Single-Input Four-Output Current Mode Filter Using Operational Floating Current Conveyor

This paper presents operational floating current conveyor (OFCC) based single input four output current mode filter. It employs only three OFCCs and two grounded capacitors and resistors each. The MOS based grounded resistors implementation is used, which adds feature of electronic tunability to the filter parameters. The filter also enjoys low component spread and low sensitivity performance.The effect of finite transimpedance and parasites of OFCC on the proposed circuit is also analyzed.The functionality of the proposed circuit is demonstrated through SPICE simulations using 0.5 μm CMOS process model provided by MOSIS (AGILENT).


Circuit Description
It may be noted that the voltage at port  is the same as input voltage at port , so voltage tracking action is available at input port.The output current flowing through port  is copied to ports 1 and 2 in phase and to ports 3 and 4 out of phase, thereby offering current tracking at the output ports.Figure 2 shows the CMOS based schematic of OFCC which is based on topology proposed in [29].

Proposed Filter.
In this section the implementation of OFCC based current mode single-input four-output filter, as shown in Figure 3, is proposed.It employs three OFCCs, and two grounded capacitors and resistors each.The analysis of the circuit gives the following transfer functions: where Thus the proposed circuit provides low pass, band pass, and notch (band stop) response simultaneously at high impedance.It may be noted that the current  HPF is not explicitly available at high impedance and therefore cannot be used directly.However, by connecting  NOTCH and  LPF1 , the high-pass response can be made available at high output impedance.This arrangement makes low pass, band pass, and high-pass responses available simultaneously.Similarly, the all pass function can easily be obtained by connecting band pass and notch output currents, that is,  AP =  BP +  NOTCH , and the corresponding transfer function is obtained as It may be noted that all the five filter responses can be made simultaneously available by adding  stages in 2nd OFCC (Figure 3).Further, there is no matching constraint for realization of filter responses.
All the responses are characterized by pole frequency ( 0 ), bandwidth ( 0 / 0 ), and quality factor ( 0 ) given as Equation ( 6) reveals that the pole frequency ( 0 ) and quality factor ( 0 ) can be adjusted by  2 , without disturbing  0 / 0 .The orthogonal adjustability of  0 and  0 can be achieved by simultaneous adjustment of  1 and  2 such that the product  1  2 remains constant and the quotient  1 / 2 varies and vice versa.With moderate values of ratios of component values (( 1 / 2 ) = ( 1 / 2 ) =  0 ), that is, from low component spread [34], high values of -factor can be obtained.Hence the component spread is of the order of √ 0 .The proposed filter uses grounded resistors which can easily be implemented using the MOS based structure given in Figure 4 [33].It uses two diode connected matched transistors, operating in saturation region.Assuming  1 = − 2 , the value of resistor is given by where  is carrier mobility,  ox is gate capacitance per unit area,   is threshold voltage, and ,  are the channel width and length respectively.
Figure 2: CMOS schematic of OFCC [29].The sensitivity analysis of the proposed circuit is as follows: Thus the all passive sensitivities are not more than unity in magnitude.So the proposed filter circuit can be classified as insensitive.

Effect of Finite Transimpedance Gain.
In this section the effect of finite transimpedance gain of OFCC is considered and compensation is employed for high frequency applications.Ideally the transimpedance gain   is assumed to approach infinity.However, in practice,   is a frequency dependent finite value.Using single pole model for transimpedance gain,   () is expressed as [29]   () =  to 1 + (/ tc ) .
The parameter  to is the DC open loop transimpedance gain and  tc is the transimpedance cutoff frequency.For high frequency applications, the transimpedance gain,   (), is approximated as where Taking single pole model of   () into account, the low pass transfer function of (2) modifies to where The inclusion of a capacitor of value   connected between the terminal  and ground in second and third OFCCs cancels the coefficient of "" thereby making both  1 () and  2 () equal to unity.Thus the transfer function of low pass filter after compensation reduces to (2).Similar analysis is valid for other responses as well.

Figure 3 :
Figure 3: Proposed OFCC based single-input four-output current mode filter.

Figure 5 :
Figure 5: Proposed OFCC based current mode filter with parasites.

Figure 8 :
Figure 8: Magnitude and phase plots for all-pass response.

Table 1 :
Transistors aspect ratios of the circuit shown in Figure2.

Table 2 :
Bias voltages/resistor values for orthogonal adjustment of  0 and  0 . 1 and  2 refer to bias voltages corresponding to resistance   . *

Table 3 :
Bias voltages/resistor values for orthogonal adjustment of  0 with  0 . 1 and  2 refer to bias voltages corresponding to resistance   . *