Voltage references are required in many types of electronic equipment. As is well-known they are commonly designed to produce outputs proportional to the semiconductor band-gap voltage (e.g., the silicon band-gap voltage at approximately 1.26V), which is inherently well-defined and insensitive to temperature.
In practice the band-gap voltage is obtained by the summation of two components, a first proportional to the difference in the bias voltages of a pair of junction diodes operating at different current densities, commonly referred to as the PTAT (proportional to absolute temperature) component and a second proportional to the full junction voltage of one of the diodes, or a similar diode, commonly referred to as the CTAT (complementary to absolute temperature) component.
A significant problem is that the PTAT component, attaining 18 mV for each factor of two in the current density ratio, cannot, practicably, be made large. Voltage offsets in the summation circuits can therefore introduce relatively large errors. Likewise, the reference output may be sensibly degraded by low frequency noise components introduced by the summation circuits.
Another problem is that the silicon band-gap voltage of 1.26V is higher than the maximum operating voltage permitted for the most recent CMOS circuits. Some well-defined fraction of this must therefore be generated. The deleterious effects of offset voltages and low frequency noise are commonly mitigated by using large area active and passive elements in the processing circuits. A disadvantage of this approach is that the offsets and noise remain only statistically predictable and are subject to variations and changes in the manufacturing process. A second disadvantage is that the physical area required may become prohibitively large. Further disadvantages are that large devices are more susceptible to leakage current, which is another noise and error source, and are more susceptible to perturbing signals.
In another approach, switched capacitor reference generators employing offset compensation, which aim to sensibly reduce errors and low frequency noise without recourse to large area devices, have been developed. However, while conventional switched capacitor reference voltage generators may show significant reduction of the error produced by an offset voltage of the summation circuit, they generally do not fully eliminate it.
Furthermore, in some conventional switched capacitor reference generators the residual error becomes increasingly significant as the reference voltage is reduced. Other conventional switched capacitor reference generators may be adapted to produce reference voltages below the band-gap level, while maintaining relatively low sensitivity to the offset voltage. The conventional generators, however may use voltage subtraction means to reduce the CTAT component, making the output increasingly sensitive to capacitor matching errors as the reference voltage is reduced.
A further disadvantage of conventional switched capacitor reference generators is that the output may be discontinuous, alternating between a “pre-charge” state and a “valid” state, such that further sampling may be needed to provide a continuous voltage. Another, related, disadvantage is that, as the amplifier must charge feedback capacitor means to pass from the pre-charge to valid states, the bandwidth must be large compared with the clock frequency, producing high noise levels.
It may therefore be advantageous to provide a reference voltage generator adapted to produce continuous outputs much smaller than the band-gap voltage, which is fully insensitive to any voltage offset in the associated summation circuit, which produces a small CTAT component without recourse to voltage subtraction means and which generates low noise without further filtering.