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A new current-mode first-order log-domain multifunction filter is presented in this paper. This filter has single input and provides three outputs (low-pass, high-pass and all-pass) using a first - order low-pass filter and five of current mirrors as building blocks. The proposed filter employs only MOSFETs and a grounded capacitor. The first - order filters are used in audio and video applications extensively. The MOSFETs of the core section are operated in weak inversion thereby making the circuit suitable for low-voltage, low-power applications. The SPICE simulations have shown good performance of the proposed filter.

In 1979, Adams [

Initially, log-domain filters were synthesized by using the exponential nature of bipolar transistor, but in 1994 Toumazou, Ngarmnil and Lande [

The first-order filters have been extensively used in audio and video applications where circuit simplicity and power consumption are important parameters. Thus, during the last few decades, voltage-mode and current-mode first-order filter circuits have found significant place in literature. Among the voltage-mode and current-mode circuits, the latter fulfill the contemporary requirements such as low-power consumption, low-voltage operation, large dynamic range etc.; therefore, current-mode (CM) circuits have received much attention and from time to time, a number of current-mode first- order multifunction (low-pass, high-pass and all-pass) filters [^{1} filter using MOS transistors and single grounded capacitor. In this circuit [

This paper proposes a MOS based multifunction first-order filter which is capable of realizing all possible first-order filters namely, low-pass, high-pas and all-pass from the same configuration. In the proposed circuit, the MOS transistors forming the core low- pass filter are operating in subthreshold region wherein MOS transistors have exponential characteristics. The validity of the proposed configuration has been confirmed through SPICE simulation results. The SPICE simulations show that the proposed circuit offers a performance which makes it suitable for low voltage, low power operation.

The core block of the proposed circuit is a first-order low-pass filter which has been obtained by an appropriate modification of the four-MOSFETs translinear circuit used earlier as a normal product computation function [

The proposed circuit offers the advantage of the MOS transistors operating in subthreshold region [

In _{1}-M_{2}-M_{3}-M_{4} along with a capacitor C constitute the basic first-order low-pass core. The transfer function of this circuit can be determined as follows. For the translinear loop comprised of M_{1}-M_{2}-M_{3}-M_{4}, we have the following equation for the close loop containing of V_{GS} of the four-MOSFETs.

If the MOS transistors are operated in weak inversion region they would have exponential relationship between drain current and gate source voltage [

where _{T} = kT/q = 26 mV at room temperature is known as thermal voltage.

Now Equation (2) can be rearranged as

Therefore Equation (1) can be written as

Equation (4) can be simplified as

From Equation (5), we finally obtain

The value of

Rearranging Equation (7), we get the transfer function of the circuit as

Equation (8) represents the transfer function of the first-order low-pass filter and can be expressed as:

where

Now the low-pass output is given by

whereas the other two current outputs namely, I_{hp} and I_{ap} are obtained by

for which the required operations are carried out by the appropriate current mirrors and current repeaters as shown in _{9}-M_{10}-M_{17}, M_{13}-M_{14}-M_{8} and M_{5}-M_{6}-M_{7} are current steering circuits and the MOSFET pairs M_{11}-M_{12} and M_{15}-M_{16} are simple current mirrors.

Thus, the transfer functions of the low-pass, high-pass and all-pass filters realized by the proposed circuit are given by

From Equations (13)-(15), it turns out that the cutoff frequency (in case of low-pass and high-pass) and phase (in case of all-pass) can be electronically tuned by changing

the value of I_{f} since

The proposed circuit was simulated in SPICE employing TSMC 0.35 μm Level 3 CMOS process parameters [_{DD} = −V_{SS} = 0.5 V, C = 3 pF, I_{f} = 70 nA, I_{o} = 0.3 µA and I_{in} = 0.2 μA. The aspect ratios of the transistors were taken as shown in _{1}-M_{2}-M_{3}-M_{4} of the basic low-pass core) i.e. V_{GS} < V_{T} is satisfied.

The result of SPICE simulations of the circuit of

Parameters | NMOS | PMOS |
---|---|---|

L | 1U | 1U |

W | 6U | 6U |

TOX | 7.9E−9 | 7.9E−9 |

NSUB | 1E17 | 1E17 |

GAMMA | 0.5827871 | 0.4083894 |

PHI | 0.7 | 0.7 |

VTO | 0.5445549 | −0.7140674 |

DELTA | 0 | 0 |

UO | 436.256147 | 212.2319801 |

ETA | 0 | 9.999762E−4 |

THETA | 0.1749684 | 0.2020774 |

KP | 2.055786E−4 | 6.733755E |

VMAX | 8.309444E4 | 1.181551E5 |

KAPPA | 0.2574081 | 1.5 |

RSH | 0.0559398 | 30.0712458 |

NFS | 1E12 | 1E12 |

TPG | 1 | −1 |

XJ | 3E−7 | 2E−7 |

LD | 3.162278E−11 | 5.000001E−13 |

WD | 7.04672E−8 | 1.249872E−7 |

CGDO | 2.82E−10 | 3.09E−10 |

CGSO | 2.82E−10 | 3.09E−10 |

CGBO | 1E−10 | 1E−10 |

CJ | 1E−3 | 1.419508E−3 |

PB | 0.9758533 | 0.8152753 |

MJ | 0.3448504 | 0.5 |

CJSW | 3.777852E−10 | 4.813504E−10 |

MJSW | 0.3508721 | 0.5 |

MOS Transistors | W/L (μm) |
---|---|

M_{1}-M_{7}, M_{9}-M_{17} | 6/1 |

M_{8} | 5.87/1 |

low-pass, high-pass and all-pass. Figures 3(a)-(c) shows the transient response of the low-pass, high-pass and all-pass filters designed for cutoff frequency of f_{0} = 85 KHz by taking I_{f} = 70 nA. _{f} = 20 nA, 45 nA, 70 nA, 95 nA and 120 nA) and

The SPICE simulation results, thus confirm the validity of the proposed filter.

This paper presented a log-domain multifunction first-order filter using only MOSFETs and grounded capacitor. The circuit is capable of realizing all first-order filters namely, low-pass, high-pass and all-pass from the same configuration with electronic tunability of the radian frequency_{f} =120 nA, appears suitable for low voltage, low-power applications. This paper has therefore, added a new CMOS multifunction first-order filter to the existing repertoire of log-domain filters (as in [

Authors are thankful to “Analog Signal Processing Research Lab”, Electronics Engineering Department, Institute of Engineering and Technology, Lucknow for conducting this research. The authors are also thankful to World Bank assisted project “Technical Education Quality Improvement Program” for funding this lab.