A Voltage-Mode First Order Allpass Filter Based on VDTA

This article presents a new voltage-mode first order allpass filter (APF) employing voltage differencing transconductance amplifier (VDTA). The advantages of the circuit are that: the circuit description is very compact, consists of merely a VDTA and a capacitor: the phase shift can be electronically adjusted by current bias: it provides the lower THD of the output signal. Without any component matching conditions, the proposed circuit is very appropriate to further develop into an integrated circuit. Moreover, the proposed APF can provide the output current with high output impedance without modification of the circuit topology. The PSpice simulation results are depicted. The given results agree well with the theoretical anticipation. The maximum power consumption is 400μW at ±1.25V power supplies.

In 2011, the voltage differencing transconductance amplifier is presented [27].It consists of 2 ports input voltage, 2 port output current and 2 external current bias.The voltage differencing of V n with V n is transferred to current at the terminal z by first transconductance gain and the voltage at z port is transferred to current at the seems to be a versatile component in the realisation of a class of analog signal processing circuits.The fact is that the device can operate in both current and voltage-modes, provides flexibility and enables a variety of circuit designs.In addition, output current of VDTA can be electronically adjusted.The aim of this paper is to propose a voltage-mode first order allpass filter, emphasizing on the use of the VDTA.The features of the proposed circuit are that: the phase shift can be tuned by current bias: the circuit description is very simple, it uses 1 VDTA and a capacitor as passive elements, which is suitable for fabricating in monolithic chip or off-the-shelf implementation: phase shift can be independently adjusted.The performances of proposed circuit are illustrated by PSpice simulations, they show good agreement as mentioned.

2.
Theory and Principle g m1 and g m2 are the transconductances of the VDTA, I b1 and I b2 are currents bias employed to adjusted g m1 and g m2 , respectively.They can be found to be: and where µ i is the mobility of the carrier for NMOS and PMOS transistors, C ox is the gate-oxide capacitance/unit area, W and L are the effective channel width and length, respectively.The symbol of VDTA is shown in Fig. 1, where V p and V n are input terminals and z, x+ and x-are output terminals.

Proposed Voltage-Mode First-Order All-Pass Filter
The proposed voltage-mode first-order APF is illustrated in Fig. 2. The proposed circuit consists of only one VDTA and a capacitor.Not only the output voltage is achieved, the output current with high output impedance is also achieved which is well-known as transconductance-mode.Considering the circuit in Fig. 2, the current of z terminal can be found to be: From the properties of VDTA, Eq. ( 4) can be rewritten as: From Eq. ( 5), the voltage and transconductanc transfer functions can be express to be: and For easy consideration, if g m2 =g m1 =g m , the transfer functions can be rewritten to: and From Eq. ( 8), the pole frequency, voltage gain and phase response of the proposed circuit are: and For transconductance-mode, the pole frequency and phase response are same to the voltage-mode APF.But the transconductance-gain is written as: From properties of VDTA as shown in Eq. ( 2) and Eq. ( 3), the pole frequency and phase response for both mode can be modified to be: and It can be seen that the circuit gives a phase shift from 180 • -0 • .Moreover, the angle pole frequency can be electronically controlled by I B .For transconductancemode, the transconductance gain is rewritten as: The ω p sensitivities of the filter can be written to: Fig. 2: Proposed voltage-mode first-order APF.

Non Ideal Case
In practice, the influences of voltage and current tracking errors and also the parasitic terminal impedances of VDTA will affect the filter performance.In this Section, these parameters will be taken into account.For non-ideal the VDTA can be respectively characterized with the following equations: where α P and α N are the transconductance error gains from P and N ports to z port.β is the transconductance error gain from z port to x port.The influences of parasitic impedances are resistive and capacitive parts affecting the P, N, Z and X ports of VDTA.
Let us denote them R P , C P , R N , C N , R z , C Z , and R X , C X , respectively.Considering into these effects, the voltage and transconductane transfer functions will be modified to the more general forms: and where In this case, the pole frequency, voltage gain and phase response are modified to: and The transconductance-gain from Eq. ( 20) is written as: It should be mentioned that the non-ideal parameters of the VDTA affect the pole frequency, voltage gain and phase response.

Simulation Results
The  22).Phase response for different I B is shown in Fig. 5.This result confirms that the angle natural frequency can be electronically controlled by setting IB as shown in Eq. ( 15).The time-domain response of the proposed APF is shown in Fig. 6, where a sine wave of 150 mV/5 MHz was applied as the input to the filter.Fig. 7 shows the total harmonic distortion (THD) variation with respect to amplitude of the applied sinusoidal input voltage at the pole frequency of the all-pass filter.The tuning of pole frequency by I B is confirmed by the result in Fig. 8.

Conclusion
An electronically tunable voltage-mode first-order allpass filter has been introduced via this paper.The   proposed configuration is very simple and can be electronically controlled.It consists of single VDTA and single capacitor.So it is easy to fabricate in IC form to use in battery-powered or portable electronic equipments such as wireless communication devices.In addition, the output current with high output impdenace is achieved.The PSpice simulation results were depicted, and agree well with the theoretical anticipation.

About Authors
Suwat
performances of the proposed voltage-mode first order allpass filter have been tested by PSpice simulation.This work employed a VDTA realized by a CMOS technology.The NMOS and PMOS transistors employed in the proposed circuit as shown in Fig. 2, were simulated by respectively using the parameters of the 0.25 µm TSMC CMOS technology (level 7) with ±1.25 V supply voltages.Fig. 3 depicts the schematic description of VDTA used in the simulations.The aspect ratios of PMOS and NMOS transistor are W/L = 8 µm/0.25 µm and W/L = 5 µm/0.25 µm, respectively.C = 10 pF, I B = 80 µA.Simulated gain and phase responses of the APF are given in Fig. 4. It can be found that the simulated gain is slightly deviated from ideal responses due to the error terms as expressed in Eq. (

Fig. 6 :
Fig. 6: Time domain response of the circuit in Fig. 2.
Since the proposed APF is based on VDTA, it is realized by CMOS technology, a brief review of VDTA is given in this Section.The characteristics of the ideal VDTA are presented by the following hybrid matrix: Winai JAIKLA was born in Buriram, Thailand.He received the B. S. I. Ed. degree in telecommunication engineering from King Mongkut's Institute of Technology Ladkrabang (KMITL), Thailand in 2002, M. Tech.Ed. in electrical technology and Ph.D. in electrical education from King Mongkut's University of Technology North Bangkok (KMUTNB) in 2004 and 2010, respectively.From 2004 to 2011 he was with Electric and Electronic Program, Faculty of Industrial Technology, Suan Sunandha Rajabhat University, Bangkok, Thailand.He has been with Department of Engineering Education, Faculty of Industrial Education, King Mongkut's Institute of Technology Ladkrabang, Bangkok, Thailand since 2012.His research interests include electronic communications, analog signal processing and analog integrated circuits.He is a member of ECTI, Thailand.