A Wide-Range Tunable 25MHz-1lOMHz BiCMOS Continuous- Time Filter

A B S T R A C T A high speed bandpass continuous-time biquad filter is implemented in a 0.8l_tm BiCMOS technology. It demonstrates a wide range of frequency tunability from 25MHz-1lOMHz. The filter intermodulation distortion stays below-52dB at 5OMHz with 1OOmVP input differential signals. The power consumption (& = 5OMHz) is 90mW with a single 5V supply. 1. INTRODUCTION The transconductor-capacitor (TC) or Gm-C technique is a well-known approach for implementing high-speed continuous-time filters. They were commercially used quite early with bipolar technology [7]. They have been developed in different technologies such as CMOS [8], bipolar [9] and BiCMOS [5],[2],[3]. They have been chosen for many industrial applications including the read channel of disk drives [9],[2], high-speed data links [10] and digital TV and/or HDTV [3]. In this article we present a biquad bandpass filter implemented in BiCMOS technology for the realization of on-chip selectivity for radios. For many high-speed transconductor-C filters a simple single-stage transconductor (sometimes just an inverter structure [ 1]) is used. However, the DC gain of this kind of transconductor is very low and usually parasitic capacitances are high. Therefore, normally a very wide range of tuning is required to compensate the non-ideal effects such as parasitic capacitances and poor output impedances. Another class of transconductors with the general structure shown in Fig. 1 can be recognized as transconductor-Miller-integrator (TMI) or transconduc-tor-C-amplifier (transconductor-C-opamp) [2],[3]. Basically it consists of two stages. The first stage is a conventional transconductor and the second stage is a high gain amplifier or opamp with feedback from a Miller capacitor usually in series with a resistor (triode mode NMOS device as shown in Fig. I). Since an " opamp " (operational amplifier) usually refers to a multiple stage amplifier with a low-impedance output stage, the more general term " TC-amp " (for transcon-ductor-C-amplifier) is used in this paper. Some important features for this class of transconduc-tors are: i) Since the gain of second-stage amplifier is high, the voltage swing at the input of the second-stage amplifier is low (behaving as a virtual ground). Thus a very small portion of the first stage transconductor's output current flows to the parasitic capacitances connected between the amplifier inputs and ground. The low input voltage swing at the second-stage amplifier input lowers its non-linearity contribution in the entire circuit as opposed to ordinary open-loop multi-stage amplifiers in which the very last stage is responsible for …