot be reduced below the threshold voltage that is required to form a conducting channel between the source and the drain terminals. In general, the reference voltage cannot be reduced below 1.3 to 1.5 volts without significantly degrading the performance of the transistors.

There is often a need to operate from a low voltage. For example, single battery cells often have voltages of about 1.5 volts. Solar cells also have relatively low voltages.

The present invention combines a voltage boosting system in combination with a corresponding boost in substrate voltages. This forms a comparator circuit can operate with the supply voltage reduced to approximately 1.0 volt. The substrate voltages are boosted when the PN junctions in p-channel MOSFET transistors are forward-biased.

The present disclosure describes a low voltage comparator for amplifying a voltage difference between a pair of differential inputs. The comparator includes a circuit for supplying a low voltage with correspondingly low current, a boost capacitor that charges to a boost voltage in excess of the low voltage, a differential pair of MOSFET transistors. Each one of the transistors includes a gate, a source, a drain, and a body terminal. The comparator also includes a latch mechanism built from a cross-coupled pair of MOSFET transistors for latching the compared result. A well capacitor is used to charge the body terminal of the differential pair. The boost capacitor and the well capacitor also boost the common source voltage and reduce the threshold voltage of the differential pair to allow the supply voltage to be at relatively low level.

In a preferred embodiment, the supply voltage is at approximately 1.0 volt. The boost capacitor is between 50 and 150 fF (1 fF=10.sup.-15 F). The well capacitor is between 50 and 150 fF. In a further preferred embodiment, the differential pair is made from two p-channel MOSFET transistors with n-type wells while the cross-coupled pair is made from two n-channel MOSFET transistors with p-type substrates. In an alternative embodiment, a coupling capacitor is inserted between the substrates of the differential pair and the negative end of the boost capacitor to pull the body voltage up.

Other embodiments and advantages will become apparent from the following description and from the claims.

These and other aspects of the invention will now be described with reference to the attached drawings, in which:

FIG. 1 is a schematic diagram of a typical prior art differential comparator;

FIG. 2 is a schematic diagram of a preferred embodiment of the low-voltage comparator; and

FIG. 3 is a schematic diagram of an alternative embodiment of the low-voltage comparator.

The present disclosure describes a low-voltage comparator 200 that has a booster structure to boost its source terminal voltages.

FIG. 2 is a schematic diagram of a preferred embodiment of a low-voltage comparator. The low-voltage comparator includes a number of advantageous features that allow the comparator to operate with lower supply voltage. The advantageous features include a forward bias increase element which allows the MOSFET to be operated with a lower forward bias voltage than would normally be possible.

A voltage boosting structure enables boosting the voltage from the actual supplied voltage to some higher voltage. A special technique involving substrates or wells of the MOSFET transistors and a capacitor tied to the common wells forward-biases the PN junctions and pulls the body terminals up. Hence, the voltage at the body terminal closely follows the boosted voltage.

The system operates with a series of switches controlled by two clocks, .phi.1 and .phi.2 which are out of phase with one another. These clocks are used to selectively open and close a number of the switches shown in the figure. The clock .phi.2 is an inversion of the clock .phi.1. The switches are used to transition between modes.

S1 switches are used during preset mode while the S2 switches are used during comparison mode. The switches are also labeled as n or p type. The switch types correspond to the channel type of MOSFET transistors that is used to create the switch. Therefore, for example, an S1p switch can be implemented, in practice, as a p-channel MOSFET transistor.

Voltages to be compared are applied to the input terminals V.sub.in.sup.+ and V.sub.in.sup.-. The circuit is preset when all the S1 switches 201 through 210 are closed and the S2 switches 212 and 214 are open. In preset mode, the internal nodes 221, 222, 226 and 228 are grounded. The floating well node 224 is charged to 1.0 volt, and the node 220 is also charged to 1.0 volt.

The low-voltage comparator 200 is configured as a differential amplifier with a pair of p-channel MOSFET transistors 240 and 242. Each of the two p-channel MOSFET transistors 240 and 242 has a threshold voltage between approximately 0.7 and 0.9 volts.

Transition to the comparison mode is made by opening all the S1 switches 201 through 210, and closing S2 switches 214 and 212. The boosting circuit operates as follows. At an initial time, the switches 201 and 210 are closed thereby charging the capacitor 230 to a 1.0 volt charge. At the same time, 1.0 volt is placed on the well capacitor 232 which drives the C.sub.well by closing the switch 208. The charge is subsequently boosted by opening the switches 201, 208 and 210. This pumps the charge out of the boost capacitor 230 that was charged to 1.0 volt during preset. This charge is additive to the node 220, and hence drives the voltage on node 220 up above the precharged 1 volt. The voltage on node 222 follows the voltage on node 220 because the switch 212 is closed. The node 222 is a common source terminal of the differential pair 240 and 242.

The comparison starts when the common source voltage of the differential pair approaches V.sub.in +V.sub.threshold value of the transistor which has the smaller V.sub.in. In this case the V.sub.threshold is between 0.7 and 0.9 volts. Before the common source voltage reaches that value (V.sub.in +V.sub.threshold), no drain current flows in either transistors 240 or 242. Hence, output nodes 226 and 228 are pulled to the ground.

When the common source voltage reaches that value (V.sub.in +V.sub.threshold), all the drain current will flow through the p-channel of the MOSFET transistor that has the higher V.sub.in value. Once that occurs, the node with the higher input gate voltage charges up to the level of the common source voltage minus the threshold voltage. This voltage level is well above the threshold voltage of the n-channel MOSFET transistors 244 and 246. Since each node 226 or 228 is tied to the cross-coupled gate of the n-channel MOSFET transistor, only the node corresponding to the higher input voltage is pulled to the ground.

The voltage boosting at the common source terminal 222 forward-biases the PN junctions of the p-channel MOSFET transistors 240 and 242. The forward biasing of the PN junctions allows the current to flow from the common source 222 to the body terminals 224 of the transistors 240 and 242. As a source to body voltage approaches 1.4 volts, the current into the substrate becomes substantial. The current flowing through the PN junctions charges the n-well 224 of the transistors 240 and 242. The voltage charged by the well capacitor 232 is pulled up above 1.0 volt. The charging of the well capacitor 232 and the subsequent boosting of the voltage at the n-wells allow the common source voltage to rise to about 2.0 volts without limitations.

The boosting of the voltages at the common source and the wells of the differential pair reduces the necessary bias voltage, and allows the comparator circuit to operate with a supply voltage as low as 1.0 volt. The input voltages can swing from 0 to 1.0 volt without being in a "dead zone" of the threshold voltage.

The embodiment saves power consumption by automatically adjusting the output current depending on the input signal. In a preferred embodiment, the energy consumption is almost independent of the input voltage and is less than C.sub.boost *V.sub.dd.sup.2 per comparison. If the boosting capacitor 230 is 100 fF and V.sub.dd is equal to one volt, the energy consumption is 100 fJ per comparison. Measured at a data rate of 1 MHZ, the power consumption is only 100 nW per comparison.

In an alternative embodiment shown in FIG. 3, a coupling capacitor 234 is inserted between the substrates of the differential pair and the negative plate of the boost capacitor. This coupling capacitor pulls the n-well voltage up during boosting and allows the boosted common source voltage at node 222 to go as high as 2.0 volts with limitations. This is an alternative way to pull the n-well voltage up during boosting.

A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, while the invention has been described in terms of p-channel MOSFET transistors in differential pair and n-channel MOSFET transistors in cross-coupled configuration, the dopants of the channels can be switched in the pairs. Accordingly, other embodiments are within the scope of the following claims.