Current mode circuits are very popular due to some of their advantages over their voltage mode counter parts. They are very suitable for high frequency, low voltage and low power applications in portable equipments such as mobile phones, aviation industry, water quality monitoring equipments, medical instrumentation and in satellite communication equipments. Current mode approach in analog processing blocks offers high slew rates, low node impedances, greater linearity, wider bandwidth and sometimes electronic tunability compared with voltage mode circuits. There have been steady and gradual interests to find new schemes for analog building blocks which may serve as an excellent current mode device. Some of the popular and off-the-shelf items have been Current Feedback Amplifiers (AD844) [1] , Current Conveyors [2] , Operational Transconductance Amplifiers (CA 3080) [3] etc., which have been used for decades as excellent current mode devices. Recently introduced current mode building blocks are Current Differencing Transconductance Amplifiers (CDTAs) [4] -[7] , and Current Follower Transconductance Amplifiers (CFTAs) [8] -[23] which provide simple design facility with ample numbers of choices for output current ports for excellent current mode signal processing. CFTA in addition to possessing single current input compared with CDTA also exhibits excellent current mode characteristics along with electronic control of parameters of interest and is therefore attracting the attention of analog circuit designers. CFTA is extensively used in the realization of signal processing as well as signal generation in current mode circuits [7] -[23] . In this paper, a new circuit using two CFTAs and two grounded capacitors along with a grounded resistor is presented. The proposed circuit realizes basic current mode filter functions such as low pass (LP), band pass (BP), and high pass (HP) for a single input current source. Also, band reject (BR) and all-pass (AP) filter functions can be realized by proper combinations of current responses. Various papers are published on different applications of CFTAs such as current-mode (CM) or voltage-mode (VM) universal filters, oscillators, amplifiers, etc. [7] -[23] . In this paper, a new current-mode second-order CFTA biquad filter is presented which uses two active elements and three passive grounded elements. The main attractive feature of this circuit is the facility of linear control of pole frequency ω₀ and quality factor Q₀. While pole frequency can be changed electronically through g_m, the quality factor Q₀ can be adjusted independently through grounded resistor. The grounded resistor can further be realized through an additional transconductance amplifier making this control again an electronic one. The simulation results are obtained using PSPICE software. Simulation results validate the theoretical findings.

The Current Follower Transconductance Amplifier (CFTA) [6] , whose schematic symbol and behavioral model are shown in Figure 1(a)-(b), consists of an input current follower that transfers the input current to the z terminal and an output transconductance amplifier stage, which is used to convert the voltage at the z-terminal into output currents. Relations between the individual terminals of CFTA can be described by the following hybrid matrix:

Assuming the standard notations, the terminals defining relations of this device can be characterized by the following set of equations

where g_m is transconductance gain of the CFTA which is directly proportional to the external bias current I_B, which is given by Equation (3)

where the V_T is the thermal equivalent voltage (= 26 mV at 27˚C) and I_B is the control bias current adjusting the transconductance g_m of the CFTA. The operation of the CFTA will be dependent on temperature variations. z_z is external impedance connected to the z-terminal.

The proposed current mode biquad filter is shown in Figure 2. It consists of two CFTAs, one grounded resistor and two grounded capacitors. The internal construction of CFTA used for simulation is shown in Figure 3.

A routine analysis of the proposed circuit as shown in Figure 2 yields the following current transfer functions.

Adding the output current reponses I_lp and I_hp, one obtains

whereas for current-mode all-pass filter response

where is given by

From (4)-(9), the pole frequency and quality factor Q₀ are given as

For and Equations (10) and (11) become

and

Equations (12) and (13) show that can be linearly and electronically varied through g_m .Whereas quality factor Q₀ can be linearly changed through grounded resistor R₁ independent of.

There are Current Tracking Errors of the proposed circuit. The characteristic equations of CFTA are

where where denotes the current tracking error of the current follower from the terminal f into the terminal-z (). from the non-ideal properties of CFTAs, the characteristic equation of the proposed circuit according to Figure 2 is

It is thus, seen that the proposed circuit is capable of realizing all the five basic functions, without requiring any component-matching or realization conditions. From (4)-(9), the various filter parameters are given by

The effect of changes in active and passive element values is determined by evaluating sensitivity coefficients which are found to be

From (12)-(20), it is clear that all the sensitivities of the various parameters of the filters realized from the proposed configuration are very low.

The performance of the proposed circuit in Figure 2 has been verified by PSPICE simulation results. The PMOS and NMOS transistors have been simulated by respectively using the parameters of a 0.25 µm TSMC CMOS technology [9] as shown in Table 1. The aspect ratios of PMOS and NMOS transistors are indicated in Table 2. The circuit was biased with ±1.5 V power supply voltage, V_BB = −0.55V, I_B1 = I_B2 = 80 µA. The values of the capacitors and resistor were chosen as: C₁ = C₂ = 1nF, R₁ = 0.5 kΩ. In Figure 4(a), simulated results of proposed LP, HP, BP, BR filters are shown using PSPICE 16.3 which shows the validity of design equations of filter shown in Equations (10) and (11). Pole frequency was found to be f₀ = 245 kHz and quality factor Q₀ = 0.7 which are very close to theoretical values. In Figure 4(b), phase response of current mode all-pass filter is shown. In Figure 4(c), simulation results are shown for the pole frequency f₀ = 245 kHz and quality factor Q₀ which varies from 0.7 to 3. The results show independent control of Q₀. Similarly, in Figure 4(d) for the quality factor Q₀ = 0.7, pole frequency f₀ is varied from 245.6 kHz to 245 MHz. However it is seen that H₀ in this case is slightly changing with frequency which may be the effect of output parasitic capacitance of CFTA or due to tracking errors between I_f and I_z. Hence, from the PSPICE simulation results it is proved that the practical results nearly match to the theoretical results.

In this paper, a new current-mode second-order biquad filter using CFTAs (Current Follower Tranconductance Amplifier) is presented. The circuit realizes all types of filter functions, i.e. LPF, BPF, HPF, and BRF simultaneously for a single current input. It consists of two CFTAs, one grounded resistor and two grounded capacitors. Hence, the low numbers of active and passive elements are used. The circuit has the features of low voltage supply, and large bandwidth. It has low-input and high-output impedances and also is convenient for electronic controllability through transconductance gain g_m of CFTAs, so it can be used in battery-powered or portable electronic equipments. The only apparent drawback is the use of a grounded resistor. However, grounded resistor can be simulated with additional transconductor and its value can be controlled through g_m3. Hence, electronic control of quality factor is possible.