The invention relates generally to n-type negative resistance device (NNRD)-bootstrapped current sources, and to NNRD-bootstrapped voltage references derived from the current sources. The invention relates more specifically to embodiments of NNRD-bootstrapped DFET current sources, one being a self-bootstrapped reference, the other bootstrapping an NNRD in relation to a load impedance through an additional circuit constraint imposed by a current mirror.
In many material systems or device technologies current- and voltage-source devices exist as highly imperfect approximations which do not enable generation of stable, well regulated, precision signals. Compared with silicon, GaAs in particular, and the III-V compounds and alloys in general, do not at present permit fabrication of devices enabling generation and maintenance of precision signal levels. Voltage-source devices such as Zener diodes are of low quality in these materials and are not process-compatible with fabrication of active devices. Current-source devices such as MESFETs exhibit a high degree of threshold voltage instability and drain-source conductance, and relatively low gain, rendering them unsuitable as the basis for precision current source circuits. GaAs Heterojunction Bipolar Transistors (HBTs) exhibit smaller conductance than GaAs MESFETS and HBT technology may be suitable for either bandgap or Vbe reference circuits. However, HBT reference circuits exhibiting the capability for stability and precision found in silicon reference circuits have not yet been demonstrated or proposed.
At present, existing monolithic voltage GaAs MESFET references can support 8 to 10 bits of precision in high-speed digital-to-analog (D/A) converters. To extend the precision of D/A conversion in these inherently high-speed technologies to levels achieved in silicon integrated circuits (operating at lower frequencies), one requirement is to provide improved integrated current and voltage references. The impending monolithic integration of resonant tunneling diodes (RTDs) with GaAs MESFET transistors provides a new means for providing, within a high-speed GaAs MESFET integrated circuit, a device with current-source properties which can be exploited to yield a constant current in the presence of large fluctuations in supply voltage. This is achieved by using high-gain amplification to bootstrap the operating point of the reference circuit to the local, high resistance operating point of the RTD.
Silicon integrated circuit technologies typically incorporate bipolar devices such as forward biased p-n junction diodes for precision level shifting, and derive reference voltages proportional to breakdown voltages in Zener diodes, or to the bandgap energy of the host semiconductor. In other integrated circuit technologies such as those employing GaAs MESFETs, heterojunction bipolar transistors, MODFETs (or HEMTs), voltage references with a high degree of supply rejection (similar to that obtained in precision silicon circuits) have not yet been demonstrated.
FIG. 1 shows an example of a GaAs integrated circuit representative of prior art found in U.S. Pat. No. 4,686,451 issued to Li et al. for "GaAs Voltage Reference Generator", implementing Schottky diodes D1, D2, D3, D4, and MESFETS M1 and M2. For the case that the diode conductances g.sub.D1 .perspectiveto.g.sub.D2 .perspectiveto.g.sub.D3 .perspectiveto.g.sub.D4 (.perspectiveto.g.sub.D) are all of the same order as the transistor transconductances g.sub.m1 .perspectiveto.g.sub.m2 (.perspectiveto.g.sub.m), and that the transistor drain-source conductances g.sub.1 .perspectiveto.g.sub.2 (.perspectiveto.g) stand in the relation g.sub.m .perspectiveto.g.sub.D &gt;g, then the fractional shift in the reference voltage due to supply fluctuations is ##EQU1##
The supply rejection for operative circuits of the type shown in FIG. 1 is approximately 50 db. Equation (1) indicates that the regulation can only be improved by either increasing device areas, at the expense of increasing power dissipation, or by turning on the devices to a greater degree at the expense of requiring larger supply voltage.
It will be appreciated by those of skill in the art that this method of bootstrapping is inefficient in the sense that the series gate-source connection employed in circuits described by Yang and Allstot, "Improved Self-bootstrapped Gain Enhancement Technique for GaAs Voltage Reference Generator" and in U.S. Pat. No. 4,686,451, between nodes A and B of FIG. 1, is a low impedance port. Connecting forward biased (thermionic) diodes at this port results in a bootstrapped operating point which depends upon the intersection of two low-impedance loadlines. Independent shifts in either loadline result in large changes in the state of the circuit. This is in contrast to the case of a high impedance device biased by a low impedance loadline, where independent shifts in the loadline create minimal shifts in the state of the high-impedance device/network.
It should further be observed that forward-biased Schottky diodes do not exhibit the abrupt turn-on shown by similarly biased p-n junction diodes, since current conduction in the former is by a combination of tunneling (soft turn-on) and thermionic emission (hard turn-on), while in the latter the current is conducted by thermionic emission only (for non-degenerately doped devices). Therefore, the gate-source voltages, and hence the currents, will shift substantially in response to changes in supply voltage.