Gm-C integrators for low-power and low voltage applications. A gaussian polyphase filter for mobile transceivers in 0.35
In CMOS transconductors, large tunability needed to correct for temperature and process variations gives a significant reduction in voltage swings at low supply voltages and consequently dynamic range reduction. The new technologies optimized for digital applications are impaired by second order effects like velocity saturation and mobility reduction. Most of the concepts used in the past cannot be used anymore. New transconductor concepts which do not rely upon the ideal square law of a MOST, are needed. Another issue is to achieve large tunability without conflicting with the large swing requirement. In the beginning of this chapter a low voltage, large swing, tunable transconductor has been presented. It features a constant input window for all tuning conditions. This structure overcomes the problems related to non-idealities of the modern MOS transistor in terms of tunability range. The transconductance can be digitally tuned, in ten coarse steps, and continuously, between coarse steps, in the range 30m A/V¸ 85m A/V. If required, the quality factor can be adjusted such that Gm tuning and Q tuning are independent. Total harmonic distortion stays below -50dB for input amplitudes of 1.8Vpp in all tuning conditions and well below -60dB for amplitudes lower than 1Vpp. Large swing property yields a large dynamic range over power ratio. In worst case the noise excess factor is close to 6 and power dissipation is 1.48mW from 3.3V supply. The transconductor can be used as a Gm-C integrator for filter applications.
Positive feedback is a promising technique for enhancing gain in sub-micron CMOS because current matching in modern technologies improves. It avoids cascoding for having large gains and can be used for low-voltage applications. The second type of integrator considered in this chapter is a current Gm-C integrator with local positive feedback for enhancing the gain. The reason for using this integrator consists in the low-voltage, high linearity and very high frequency of operation with a high power efficiency. It is compatible with standard digital technology has a high quality factor Q and can work down to 1.5V power supply voltage. By using the DR*GBW product concept from Chapter 3 it is shown that the current Gm-C approach has better power figures for the same working conditions as Gm-C approach.
The fundamental limits have shown the difference between an analogue filter and a digital one when signal to noise ratio and power/pole are taken as variables in the comparison. However, it is not clear for which S/N ratio the transition between an efficient analog filter and a digital filter takes place. According to the previous conclusions, this transition could be somewhere in between 50dB and 60dB which makes difficult to decide whether or not a digital filter is power efficient when compared to an analog filter. The two integrators presented above are used to realize the video filter from Chapter2 in an analog way and to make a comparison in power to the digital approach. Matching in this type of filters is not important. Therefore we are dealing with noise driven power consumption. It turns out that digital approach has less power consumption per pole than the analog counterparts for a 0.5mm CMOS process. This explains why, in the future, digital filters will be used even for low DR applications.
There are filter applications where matching requirements and noise requirements have the same importance, with constraints on power consumption and linearity. Channel selectivity in receivers has been realized until recently using SAW filters. Those components are external components and therefore integration on chip of selectivity has become a major concern in receivers. From Chapter 3 we already know that selectivity increases the noise power and requires extra power consumption to achieve it. A polyphase filter is an example of a selective filter without the need of high Q bandpass sections. Here selectivity is rather ensured by using polyphase signals where matching driven power consumption comes as a variable. Polyphase filters can discriminate between positive and negative frequencies and therefore, using this property, selectivity can be achieved. By using a low power integrator, we have shown how to realize a polyphase filter needed for image rejection in a mobile transceiver. The filter has a central frequency of 1MHz, a gaussian to -6dB transfer and a pass-band from 500KHz up to 1.5MHz. The filter has been simulated in a 0.35mm CMOS technology with a supply voltage of 2.5V. The image rejection can be made better than -52dB with a power consumption of 15mW with a dynamic range of 69dB. When compared to active-RC realizations with opamps it shows power figures better with a factor six.