1. Field of the Invention
The present invention relates to a frequency converter and a radio communication apparatus with the same.
2. Description of the Related Art
In recent years, miniaturization, reduction in costs and low power consumption of radio terminals such as portable telephones, and portable information terminals have been developed energetically. In order to satisfy request of both of miniaturization and reduction in costs of the radio terminal, there is employed a method of realizing a circuit (a circuit handling a signal of a RF band) used for the radio unit of the terminal by IC (integrated circuit) as much as possible.
On the other hand, the low power consumption of the radio terminal is realized mainly by improvement of the property of a device including active elements such as transistors and passive elements such as capacitors and improvement of a circuit technique thereof.
As one of functional circuits in the radio unit of the radio terminal is used a frequency converter which converts a received signal into a lower frequency signal, and a transmission signal into a higher frequency signal. For example, a single balance mixer (SBM) and a double balance mixer (DBM) are known as conventional frequency converters.
The frequency converter such as SBM and DBM includes a problem to produce distortion, particularly the third order distortion IM3 (the third order intermodulation distortion) in a frequency conversion output signal due to the non-linearity of the Gm amplifiers and differential pair of transistors. The simplest method for reducing the distortion by a circuit technique is to increase an operating current flowing through the Gm amplifiers and a differential pair of transistors constructing a current switch.
However, in a method for making distortion reduce by increasing the operating currents of the Gm amplifiers and differential pair of transistors, if input referred third order incept point (IIP3) generally known as index of distortion must be improved by 6 dB, it is necessary to increase about 2 times the operating current. This is undesirable in terms of the low power consumption.