In wireless receivers, a wide range of signal levels needs to be handled. Some of the reasons for this wide range of signal levels are that changes in the path loss are frequent and so are changes in distance from the transmitter to the receiver. One of the most common methods of accommodating this high dynamic range requirement is to use a variable gain element. This element (or elements), with some control circuitry, acts in such a manner that the signal level at the receiver output is kept constant.
One method of implementing a variable gain element known to those skilled in the art is Lo use dual-gain emitter-coupled differential pairs. This method is shown on page 46 of "RF-Heterodyne Receiver Design Technique for Digital Cellular Wireless Systems" published in 1995 by Josef Fenk of Siemens. This publication is hereby incorporated by reference. With one single amplifier stage, two different values of gain may be realized In order to accomplish multiple gains, several stages of dual-gain amplifiers are stacked in series. This method is also well known to those skilled in the art. With this method, stacking three dual-gain amplifiers cells in series will result in eight different gain values. Several points should be noted concerning this realization. First, in order for it to work, it is necessary that proper DC level shifting be performed at the interface of different dual-gain amplifiers. Second, when this circuit is in operation, one differential pair in each dual-gain cell must be turned on for each gain setting. This results in unnecessary power consumption.
Thus, there exists a need in the industry to provide an improved switchable gain amplifier particularly well suited for an environment where multiple levels of gain are required.