1. Technical Field
This invention relates to a transconductance stage and methods for its use, and more specifically to a highly linear tunable six inverting amplifier transconductance stage and methods for its use.
2. Background Art and Technical Problems
In modem electronic circuits, there is a need for filter blocks which are highly linear, low power, and capable of high frequency. Filter blocks are especially advantageous and are used extensively in conjunction with the implementation of transceivers in the telecommunication and network fields. Analog transceiver chains contained in such transceivers generally contain several filtering blocks that allow the removal of parasitic signals so that a "clean" signal is delivered to the digital signal processing circuit. These filtering blocks are often distributed throughout the transceiver system and encompass filters extending in frequency range from very high frequency to very low frequency. By way of illustration, FIG. 1 shows an analog receiver chain 7 including a low noise amplifier (LNA) 25, a first filter 26, a mixer 27, a variable gain amplifier (VGA) 28, and a second filter 29.
Such filters can be implemented by using transconductance stages. Solutions to the issues of linearity and frequency control in high frequency filters have been disclosed in Nauta, "A CMOS Transconductance-C Filter Technique for Very High Frequencies," IEEE Journal Solid-State Circuits, Vol. 27, No. 2, February 1992, the disclosure of which is hereby incorporated by reference. The six inverting amplifier structure disclosed in Nauta contains no internal capacitive nodes, so that the circuit is suitable for high frequency applications. In addition, the disclosed structure contains only two transistors in series. The structure is therefore suitable for low voltage applications. Unfortunately, in the structure disclosed in Nauta, the only way that the cut-off frequency and DC gain can be controlled is by adjusting the power supply for the circuit. Adjusting the power supply of an integrated circuit is not a reasonable or practical method of operation, since integrated circuits in most applications operate from a fixed supply voltage. In addition, the Nauta circuit has very poor immunity to power supply noise because the devices are connected directly to the power supply. Because the circuit is controlled by changes in power supply, any noise in the power supply is reflected as a change in the output performance.
A need therefore exists for an improved transconductance stage which can be used for high frequency applications, in which the cut-off frequency can be controlled, which is highly linear, which has low power consumption, and for which the DC gain can be controlled.