Transconductor devices, such as transistors, are generally non-linear devices. For this reason, an intermodulation results in the occurrence of intermodulation bands in the communications channel, causing a degradation of the signal/noise ratio, or a phase-trajectory error.
When two signals or tones having the same amplitude and different frequencies are applied to a non-linear device, such as a simple transistor biased ideally via an anti-surge inductor, two intermodulation components of order 2 (IMD2) in the output signal having frequencies respectively equal to the sum and to the difference of the frequencies of the two input signals are obtained. Moreover, the 2nd order intermodulation component, whose frequency is equal to the frequency separation of the two tones, may be situated within the useful band of the signal and may consequently interfere with this signal.
Current radio frequency receivers use architectures of the zero intermediate frequency type (ZIF architecture), which consequently offer a direct conversion into base band, or low intermediate frequency architectures. In these types of architecture, the radio frequency input signal is directly converted into base band, or in the neighborhood of the base band (intermediate frequency signal) in a single step by way of a frequency transposition using a local oscillator signal.
This type of architecture does not require an external intermediate frequency filter, and allows a single local oscillator to be used. For these reasons, these architectures have a lower cost and are widely used. One drawback of these architectures resides in the presence of a 2nd order intermodulation level, also called second order intermodulation distortion (IMD2) which, as indicated previously, introduces undesirable spectral components into the base band and, consequently, degrades the sensitivity of the receiver.
In radio frequency receivers, the 2nd order intermodulation components are generated by non-linearities, unmatched components and undesirable signal leakages within the radio frequency stage. However, the main cause of the appearance of IMD2 resides in the non-linearity of the transconductors.
Furthermore, the specifications on the values of power supply voltage and on the current consumption are becoming more drastic for radio frequency receivers. Thus, the receiver needs to be capable of simultaneously supplying several blocks of stacked cells with a low power supply voltage and of reducing the current consumption.
Various approaches exist for forming a transconducting device, in particular in a radio frequency receiver. These approaches differ in the relationship between the transconducting device and the mixing device (frequency transposition device) and/or the type of architecture of the transconducting device.
As far as the connection between the transconducting and mixing devices is concerned, a first architecture includes coupling the transconducting element to the mixing device via an AC coupling, generally via a capacitive stage, thus blocking the DC component. Then only the high-frequency components of the signal migrate towards the input of the mixer. Consequently, the IMD2 components produced by the transconductor stage are eliminated, or at least greatly weakened, at the input of the mixer.
Such an architecture has a drawback in terms of power consumption. This is because two different biasing approaches are needed: one for the transconductor stage and the other for the mixer-load stack. Consequently, this results in a high current consumption.
In contrast, for an architecture in which the transconductor stage is directly connected to the input of the mixer via a coupling of the DC type, only one biasing approach is required to bias both the transconductor stage and the mixing stage. The current consumption is therefore low.
In this architecture, all the frequency components of the signal migrate towards the input of the mixer. Consequently, the IMD2 frequency components produced by the transconductor stage are unsuppressed at the input of the mixer stage.
Such an architecture also has another drawback. The problem is that the more the value of the power supply voltage is reduced, the more difficult it becomes to form a transconductor stage-mixer/load stage stack, between the power supply terminal and ground because of the voltage drops across the terminals of each of the elements of the stack.
As far as the type of architecture of the transconductor stage itself is concerned, an architecture of the pseudo-differential type and an architecture that is fully differential may essentially be distinguished. In a pseudo-differential architecture, the biasing of the stage is effected by applying two DC voltages to the bases of the transistors of the transconductor stage. Such a structure has the disadvantage of exhibiting a low emitter impedance both at low and high frequencies. Consequently, the low-frequency currents, and therefore the IMD2 currents, are high at the output of the transconductor stage.
A fully-differential transconductor structure differs from the preceding structure in the sense that the biasing of the transconductor stage is effected by a current source connected to the transistor emitters. The result of this is then that the emitter impedance of this architecture is high at low frequency. Consequently, the IMD2 currents are low at the output of the transconductor stage.
Such an architecture has the drawback of making the stacking of all the cell blocks (transconductor stage, mixer-load stage) difficult with a low power supply voltage.