Threshold voltage correcting reference voltage generator

Technical Field

0001This invention relates to MOS analog sampling circuits of the charge metering type, and to avoidance of inaccuracy in their operation due to variation in the threshold voltage of MOS transistors from circuit to circuit.

Background Art

0002Charge metering sampling circuits and some of their applications are set forth in application, serial numbers 07/968,697, (now U.S. Pat. No. 5,400,028) and in copending U.S. Patent Application Nos. 07/968,698 and 07/968,699 which are all incorporated herein by reference. Charge metering circuits are of inverting and noninverting types, and a widely useful configuration consists of an inverting stage followed in cascade by an noninverting stage. The inverting stage has an output which is independent of transistor threshold voltages, but the noninverting stage output is reduced by the zero-current threshold voltage of an MOS transistor in that stage, such that, for example, an increase of 0.1 volt in the threshold reduces the output voltage by 0.1 volts. In many applications a moderate variation in the output, such as would be caused by the variation in thresholds between devices on the same integrated circuit chip, would be tolerable. However, the variation in thresholds between different chips is larger and would need correction.

0003An example of the aforementioned applications is that of the track-and-hold and analog latch stages of a sampled-ramp data line driver circuit for active matrix liquid crystal displays, as set forth in application serial number 07/968,699. Because of the periodic inversion of the liquid crystal cell voltage, the RMS voltage which controls the state of the liquid crystal is insensitive to small output voltage level shifts such as would be caused by threshold variations within a single chip. However the overall threshold variation between circuits on different chips is excessive and must be corrected. (Nevertheless, the present invention is capable of correcting for threshold variations between regions of a chip or on a circuit by circuit basis if required.) The correction must take into account the target circuits' operating conditions which affect their thresholds; the equilibrium current (zero), the source-to-substrate voltage at some input condition, and the source voltage being in equilibrium with the channel potential.

0004Inverting stages have a feature which offers a unique opportunity to correct for threshold variations in the noninverting stages: A reference voltage is applied to the inverting stages which precisely determines their initial output voltage level. The above referenced patent and patent applications point out that the required correction can be accomplished by making the reference voltage applied to the inverting stage depend on the threshold voltage with the correct sense to compensate for the threshold dependence of the output of the noninverting stage.

0005A conventional approach to providing the threshold correcting reference voltage would involve the use of analog processing using operational amplifiers. The latter are undesirable because they occupy large silicon area, have substantial quiescent power dissipation and, absent added circuit complexity and component trimming after manufacture, exhibit significant offset voltage errors.

Summary of the Invention

0006It is an object of the invention to provide a charge metering circuit that generates the threshold correcting reference voltage directly under the required operating conditions without the need for further processing.

0007It is another object of the present invention to merge the disclosed circuit with the target circuits and share some of the components of the target circuit, thus reducing the required silicon area.

0008It is a further object of the present invention to provide a circuit which has no quiescent power dissipation and introduces no offset voltage errors.

0009The invention is preferably embodied in CMOS integrated circuits, but may alternatively be implemented with NMOS or PMOS as would be understood by one skilled in the art. Also, while the invention is described here in terms of preferred transistor types, other choices of NMOS or PMOS for each transistor of the invention are possible with suitable changes in the applied voltages and clock signals. However, the type of the metering transistor in the circuits of the invention should agree with the type of the metering transistor of the noninverting stage of the target circuits, the threshold voltage of which the invention is to correct. In the following description of the invention the metering transistors are assumed to be NMOS.

0010In an embodiment of the invention, a capacitor is charged from a first voltage source to a voltage above the desired corrected reference voltage by means of a charging transistor controlled by a clock pulse. A metering transistor has its drain and gate electrodes connected to the capacitor either directly or through a blocking transistor. The source of the metering transistor is connected to a second voltage source which has a value below the desired corrected reference voltage. The charging transistor is then turned off and the blocking transistor, if present, turns on. Charge flows from the source to the drain of the metering transistor, reducing the capacitor voltage until the metering transistor just turns off. At this point the capacitor voltage is higher than the second voltage source by the threshold voltage of the metering transistor. This capacitor voltage is the desired corrected reference voltage; its value depends directly on the magnitude of the metering transistor threshold voltage. Finally the blocking transistor, if present, turns off to isolate the capacitor. The purposes of the blocking transistor are to avoid the direct flow of current between the first and second voltage supplies when the charging transistor is turned on, and to protect the output from errors due to subthreshold conduction in the metering transistor. Referring back to the requirement that the operating conditions of the metering transistors of the invention and of the noninverting stage of the target circuits be the same; both have zero current at equilibrium and have the same source to substrate voltage (within tolerances) when the input voltage change of the target circuit is zero. In equilibrium both have the source voltage in equilibrium with the channel potential.

0011In the instance where each target circuit has a separate threshold correcting reference generator circuit, the capacitor, the charging transistor and the first voltage supply already exist in the target circuit so only the blocking transistor, the metering transistor and the second voltage source need be added. In the instance where the threshold correcting reference generator serves multiple target circuits, it is uneconomical to provide a separate capacitor in the reference generator. Instead, a second embodiment is employed in which the combination of the capacitors of the target circuits constitutes the capacitor for the reference generator. Transistors already present in the target circuits provide the necessary isolation.

Brief Description of the Drawings

0012Fig. 1 is a schematic diagram of a rudimentary embodiment of the invention.

0013Fig. 2 shows a preferred embodiment of the invention.

0014Fig. 3 is a timing and waveform diagram for the circuit of Fig. 2.

0015Fig. 4 shows the invention serving, and merged with, a single target circuit.

0016Fig. 5 shows the invention serving, and merged with, multiple target circuits.

Detailed Description of the Invention

0017In the figures, similar reference numbers designate corresponding components.

0018A rudimentary embodiment of the circuit of the invention is shown in Fig. 1. The circuit 10 comprises NMOS metering transistor 11, PMOS charging transistor 12, capacitor 13, first voltage supply V1 connected to terminal 14, second voltage supply V2 connected to terminal 16 and output terminal 100. Charging clock pulses are applied to terminal 15. Fig. 2 shows a more complete and preferred embodiment, having in addition to components corresponding to Fig. 1, PMOS blocking transistor 27 and blocking clock pulses applied to terminal 28. Operation will be described for Fig. 2 with the aid of the timing and waveform diagram of Fig. 3. The operation of Fig. 1 will be understood by simply deleting references to transistor 27 and its blocking clock pulses at 28.

0019The function of the circuits of Figs. 1 and 2 is to act upon the voltage supplies V1 and V2 to produce at the output a reference voltage which is greater than V2 by the zero-current threshold voltage of metering transistor 11 (Fig. 1) or 21 (Fig. 2). Voltage supply V1 is at least incrementally greater than the largest output voltage to be obtained. The value of V2 is the difference between the desired minimum value of the output and the minimum of the range of the threshold voltage of Transistor 21. At the start of an operating cycle transistors 22 and 27 are off. Transistor 22 is turned on momentarily by charging clock pulse 35 (Fig. 3) at terminal 25, thus charging capacitor 23 toward V1 as shown as rising portion 391 of waveform 39 at terminal 200. After transistor 22 turns off, unblocking clock pulse 38 at terminal 28 momentarily turns Transistor 27 on. The output voltage decays, stabilizing when the channel Potential of 21 is in equilibrium with V2 so that transistor 21 just turns off. This is shown as falling portion 392 of waveform 39 at terminal 200. At that point, since the gate of transistor 21 is at the output potential (the drop across transistor 27 being negligible), the output is constrained to be greater than V2 by the threshold voltage of transistor 21. To the extent that the threshold voltage of 21 accurately tracks that of the metering device in the noninverting stage of a target circuit, the final output voltage has the value required to correct the target circuit outputs for threshold variations.

0020Fig. 4 shows a preferred embodiment in which reference generator 40, in accordance with the invention, serves a single target circuit 440 and is merged with it such that charging transistor 42 and capacitor 43 are shared by reference generator 40 and target circuit 440. The target circuit metering transistor of which threshold variations are to be corrected is transistor 441. Operation of reference generator 40 is the same as described above and occurs at the time in the operating cycle of target circuit 440 when the output capacitor 43 of the first stage is to be charged to the reference voltage. For optimum threshold tracking, metering transistor 41 should be located as close as possible to the metering transistor 441 of the target circuit, the threshold variation of which is to be corrected.

0021Fig. 5 shows a preferred embodiment in which a reference generator 50, in accordance with the invention, serves several target circuits 550. In this case the load capacitance of the reference generator (corresponding to 13, 23 and 43 of the previous embodiments) is provided by the parallel combination of capacitors 53' of all of the served target circuits, and is equal to the sum of their capacitances. Thus, the reference generator is merged with the target circuits. This case differs from that of Fig. 4 in that charging transistor 52 is not shared and a transistor 590 of each target circuit is interposed between reference generator output terminal 500 and the shared capacitors 53'. In operation, 590 is turned on approximately simultaneously with 52 and is turned off after the reference voltage has reached its final value across 53' but preferably before 52 turns off because that sequence minimizes possible charge injection errors. Metering transistor 51 of reference generator circuit 50 should be located as close as possible to a central position relative to the served target circuits to optimize threshold tracking.