Sample-and-hold amplifiers, and their close cousins, track-and-hold amplifiers, are widely used electronic building blocks. They are employed, for example, to obtain instantaneous samples of time-varying signals, for signal conversion and discrete signal processing systems.
The sample-and-hold amplifier field is well-developed, with a great variety of architectural variations and specific designs being available, implemented both monolithically and in discrete systems. In general, SHA's may be classified into two categories: (1) closed loop SHA architectures, tending to provide greater accuracies at lower speeds and (2) open loop schemes, which offer the greatest bandwidths but seldom provide accuracy beyond the eight-bit level.
Although SHA's are typically configured to handle signals in the voltage mode, the advantages afforded by current-mode analog systems become more pronounced when seeking greater and greater system bandwidths and clocking frequencies. Conventional voltage-mode SHA's are extremely cumbersome to adapt into current-mode systems, however. They require current-to-voltage and voltage-to-current conversions. In a system where both high accuracy and high speed are important, these conversions can be very difficult to implement, and they add significant circuit complexity and power dissipation.
An additional feature required by many analog systems is gain in the SHA. This is realizable in the closed loop schemes by putting attenuation in a feedback network, as would be done with any operational amplifier. Just as in conventional operational amplifier theory, however, this runs into some rather severe limitations at high frequencies, where the gain-bandwidth product of the amplifier imposes the trade-off that the greater the gain used, the lower the bandwidth of the SHA will be. This can have serious ramifications, not only on the bandwidth of the SHA, but also on the acquisition time and the sample-and-hold settling time.
Additional general principles and background on SHA circuits may be obtained from extensive published texts and articles on the subject.
Accordingly, it is an object of the present invention to provide an improved sample-and-hold amplifier which implements a closed-loop architecture, but: which still provides excellent bandwidth and sampling frequency.
Another object of the invention is to provide a sample-and-hold amplifier which generates a current output.
A further object of the invention is to provide a sample-and-hold amplifier which generates both a current output and a voltage output.
Yet another object of the invention is to provide a sample-and-hold amplifier which will accept a current or voltage input and provide both a current and voltage representation at the output.