Active Capacitance Circuit Research Articles

A Planar Balanced Quadrature Coupler With Tunable Power-Dividing Ratio

This paper reports on, for the first time, planar balanced quadrature couplers with wide tunable power-dividing tuning ratio using two tuning capacitors. The differential-mode operation, common-mode rejection, and cross-mode conversion suppression are obtained for all power-dividing ratios. Closed-form design equations are derived. For verification, a 2-GHz tunable balanced quadrature coupler is designed and measured. The measured power-dividing ratio is in the range of −11.3–10.2 dB with better than 15-dB return loss and isolation. Furthermore, the insertion loss and phase difference resulting from the varactors and transmission lines are compensated using an active capacitance circuit with negative resistance. The worst case of phase imbalance is improved from 25° to 2°. The insertion loss is improved by ∼1.2 dB and nearly close to the theory value.

Design of an Active Tunable Bandpass Filter for Spectrum Sensing Application in the TVWS Band

In this paper, we propose an active tunable bandpass filter (BPF) for efficient spectrum sensing in the TV White Space (TVWS) band. By designing a narrow bandwidth, it is possible to improve the sensing probability. The basic circuit configuration involves switching the PIN diode compromising capacitor bank to change the capacitance of the LC resonant circuit. To cover the whole TVWS band effectively, we add a varactor diode, and the bandwidth is set to 25-MHz. We improve the insertion loss by using the active capacitance circuit. The tunable BPF in the TVWS band with a 20-MHz interval is designed to have 11 channels with a bandwidth of 25 MHz and a low insertion loss of 1.7-2.0 dB.

A Tunable Combline Bandpass Filter Loaded With Series Resonator

This paper proposes a varactor-tuned microstrip combline bandpass filter (BPF) that is loaded with lumped series resonators instead of the short-circuited end of combline. This configuration has the advantage of enhancing the tunability of the suggested BPF by varactor diodes in series resonators, which leads to a wide tuning range even with a small capacitance ratio (Cratio) of the varactor. Therefore, a low controlled bias voltage varactor-tuned BPF can be achieved. In addition, due to the controllable slope parameter of the proposed resonator, the coupling between resonators can be controlled. As a result, the proposed tunable BPF can keep nearly constant bandwidth over the wide tuning range. The insertion loss resulting from the varactors is compensated using an active capacitance circuit with negative resistance. Theoretical analysis and design approach are described. Experimental results of a demonstrative BPF show a 500-MHz tuning range (1.7-2.2 GHz) with Cratio of only 2.6 (1-5-V bias voltage) while maintaining nearly constant absolute bandwidth and low insertion loss. The measured performance of the fabricated filter shows good agreement with the simulated one.

고출력 특성을 고려한 능동 가변 대역 통과 여파기 설계

본 논문에서, 부성 저항 특성을 갖는 능동 커패시턴스 회로를 이용한 고출력 능동 가변 대역 통과 여파기는 동축형 유전체 공진기와 버랙터 다이오드를 사용하여 설계하였으며, 셀룰러 TX, RX 대역을 모두 가변할 수 있도록 설계하였다. 능동 커패시턴스 회로의 직렬 피드백 구조는 가변 대역 통과 여파기의 버랙터 다이오드로부터 생기는 손실을 보상함과 동시에 고출력 특성을 갖도록 하기 위해 <TEX>$P_{1dB}$</TEX>가 32 dBm인 GaAs HFET을 사용하였다. 버랙터 다이오드는 고선형 특성을 갖도록 하기 위해 back-to-back 구조를 사용하였다. 제작된 2단 능동 가변 대역 통과 여파기는 셀룰러 대역인 800 MHz에서 900 MHz를 가변하며, 각각 25 MHz 대역폭으로 TX 대역 836 MHz에서 0.48 dB 삽입 손실 특성을 나타냈으며, RX 대역 881.5 MHz에서 0.39 dB 삽입 손실 특성을 나타내었다. <TEX>$P_{1dB}$</TEX>특성은 TX 및 RX 대역에서 각각 19.5 dBm과 23 dBm을 얻었다. In this paper, a high power active tunable bandpass filter made of dielectric resonators and varactor diodes is designed using the active capacitance circuit generating negative resistance for tuning cellular TX, RX band. An active capacitance circuit's series feedback circuit using GaAs HFET whose <TEX>$P_{1dB}$</TEX> is 32 dBm is used for compensating the losses from the varactor diodes of the tunable bandpass filter. The tuning elements, the varactor diodes are used as the back-to-back configuration to achieve the high power performance, The designed active capacitance circuit improves the insertion loss characteristics. The designed 2-stage active tunable dielectric bandpass filter at cellular band can cover from 800 MHz to 900 MHz. The insertion losses at 836 MHz and 881.5 MHz with 25 MHz bandwidth are 0.48 dB and 0.39 dB, respectively. The <TEX>$P_{1dB}$</TEX> of the designed bandpass filter at TX and RX band are measured as 19.5 dBm and 23 dBm, respectively.

Design procedure for active microwave filters using the high quality active negative capacitance circuit for RF low‐noise bandpass applications

AbstractThis article presents a new methodology based on active band pass filter. In the proposed design, a resonator consists of an active negative capacitor together with a conventional inductor represented. The active negative capacitor is made of a field‐effect transistor that exhibits negative capacitance and negative resistance. It can, therefore, compensate the loss of an inductor. Two pole active bandpass filter centered 1.3 GHz, with 3 dB bandwidth of 640 MHz, high gain 1.5 dB, including a very low‐noise active filter which exhibits a noise figure of 2.5 dB are realized in the fist part. Compared with the conventional BPF, the proposed design shows better gain, noise figure, and the return loss level. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1086–1093, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23296

Reconfigurable RF active bandpass filter with Ba x Sr 1-x TiO 3 varactor

An RF tunable active bandpass filter employing a BaxSr1-xTiO3 (BST) based capacitor is proposed. This filter consists of an active capacitance circuit together with a BST tunable varactor. The active capacitor is made of a field effect transistor that exhibits negative resistance as well as capacitance. BST thin film was deposited by Sol-Gel solution and using the metal-insulator-metal capacitor process. The fabricated capacitor showed capacitance of 2.7–1.5 pF at 1.8 GHz for a maximum applied electric field of 34 V/µm. The measured second-order active bandpass filter showed bandwidth of 100 MHz, insertion loss of+0.48 dB at 1.8 GHz centre frequency and tuning frequency of 220 MHz with the application of 34 V/µm.

Application of RF Varactor Using BaxSr1-xTiO3/TiO2/HR-Si Substrate for Reconfigurable Radio

In this paper, the potential feasibility of integrating Ba(x)Sr(1-x)TiO3 (BST) films into Si wafer by adopting tunable interdigital capacitor (IDC) with TiO2 thin film buffer layer and a RF tunable active bandpass filter (BPF) using BST based capacitor are proposed. TiO2 as a buffer layer is grown onto Si substrate by atomic layer deposition (ALD) and the interdigital capacitor on BST(500 nm)/TiO2 (50 nm)/HR-Si is fabricated. BST interdigital tunable capacitor integrated on HR-Si substrate with high tunability and low loss tangent are characterized for their microwave performances. BST/TiO2/HR-Si IDC shows much enhanced tunability values of 40% and commutation quality factor (CQF) of 56.71. A resonator consists of an active capacitance circuit together with a BST varactor. The active capacitor is made of a field effect transistor (FET) that exhibits negative resistance as well as capacitance. The measured second order active BPF shows bandwidth of 110 MHz, insertion loss of about 1 dB at the 1.81 GHz center frequency and tuning frequency of 230 MHz (1.81-2.04 GHz).

Design of an RF low-noise bandpass filter using active capacitance circuit

A novel RF active bandpass filter (BPF) is proposed and its noise performance is optimized by noise analysis. In the proposed design, a resonator consists of an active capacitance circuit together with a conventional inductor. The active capacitor is made of a field-effect transistor that exhibits negative resistance as well as capacitance. It can, therefore, compensate the loss of an inductor. Whereas the conventional active filters using negative resistance circuits usually have a high noise figure, the proposed active filter shows good noise property achieved by adopting a novel active capacitance circuit, which is proven by noise analysis and the experimental result. The measured second-order active BPF shows bandwidth of 95 MHz, 0.1-dB insertion loss, 0.3-dB ripple, and a noise figure of 2.4 dB at the 1.9-GHz band, which agrees well with the simulated results.

Analysis of negative‐resistance characteristics in active capacitance circuit

AbstractAn active‐capacitance circuit with a series‐feedback network is analyzed in depth and its application to an active microwave band‐pass filter (BPF) is discussed. In this paper, we extract the exact equivalent values of this circuit and analyze the RF characteristics while varying the values of active devices and feedback circuits. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 240–243, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20264