This invention pertains to transconductance amplifiers and more particularly to variable gain transconductance amplifiers using a low supply voltage while operating with high swing input signals, and use of the amplifier in active filters.
The concept of transconductance amplifiers has been known for many years. The major limitation of existing transconductance amplifiers is the restricted differential input voltage swing required to maintain linearity. All prior art transconductance amplifiers have significantly higher supply voltages or reduced signal handling capabilities at lower supply voltages. Some efforts have been expanded to solve this problem, chiefly consisting of using voltage attenuators and buffers at the inputs of existing transconductance amplifiers. This technique can be used to obtain reasonable signal swings in existing transconductance amplifiers. Although such circuits are useful, a rather high price is paid for these modifications. First, the circuit requires many more components. Second, the finite bandwidth of the amplifiers will limit the frequency response of the transconductance amplifier structures. Finally, the attenuation of the input signal to the transconductance amplifier causes a serious loss in dynamic range.
Also, the use of transconductance amplifiers as components of complex filters is known in the art. However, there are no practical prior art filters with an adjustable bandwidth wherein the center frequency is not effected by the adjustment. Generally, to accomplish this task is a prohibitively difficult problem. For example, the technique of mixing a signal down to baseband, lowpass filtering using a programmable lowpass filter, and mixing back up to the desired frequency would accomplish this result. However, this approach would have a high system complexity and would generate undesired mixer spurious responses.