A classic second-order coupled-capacitor Chebyshev bandpass filter using resonator of tunable active capacitor and inductor is presented. The low cost and small size of CMOS active components make the bandpass filter (BPF) attractive in fully integrated CMOS applications. The tunable active capacitor is designed to compensate active inductor’s resistance for resistive match in the resonator. In many design cases, more than 95% resistive loss is cancelled. Meanwhile, adjusting design parameter of the active component provides BPF tunability in center frequency, pass band, and pass band gain. Designed in 1.8 V 180 nanometer CMOS process, the BPF has a tuning frequency range of 758–864 MHz, a controllable pass band of 7.1–65.9 MHz, a quality factor
The rapid development of complementary metal-of-semiconductor (CMOS) endues the integrated circuit with small size and low cost in both digital and analog applications. A wireless communication system mainly consists of three components: mixer, bandpass filter, and low noise amplifier. The bandpass filter blocks unwanted signals and selects desirable signal matched to different pass band mixers, that is, 1,920–1,980 MHz of WCDMA, 890–960 MHz of GSM, 1,575 MHz of GPS L1 BPF, and 2,400–2,483 MHz of 802.11b/g. Bandpass filter with high
Reducing resistive loss in the Chebyshev bandpass filter has been presented in improvement on pass band gain, bandwidth, and center frequency [
In this paper, a new BPF using tunable active capacitor and inductor is presented. Self-negative resistance of active capacitor is designed to compensate the positive resistance of active inductor, independent of signal frequency within its tunable range. Meanwhile, adjusting design parameters of the active component can control tunability of center frequency, gain, and bandwidth. The paper is organized as follows. Sections
The first active capacitor (AC) with negative resistance was demonstrated in [
The active capacitor and its equivalent circuit.
The AC small signal model and its equivalent circuit are depicted in Figure
The small signal modal of active capacitor.
We continue to analyze the small signal model shown in Figure
From (
Figures
Tunable AC capacitance.
Tunable AC negative resistance.
Several active inductors have been proposed [
Lossy single-ended gyrator-C active inductor.
The active inductor.
The small signal modal of active inductor.
In Figure
At node A,
At node B,
From node A, the input impedance equals
Compared with the simplified model of RLC circuit,
Two pairs of current mirrors
For the small signal model of the proposed active inductor, the input impedance equals
From the above analysis,
Figures
Inductance of tunable AI.
Resistance of tunable AI.
The 2nd-order active BPF is designed based on the classic Chebyshev BPF structure [
The proposed tunable BPF.
Equivalent circuits of the resonator.
BPF performance.
Capacitance versus frequency (before and after using
Conductance versus frequency (before and after using
Inductance versus frequency (before and after using
Resistance versus frequency (before and after using
As shown in Figures
In Figure
If
By adjusting
In order to find a match between the negative resistance and the positive resistance,
The active inductor in this application provides a relative fixed value of inductance and resistance. By adjusting the bias voltage
Tunable BPF gain versus signal freq.
It is observed from Figure
Table
Tunable BPF performance.
6 Tunable BPF cases | Active inductor | Active capacitor | Theoretical |
Practical |
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BPF | |||||||
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Gain (dB) | BW (MHz) | |||
Case |
15.78 | 121.64 | 0.618 | 57.05 | 168.17 | −164.72 | 189.10 | 766.13 | 758.58 | 7.56 (0.99%) | 3.45 (2.1%) | 107 | 18.1 | 7.1 |
Case |
15.79 | 121.94 | 0.627 | 55.98 | 169.85 | −166.67 | 186.54 | 778.71 | 770.31 | 8.40 (1.1%) | 3.17 (1.9%) | 39 | 11.31 | 19.98 |
Case |
15.80 | 122.15 | 0.633 | 55.28 | 171.03 | −168.30 | 184.35 | 788.31 | 778.48 | 9.82 (1.2%) | 2.74 (1.6%) | 22 | 8.54 | 34.83 |
Case |
15.81 | 122.74 | 0.648 | 53.48 | 174.27 | −170.42 | 182.11 | 806.40 | 800.45 | 5.95 (0.74%) | 3.85 (2.2%) | 12 | 6.55 | 65.34 |
Case |
15.85 | 123.97 | 0.678 | 50.28 | 181.01 | −173.11 | 178.71 | 839.47 | 844.63 | 5.16 (0.61%) | 7.90 (4.4%) | 14 | 7.59 | 59.37 |
Case |
15.86 | 124.54 | 0.692 | 49.02 | 184.10 | −176.32 | 175.83 | 857.57 | 864.30 | 6.74 (0.79%) | 7.78 (4.2%) | 41 | 13.24 | 21.24 |
Table
The previously reported several works by using the same structure of classic Chebyshev bandpass filter.
[ |
[ |
[ |
[ |
[ |
[ |
This work | |
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Technology process | CMOS |
CMOS |
BJT |
BJT |
BJT |
CMOS |
CMOS |
Active component | Active capacitor | — | Active inductor | Active inductor | Active inductor | — | Active capacitor/inductor |
Order | 2 | 2 | 3 | 2 | 1 | 3 | 2 |
Center frequency (MHz) | 5300 | 23500 | 1950 | 2100 | 600 | 2368 | 758 |
BW (MHz) | 1700 | 4000 | 10 | 15 | 300 | 60 | 7.1 |
Pass band gain (dB) | 0.77 | 1.65 | −8 | 0 | 0.1 | 1.8 | 18.1 |
Stopband rejection (dB) | 36.8 | 15.2 | — | — | — | 30 | 50 |
Power (mW) | 2.2 | 4.2 | 4 | — | 120 | 8.8 | 25.5 |
Quality factor | 3 | 6 | 195 | 140 | 2 | 40 | 107 |
Tunability | |||||||
Center freq. (MHz) | — | — | 1800~2050 | — | — | — | 758~864 |
Gain (dB) | — | — | −8 | — | — | — | 6.5~18.1 |
BW (MHz) | — | — | 10 | — | — | — | 7.1~65.9 |
Quality factor | — | — | 180~205 | — | — | — | 12~107 |
In this paper, a classic Chebyshev BPF adopting active capacitor and active inductor for tunability, low cost, and smaller size is presented. The tunability of BPF center frequency and pass band is achieved by controlling the active capacitance, which is tunable by adjusting the DC bias voltage. The negative resistance of active capacitor compensates 95% above the resistive loss of active inductor in the tunable center frequency range. A pass band gain of 18.1 dB and stopband rejection of 50 dB are obtained at the center frequency 758 MHz. The BPF achieves a high quality factor
The authors declare that there are no conflicts of interest regarding the publication of this paper.