Systems involving thermal printers or LED imaging, for example, often require accurately apportioned, temperature stable multiple current sources and/or sinks. Typically, it is preferred that such current sources and/or sinks be provided in integrated circuit form. Also, many of these and other types of systems require considerable logic-signal processing. The various functions required, along with low power consumption, are often provided by CMOS devices in combination with accurately matched output-current drivers. The latter components produce local thermal gradients on the silicon substrate, where the preferred integrated circuits are utilized. The thermal gradients often cause undesirable changes in the magnitudes of current flowing through various current sources or sinks located on the substrate. MOSFET technology is often used to attempt to satisfy applications requiring multiple current sources and/or sinks.
Known MOSFET current sources and sinks do not meet the operating requirements of many present applications, and do not provide a relatively high degree of accuracy in matching the magnitudes of the slave currents to the magnitude of the master current. In such integrated circuits, matching of devices on the integrated circuit chip varies with current density, with higher current densities generally providing better matching in the square-law region. However, such high-density operation requires the use of relatively large operating voltages, and relatively large positive gate-to-source voltage temperature coefficients pertain. Also, to minimize the required area on the silicon integrated circuit substrate, small channel lengths are typically used, which result in both poor matching and low dynamic output resistance (rout). As a result, the integrated circuit current mirror, for example, is very sensitive to load and supply voltage variations.
In many applications involving monolithic integrated circuit current mirrors for use as current sources or current sinks, such devices must also be programmable, typically in a digital fashion (programmably turned on or off). In such devices, the magnitudes of the output currents are significant, and local thermal gradients will vary throughout the chip, dependent upon the programming word applied at a given time for turning on or off various ones of the devices on the integrated circuit chip, or by some other power source causing varying thermal gradients on the integrated circuit substrate. As a result of the local thermal gradients, the accuracy of the current ratios or magnitudes is often diminished. Programmable monolithic integrated circuit current mirrors or sinks may include a large number (e.g. 84) of slave outputs. Such devices would require prohibitively complex interconnections within the integrated circuit should one attempt the normal practice of interdigitating devices throughout the chip, for obtaining temperature averaging, to reduce errors in slave current magnitudes due to the previously mentioned thermal gradients.
There have been many attempts in the prior art to reduce the effects of temperature gradients on the performance of transistor amplifiers, particularly integrated circuit current mirror transistor amplifiers. Examples of such prior attempts follows.
Schade, U.S. Pat. No. 4,243,948, entitled "Substantially Temperature-Independent Trimming of Current Flows", issued Jan. 6, 1981, teaches in an electronic device, a circuit including a positive-temperature-coefficient resistor and semi-conductor diode connected in parallel with a circuit for generating trim current. The latter circuit either consists of a relatively large, zero-temperature-coefficient adjustable resistance, or includes such a resistance connected in series with a zero-temperature-coefficient voltage source. In this manner, the trim for the current flow in the series-connected circuit is substantially unaffected by temperature gradients or changing temperature.
Wheatley, U.S. Pat. No. 4,051,441, entitled "Transistor Amplifiers", issued on Sept. 27, 1977, teaches in an NPN current mirror amplifier the use of emitter degeneration resistances that have temperature coefficients of 1/T.sub.0 for a range of temperatures around T.sub.0. Each emitter degeneration resistance includes a current source in loop connection therewith for supplying substantially temperature-independent currents, respectively. At least one of the current sources is adjustable for changing the value of the current supplied to control the ratio of the collector currents of the first and second transistors, with the ratio being maintained substantially constant over a range of temperature changes in the vicinity of the transistors.
In Wheatley, U.S. Pat. No. 4,055,811, entitled =Transistor Amplifiers", issued Oct. 25, 1977, a transistor amplifier is disclosed in which the collector currents of first and second junction transistors, having base electrodes biased at the same quiescent potential, and emitter electrodes connected via a respective emitter degeneration resistance to a common point, are adjusted relative to each other by applying linearly temperature-dependent potentials to the latter, with at least one of the potentials being adjustable, for providing adjustment of the relative values of the collector currents that remains substantially unchanged over a range of temperature.