Monolithic filters in the MHz range have been reported using both Switched-Capacitors techniques [ 1,2] and Continuous-Time techniques [3,4,5,6]; their typical applications are FM receivers where a highly selective filter around 10 MHz is required. Several techniques have been proposed to realize monolithic Continuous-Time filters [4,6]. For high frequency applications the transconductor based approach seems to be the most suitable. The major difficulty in the successful realization of high-Q high-frequency Continuous- Time filters comes from the non idealities, like finite gain and parasitic poles, of the integrators that make up the filter [7]. In particular, the excess of phase at the integrator unity gain frequency due to the non-dominant poles of the transconductor is of primary importance [7]. So, either moderate Q realizations are chosen [7] or some form of Q control loop based on an external reference [4,6,8] has to be used to automatically adjust the filter response. The achievable accuracy on the filter transfer function using the latter approach has not been fully assessed in the presence of large process and temperature spreads. In any case using an extra Q-control loop complicates the design and requires extra area and power. A second limitation of Continuous-Time filters, especially in those used in high frequency applications is the limited dynamic range [3,4,5,6]. This is due to the small fraction of the total supply voltage over which the transconductors in the filter achieve the required linearity. Due to this limitation a high supply voltage is often used [3,4,5] which is not compatible with fast scaled technologies. In this paper a differential transconductance stage implemented in BiCMOS technology is described. The key features of the new stage are: - a THD less than 0.15% up to a 3 Vpp differential input signal, assuming 2% mismatch of the input devices, with a - a second pole frequency typically higher than 2 GHz, - a gain of more than 50 dB. All of these features are obtained from simulations performed using SPICE and correspond to a BiCMOS process featuring 2pm minimum channel length and 7 GHz bipolar fT. In section 11 the structure of the new transconductor is described and its operation explained. In Section I11 the non-idealities of the stage like distortion, finite gain, parasitic poles, noise and offset are discussed. In section IV the complete implementation is presented. Finally in section V the simulated performance of a bandpass filter based on the new transconductor is reported. These results demonstrate that using the new circuit a filter centered around 5 MHz with a Q of 22 should result in a Q precision better than 12% without any Q tuning

"A very linear BiCMOS transconductor for high-frequency filtering applications,"

CASTELLO, RINALDO;MONTECCHI, FEDERICO;BASCHIROTTO, ANDREA
1990

Abstract

Monolithic filters in the MHz range have been reported using both Switched-Capacitors techniques [ 1,2] and Continuous-Time techniques [3,4,5,6]; their typical applications are FM receivers where a highly selective filter around 10 MHz is required. Several techniques have been proposed to realize monolithic Continuous-Time filters [4,6]. For high frequency applications the transconductor based approach seems to be the most suitable. The major difficulty in the successful realization of high-Q high-frequency Continuous- Time filters comes from the non idealities, like finite gain and parasitic poles, of the integrators that make up the filter [7]. In particular, the excess of phase at the integrator unity gain frequency due to the non-dominant poles of the transconductor is of primary importance [7]. So, either moderate Q realizations are chosen [7] or some form of Q control loop based on an external reference [4,6,8] has to be used to automatically adjust the filter response. The achievable accuracy on the filter transfer function using the latter approach has not been fully assessed in the presence of large process and temperature spreads. In any case using an extra Q-control loop complicates the design and requires extra area and power. A second limitation of Continuous-Time filters, especially in those used in high frequency applications is the limited dynamic range [3,4,5,6]. This is due to the small fraction of the total supply voltage over which the transconductors in the filter achieve the required linearity. Due to this limitation a high supply voltage is often used [3,4,5] which is not compatible with fast scaled technologies. In this paper a differential transconductance stage implemented in BiCMOS technology is described. The key features of the new stage are: - a THD less than 0.15% up to a 3 Vpp differential input signal, assuming 2% mismatch of the input devices, with a - a second pole frequency typically higher than 2 GHz, - a gain of more than 50 dB. All of these features are obtained from simulations performed using SPICE and correspond to a BiCMOS process featuring 2pm minimum channel length and 7 GHz bipolar fT. In section 11 the structure of the new transconductor is described and its operation explained. In Section I11 the non-idealities of the stage like distortion, finite gain, parasitic poles, noise and offset are discussed. In section IV the complete implementation is presented. Finally in section V the simulated performance of a bandpass filter based on the new transconductor is reported. These results demonstrate that using the new circuit a filter centered around 5 MHz with a Q of 22 should result in a Q precision better than 12% without any Q tuning
IEEE International Symposium on
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11571/453840
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