Chapter 5: A Low-Power, Low-Voltage Bluetooth Channel Filter Using Class AB CMOS Transconductors

John Hughes, Adrian Spencer, Apisak Worapishet, Rungsimant Sitdhikorn

5.1 Introduction

The low-IF (intermediate frequency) polyphase architecture has emerged as the preferred approach for achieving the required sensitivity in fully integrated wireless transceivers [1]. The industry is currently driving down the cost and power consumption by attempting standard CMOS solutions for applications such as Bluetooth [2] and ZigBee (HomeRF Lite) [3] and this is creating new challenges for the circuit designer.

Figure 5.1 shows a typical low-IF receiver architecture. One of the keys to the success of this polyphase approach has been the ability to integrate the channel filter. However, when solutions are required using the ever lowering supply voltages demanded by standard CMOS, and with perhaps an order of magnitude lower power consumption, conventional filter techniques using operational amplifiers with passive resistors and capacitors may prove inadequate.

Figure 5.1: Typical low-IF receiver architecture

The approach adopted for this work was to make transconductor-capacitor (Gm-C) filters using class AB CMOS transconductors. While these can be expected to yield more efficient solutions than the conventional approach, they would not be expected to be as linear as the alternative passive resistors. So, a key aspect of this work was to demonstrate that suitable filter responses could be achieved with considerably lower power consumption and with adequate intermodulation performance.

In this chapter, we design a Gm-C complex low-IF channel filter for Bluetooth. The class AB transconductor is developed and simulated performance of the filter designed with these transconductors is presented.