� Abstract—A controllable modification of current feedback

amplifier is presented in this contribution. In spite of classical conception, there are two current outputs of both polarities available and two buffered voltage outputs. The control of parameters consists of adjustable current gain between X and Z ports and intrinsic input resistance of current input (X). We proposed complex model of active element, which is based on CMOS and bipolar available transistor models of process technology. Spice simulations confirmed our presumptions and provided features of designed active element.

Keywords—Spice simulation and modeling, electronic control,

current feedback amplifier, double current controlled CFA.

I. INTRODUCTION urrent feedback amplifier (CFA) is very useful active element which is today still very popular in applications. From quantity of active elements [1] for analog signal

processing, CFA is suitable for mixed mode implementations. However, common CFAs do no allow any direct electronic control of application through adjusting of parameters of active elements. Development of current conveyors (CC) [2, 3] was very important. In our point of view discovery of controllable current conveyor was very important. Fabre et al. [4] presented possibility of control of internal intrinsic resistance of current input (RX), which is adjusted by bias current. Also recent works use this approach for control of active element [5-12]. Specific oscillating [5, 6, 11, 12] and filtering [9, 10, 11] circuits with controllable features (oscillation frequency, oscillation condition, characteristic frequency, quality factor, bandwidth, etc.) are typical examples of applications. Surakampontorn et al. [13] introduced CC with possibility of current gain adjusting (between X and Z port), so-called electronically adjustable current conveyor of second generation (ECCII). Hitherto published papers, for example [14-22], were focused on

Manuscript received February 15, 2012. This work was supported in part

by the Czech Science Foundation projects no. GA102/09/1681, GP102/11/489 and action IC0803 COST no. OC09016. Described research was supported by project WICOMT CZ.1.07/2.3.00/20.0007 from the operational program Education for competitiveness and project SIX CZ.1.05/2.1.00/03.0072 from the operational program Research and Development for Innovation.

R. Sotner (corresponding author) is with the Dept. of Radio Electronics, Brno University of Technology, Brno, Purkynova 118, 612 00 Czech Republic. (e-mail: sotner@feec.vutbr.cz).

J. Jerabek, N. Herencsar, and K. Vrba are with the Dept. of Telecommunications, Brno University of Technology, Brno, Purkynova 118, 612 00 Czech Republic (e-mails: {jerabekj, herencsn, vrbak}@feec.vutbr.cz).

T. Dostal is with College of Polytechnics Jihlava, Jihlava, Tolsteho 16, 586 01 Czech Republic.

improving of approaches to current gain control in CC. Several works also combined control of two parameters. Minaei et al. [15] employed CC with possibility of current gain control and control of intrinsic resistance of current input. Marcellis et al. [18] also proposed CC, where control of current gain (X�Z) and voltage gain (Y�X) is possible. Controllable current gain (B) also found field of usability in various applications [19-25]. Controlled current resistance (RX) in CFA is quite new idea. Siripruchyanun et al. used this way in so-called CC-CFA [26]. In this paper we introduce modification of CFA, where both current gain control and also intrinsic resistance control are allowed. Our CFA is extended and we gained current outputs of both polarities and two buffered voltage outputs. Conception, presumption of design and simulation results are presented in following chapters of the contribution.

II. DOUBLE CURRENT CONTROLLED CFA We started from classical current feedback amplifier

(commercially available AD844 [27] for example) with voltage (Y), current (X) input ports, current output (Z+) and voltage buffered output (O). Our solution is extension of this conception and provides two current outputs (Z+, Z-), two buffered voltage outputs (O+, O-). Main feature with respect to classical CFA is the possibility of current gain (B) adjusting (between X and Z� ports) and control of intrinsic resistance of current input X (so-called RX). Symbol of the element is depicted in Fig. 1. We call this element as double current controlled CFA (DCC-CFA). Elementary controlled sources can serve for detailed study of function of the DCC-CFA. We have covered also important

Fig. 1. Symbol of the DCC-CFA

Fig. 2. Model of DCC-CFA based on controlled sources including passive components representing important parasities

Electronically Adjustable Modification of CFA: Double Current Controlled CFA (DCC-CFA)

Roman Sotner, Jan Jerabek, Norbert Herencsar, Tomas Dostal, and Kamil Vrba

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401978-1-4673-1118-2/12/$31.00 ©2012 IEEE TSP 2012

Fig. 3. Block conception of DCC-CFA parasitic influences of real circuit in the model shown in Fig. 2. Following hybrid matrix equations describes behavior of this element (Fig. 2):

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IIVVIV

RR

YBYB

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0100000100000000000000100000

, (1)

where YY = sCY + 1/RY, YZ+ = sCZ+ + 1/RZ+, YZ- = sCZ- +1/RZ-. Block conception of DCC-CFA we shown in Fig. 3. Two current conveyors (CC) represent the core of our model. The first CC1 provides control of RX [4-12, 28, 29] by bias current Iset_RX and the second CC2 is used for control of current gain (B) by control current Iset_B (translinear approach [14, 16, 17, 20, 25, 30, 31]). This CC2 employs two current outputs (Z�).

Complete internal structure is included in appendix (Fig. 15) and transistor dimensions are summarized in Table 2 (also placed in appendix). There were used online available process models of CMOS and bipolar technology [32], [33]. Current input resistance RX in section CC1 is given by [4-10]:

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1

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where

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MPNm I

LW

KPg _/2 ���

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Parameters of (3) are defined as KP = �0COX (�0 - channel mobility, COX - capacitance of MOS gate oxide) for used ON-Semi C5 0.5 �m technology transistor model available on [32] and W/L are gate dimensions. Current gain control in CC2 was focused to the section based on bipolar process model [33]. Current gain is given by following equation [16, 17, 20, 25, 30, 31]:

a

Bset

II

B _� . (4)

Theoretically, we can use only one CC for RX and B control (CC in Fig. 15 a is omitted) but simulations reveal that change of the bias current for RX (Iset_Rx) control and control of current gain B (Iset_B) in one element influences together and have impact on linearity of dependence of B on Iset_B, see Fig. 4. This result we obtained from simulation of separated CC2 (appendix, Fig. 15 c), where we used Ib as Iset_Rx. Increasing of Iset_Rx also degrades output impedance of current outputs. Therefore independent sections for control of RX and B are better in real case because controls of RX and B are practically independent. However, complete internal structure is quite complicated. Both current outputs Z are buffered by voltage buffers as is obvious from the Fig. 3.

0,0

0,5

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1,5

2,0

2,5

3,0

3,5

4,0

0 50 100 150 200I set_B [�A]

B [-]I set_Rx = 10 �A (one CC)

DCC-CFA for all I set_Rx)

I set_Rx = 200 �A (one CC)

I set_Rx = 400 �A (one CC)

ideal

I set_Rx = 50 �A (one CC)

Fig. 4. Dependence of B on Iset_B in one CC and DCC-CFA with both types of current control

Fig. 5. Dependence of B on Iset_B (whole DCC-CFA)

Fig. 6. Dependence of RX values on Iset_Rx

III. SIMULATION RESULTS Behavior of proposed active element was investigated by

simulations in PSpice. Proposed model takes into account � 5 pF capacitances in each available pin of active element which simulate parasites caused by bonding and ESD protection. Therefore, obtained frequency features and small signal parameters are little bit different or worse than in case without these capacitors. Important characteristics are mentioned in following discussion. The dependence of current gain B on control current Iset_B is in Fig. 5. From Fig. 5 it is clear that nonlinearity of dependence arise dramatically for Iset_B higher than 100 �A. Current gain B = 1.9 for Iset_B = 100 �A but 3.6 for Iset_B = 200 �A (theoretical value is B = 4). Dependence of RX on control current Iset_Rx is in Fig. 6.

402

Fig. 7. Frequency responses of current gain B (X � Z)

Fig. 8. Frequency responses of RX

Fig. 9. Dynamical characteristic of the current transfer between X and Z ports (X � Z�) for different B Dependence of current transfer B on frequency and Iset_B is in Fig. 7. Gain bandwidth (GBW) of the current gain response is about 40 MHz. GBW is about 70 MHz without parasitic capacitors (bonding and ESD). Similarly are shown frequency characteristics of RX in Fig. 8. We demonstrated range of RX value from 1700 � (Iset_RX = 10 �A) to 270 � (Iset_RX = 400 �A). This range was tested in our simulation. Current dynamic features (transfer from X to Z ports) of proposed active element are shown in Fig. 9 for several values of B. Linear range of operation is several tens of �A of input current (approximately �150 �A). We tested also

Fig. 10. Transient response (X � Z�) for different B

Fig. 11. Frequency responses of transfer (Y � X) for different values of RX (Iset_RX)

Fig. 12. Dynamical characteristic of the voltage transfer between Y and X ports (Y � X) for different RX transient response of the current transfer for the same values of B. Results are in Fig. 10 (amplitude of IINP = 50 �A). Bipolar transistors in Fig. 15 c (appendix) improve linearity (Fig. 5) of dependence B = f(Iset_B) and dynamical features (Fig. 9) of DCC-CFA in comparison to MOS types only in whole design. Small signal parameters are quite adequate to the used technology and approach. Output impedances have quite similar behavior for both positive and negative outputs. Values are more than 300 k� at 100 kHz (output impedance has highly capacitive character). Dependence of impedance of voltage input RY on frequency is very similar. Magnitude frequency response of the voltage transfers between Y and X ports is in Fig. 11. It is dependent on Iset_Rx which has impact on bandwidth. Impact of current gain which is controlled by auxiliary current Iset_B is negligible. Dynamics of voltage transfer is introduced in Fig. 12. The

403

Fig. 13. Frequency response of voltage follower

Fig. 14. Dynamical characteristic of the voltage buffer

Tab. 1. Small-signal DC parameters (.TF PSpice analysis)

parameter value (Iset_Rx = Iset_B = 50 �A) RY 1 T� RX 759 � RZ+ 500 M� RZ- 350 M� RO� 6 ��

KI (X�Z+) 0.985 KI (X�Z-) 0.985 KV (Y�X) 1.000

Iset_Rx influences the corners of the characteristic (limited by supply voltage), but impact is not very high. Several characteristics of voltage buffer model are also included in Fig. 13 and Fig. 14, where magnitude response and dynamical characteristic is shown. The voltage follower is based on complementary stage of two transconductors [28, 29, 34] and simple inverter. Transit frequency is quite high. There is small ripple at high frequencies and it requires capacitive compensation in some cases. Input impedance of the voltage buffer has also mainly capacitive character and it is higher than 1 M� below 30 MHz. Output resistance of voltage buffer is quite small, only 1-2 � at frequencies below 10 MHz. Detailed performances of DCC-CFA are given in Tab. 1.

IV. CONCLUSION We designed and verified improved and modified version

of controllable current feedback amplifier. Internal structure is more complicated but thanks to it we obtained two current outputs of both polarities and two voltage buffered outputs. Important features of proposed modification are control of the current gain between X and Z ports and control of intrinsic resistance of current input X. Such active element can be

applied in various systems like adjustable amplifiers, negative resistors and inductors, integrators, active filters, oscillators (quadrature and multiphase types), etc. Simulations confirmed workability and we also verified this element in application of adjustable quadrature oscillator with negative resistance [35].

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APPENDIX

a) b)

c)

Fig. 15. Proposed process model of modified controllable CFA: a) current conveyor with RX control, b) voltage buffer, c) controlled current conveyor with B control

TAB 2. DIMENSIONS OF CMOS TRANSISTORS IN FIG. 15

Transistor W/L [�m] M3, M4, M7 - M25, M47, M48, M51, M52, M55, M56 40/1 M1, M2, M35 - M44, M53, M54, M60, M61, M65, M66 20/1 M5, M6, M45, M46, M49, M50, M4b - M7b, M10b, M11b, M14b - M17b 10/1 M26-M34, M57 - M59, M62 - M64, M1b - M3b, M8b, M9b, M12b, M13b 5/1

405