VARIABLE TEMPERATURE COEFFICIENT LEVEL SHIFTER
Background of the Invention
Field of the Invention:
This invention relates generally to a voltage level shifter and, more particularly, to a circuit for generating a voltage having an independently controllable temperature coefficient and amplitude.
Description of the Prior Art:
The need often arises to provide an output current or voltage having a zero temperature coefficient, and circuits for accomplishing this are well-known. For example, reference is made to U.S. Patent 3,887,863 entitled "Solid-State Regulated Voltage Supply", U.S. Patent 3,617,859 entitled "Electrical Regulator Apparatus Including A Zero Temperature Coefficient Voltage Reference Circuit", and U.S. Patent 3,893,018 entitled "Compensated Electronic Voltage Source". Such circuits generally offset the negative temperature coefficient of a base-to-emitter voltage (VBE) of one transistor with a positive temperature coefficient derived from the base-to-emitter voltage differential (ΔVBE) between a pair of transistors. One of the problems associated with this prior art technique is that the amount of negative temperature coefficient that may be introduced into the output is severely restricted by a single VBE.
Summary of the Invention
It is an object of the present invention to provide a voltage level shifting circuit having a controllable temperature coefficient and which produces a stable independently controllable level shifting voltage amplitude.
It is a further object of the present invention to provide a voltage level shifting circuit having a controllable temperature coefficient and an independently controllable shift amplitude which is not affected by circuitry coupled to its output or otherwise associated therewith.
It is still further object of the invention to provide a voltage level shifting circuit having a controllable temperature coefficient and an independently controllable shift amplitude which does not require multiplying or the use of resistive voltage dividers.
According to a first aspect of the invention there is provided a level shifting circuit for producing an output voltage having a desired amplitude and temperature coefficient, comprising: a first supply voltage terminal; a second supply voltage terminal; a first current source coupled to said first supply voltage terminal for generating a first current having a positive temperature coefficients a second current source coupled to said first supply voltage terminal for generating a second current having a negative temperature coefficient; and first resistive means coupled between said first and second current sources and said second supply voltage terminal for combining, said first and second currents to produce a third current having a net temperature coefficient corresponding to said desired temperature coefficient and for generating from said third current a voltage having said net temperature coefficient, said voltage having said desired amplitude. According to a further aspect of the invention there is provided a method for level shifting a voltage, the amplitude of the level shift and the temperature coefficient thereof being independently controllable, comprising: generating a first current having a positive temperature coefficient; generating a second current having a negative temperature coefficient; varying the magnitude
of said first and second currents to achieve a net negative, zero, or positive temperature coefficient; and applying the sum of said first and second currents to a first resistive means the resistance of which being chosen to produce a required level shift.
Brief Description of the Drawings
The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawing in which the sole figure is a diagram, partially in block form and partially in schematic form, illustrating the invention.
Description of the Preferred Embodiment
The inventive arrangement shown in the drawing includes first and second resistors RN and RP coupled between ground and nodes 4 and 6 respectively. A third resistor RS is coupled to a source of supply voltage (V+) and to node 2 from which the circuit output is taken. Block 8 which is coupled to nodes 2, 4, and 6 as shown includes circuitry for generating a first voltage VBE and a second voltage ΔVBE, VBE corresponding to the base-emitter voltage of a transistor and having a negative temperature coefficient, and ΔVBE being the base-toemitter voltage differential between a pair of transistor and having a positive temperature coefficient. Circuits for generating these voltages are well-known and a further description is not deemed necessary here. However, the interested reader is referred to the above-cited U.S. Patent 3,887,863. Resistors RN, RP and RS may be internal to an integrated circuit chip or external thereto.
With VBE appearing at node 4, the current flowing through RN has a negative temperature coefficient and a value of VBE/RN. In like manner, with ΔVBE appearing at node 6, the current flowing through RP has a positive temperature coefficient associated therewith and a value of ΔVBE/RP. Thus, the total current flowing through resistor RS (ICNT equals VBE/RN plus ΔVBE/RP). This current has a net temperature coefficient associated with it which is controlled by properly selecting resistors Rpj and RP. For example, if RN is open (infinite impedance), the temperature coefficient of ICNT is totally due to the ΔVBE component and is therefore positive. If, on the other hand, RP is open, the temperature coefficient of ICNT is due to the VBE term and is therefore negative. Thus, by properly scaling RN and RP, the temperature coefficient of ICNT may be varied from approximately -2800 parts-per-million to +3000 parts-per-million.
Now that the temperature coefficient has been set to some desired value, the magnitude of the level shift appearing at node 2 can be set to some desired magnitude by properly selecting resistor RS. The voltage drop across RS will now have the same temperature coefficient associated therewith as was imparted to the control current ICNT. Thus, a voltage source has been created which has a controllable temperature coefficient and an independently controlled magnitude. That is, temperature coefficient is controlled by selecting RN and RP, and the magnitude of the shift is controlled by selecting RS.
Several advantages of the arrangement shown in the drawing should be noted. First, it is only the ratio of the resistors which sets the amplitude of the level shift and not the absolute values of the resistors. This reduces resistor tolerance requirements as long as the resistors are created using common resistor processing. For example,
if the values of RN and RP are high, the current will be low. However, since the value of RS will also be high, the resulting level shift remains the same. Second, the level shift voltage across resistor RS is constant regardless of fluctuations in the supply voltage V+.
The above description is given by way of example only. Changes in form and details may be made by one skilled in the art without departing from the scope of the invention.