A Class AB 6 th Order Log-Domain Filter in BiCMOS with 100–500 MHz Tuning Range Klaus Schmalz ∗ , M.A. Teplechuk ** and J.I. Sewell ** ∗ IHP, D-15236 Frankfurt (Oder), Germany, e-mail: schmalz@ihp-microelectronics.com ** Department of Electronics and Electrical Engineering, University of Glasgow, G12 8LT, UK, e-mail: M.Teplechuk@elec.gla.ac.uk , j.sewell@elec.gla.ac.uk Abstract − A differential lowpass filter with tunable cutoff frequency 100-500 MHz for application in broadband WLAN has been designed and tested. The sixth-order Butterworth filter of class AB log-domain type with cascaded biquads and equal capacitors was synthesized by XFILTER software and implemented in BiCMOS. The noise figure of the filter is 30 dB, and the IIP3 is 7 dBm. The circuit is fabricated in 0.25 μm SiGe:C BiCMOS technology. 1 INTRODUCTION In recent years the log-domain (LD) approach to the realisation of active integrated filters has been championed as a solution for low-power, high frequency applications. However, in general, for filters of reasonable order and complexity, the results from implementations targeted at high frequency uses have been quite disappointing. The low fidelity of response is attributed to transistor non-idealities, like finite beta and associated limitations in frequency response [1]. The use of compensation circuitry is possible, but introduces extra demands in area and power consumption. The selectivity requirements of filters for emerging 1Gbit/s broadband WLAN systems demand filters of reasonable order to function properly at reasonably high frequencies up to 500 MHz. This paper presents a design, fabricated in 0.25µm SiGe:C BiCMOS technology, in which the filter structure is deliberately kept simple. XFILTER [2] software enabled a number of designs to be investigated and the cascade biquad approach selected presents a filter with equal capacitance values and a low spread in transistor bias currents (hence low spread in transistor sizes). Quite acceptable performances have been obtained without recourse to excessive compensation circuitry. 2 LOG-DOMAIN FILTER DESIGN 2.1 Log-domain filter synthesis The responses of lowpass, highpass, bandpass and bandstop for both even and odd ordered filters can be realised by combination of the biquadratic and first order blocks. Fig. 1 shows the general structure of a biquadratic section, for which the general transfer function is given by: 0 1 2 0 1 2 2 ) ( ) ( d s d s n s n s n s I s I in out + + + + = . Iou Iin Io1 Io2 -Id -Io3 IC IF Io4 1 sCV T 1 sCVT Figure 1: Biquadratic section implemented in XFILT. The design equations for various responses are given in Table 1. Par. LP HP/LPN/HPN BP Component C Io d 0 VT fdintcap Io2 Io3 Io fde Id d1CVT fdidc Io1 Io n 0 d 0 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Io - fde Io4 - −Io n 2d1d 0 d 0 n 0 − n2d 0 ( ) ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ Io n1 d 0 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ fde IF - 2 n - BJT (x) IC 1.0 n 0 d 0 − n 2 1.0 BJT (x) Table 1: Design equations for log-domain biquad. ECCTD 2005 - European Conference on Circuit Theory and Design, Cork Ireland, 29 August - 2 September 2005 0-7803-9066-0/05/$20.00 ©2005 IEEE