Wireless devices have been in use for many years for enabling mobile data and communication. Such devices can include mobile phones and wireless enabled personal digital assistants (PDAs) for example. FIG. 1 is a generic block diagram of the core components of such wireless devices. The wireless core 10 includes a baseband processor 12 for controlling application specific functions of the wireless device and for generating and receiving voice or data signals to a radio frequency (RF) transceiver chip 14. The RF transceiver chip 14 is responsible for frequency up-conversion of transmission signals, and frequency down-conversion of received signals. The RF transceiver chip 14 includes a receiver core 16 connected to an antenna 18 for receiving transmitted signals from a base station or another mobile device, and a transmitter core 20 for transmitting signals through the antenna 18 via a gain circuit 22. Those of skill in the art will understand that FIG. 1 is a simplified block diagram, and can include other functional blocks that may be necessary to enable proper operation or functionality.
Third-order intermodulation (IM3) is a common interference problem in RF where two or more signals mix in a non-linear phase or “device” to form one or more new signals, and thereby creating intermodulation products. In the RX section of a transceiver chip, these intermodulation signals may fall on top of a desirable signal (in frequency domain) thereby reducing the signal to noise ratio. For the TX section, intermodulation may cause the signal to spread out causing power to leak in a neighbouring band. Various elements within a radio use transconductance cells. These transconductance cells convert voltage into current, but also add third order harmonic distortion. As an example, active mixers typical use transconductance cells as an input stage to the mixing cell and are widely used in modern communication systems in order to achieve frequency translation of the carrier signals. Intermodulation distortion in the mixer affects the dynamic range of most communication systems. The IM3 of transconductance cells is governed by the voltage to current transfer function produced by elements within the cell (i.e. transistors) and the amount of feedback in the circuit
A typical output of a transconductance circuit is given by:iout=A1vin+A3vin3+ . . .where iout is the output current, A1 is the transconductance gain of the circuit, vin is the input voltage, and An are distortion terms where n>3. However, a nearly linear relationship between iout and vin is desirable since many communications standards specify the amount of distortion that is acceptable. Failure to comply with such standards may result in non-certification of a device. It is therefore desirable to eliminate distortions.
In the past, distortion cancellation was accomplished by techniques employing bipolar transistors as shown in such references as U.S. Pat. No. 6,781,467 (Sun), U.S. Pat. No. 5,497,123 (Main et al), S. Otaka, M. Ashida, M. Ishii, T. Lakura, “A+10 dBm IIP3 SiGe Mixer with Cancellation Technique,” ISSCC2004, and B. Gilbert, “The MICROMIXER: A highly linear variant of the Gilbert mixer using a bisymmetric class-AB input stage” in J. Solid-State Circuits, vol. 32, pp. 1412-1423, September 1997. The general approach of linearization is to add circuitry such that the terms An are brought to zero and A1 remains approximately the same (in most cases A1 reduces); note, for an ideal amplifier An=0, where n is greater than or equal to 3. In Main et al and Otaka, A3 is made up of two terms that oppose each other; i.e.A3=A3+−A3−where A3+ and A3− are made equal using component values in the circuit. In Main et al a phase shifting technique is used and is applied to a mixer architecture. In Otaka, a resistor value is used set the A3 to zero. The linearization technique used by Gilbert consists of pre-distorting the signal so that the terms An are set to zero at the output after it passes thru a distorting amplifying stage, which will introduces excessive noise. Sun uses bipolar transistor technology for a low noise amplifier. All these methods reduce the amount of third order distortion, but these past implementations are bipolar based, opposed to Complementary Metal Oxide Semiconductor (CMOS) based. The advantages of CMOS technology are cost and the fact the technology improves at a rate given by Moore's Law. These techniques are also susceptible to manufacturing variations in device parameters. Specifically, the IM3 may not be reduced because of variations in transistors parameters from part to part. Other techniques use feedback to reduce the amount of IM3 in a circuit. However feedback circuits introduce noise and increase the total amount of current and area required by the circuit.
Such techniques also employ SiGe or GaAs devices and technology, which although they exhibit highly linear characteristics, the technologies are new and expensive. Consequently, the cost of manufacturing often outweighs the benefits of using such devices.
It is therefore desirable to provide CMOS technology for tuning out IM3 products in transconductance circuits. The invention described below is a tuneable method for reducing the IM3 tone in a transconductance element so to make it less susceptible to manufacturing variations and the transconductance implementation is described within CMOS technology.