A Simple Current-mode Schmitt Trigger Employing Only Single MO-CTTA Phamorn Silapan* and Montree Siripruchyanun** * Electrical and Industrial Program, Faculty of Industrial Uttaradit Rajabhat University, Muang Uttaradit 53000, THAILAND ** Department of Teacher Training in Electrical Engineering, Faculty of Technical Education, King Mongkut’s University of Technology North Bangkok, Bangkok, 10800, THAILAND Email:
[email protected]*,
[email protected]** Abstract- A new current-mode Schmitt trigger based on MOCTTA (Multiple output current through transconductance amplifier) is presented in this article. The circuit description is very simple, its construction consists of only single MO-CTTA. The hysteresis and amplitude of the output current can be tuned independently/electronically. With mentioned features, it is very suitable to realize in a monolithic chip. The PSPICE simulation and experimental results are depicted, and agree well with the theoretical anticipation. The maximum power consumption is approximately 249mW at ±5V supply voltages.
I.
INTRODUCTION
A Schmitt trigger has been firstly proposed by Otto Schmitt in 1938 [1]. Its application is to reduce the noise effect in triggering devices and analog to digital conversion [2-3]. From our survey, we found that several implementations of the Schmitt trigger employing different high-performance active building blocks, such as, OTAs [4], current conveyors [5], and Op-Amp [6], have been reported. Unfortunately, these reported circuits suffer from one or more of following weaknesses: x Excessive use of the active/passive elements, especially external resistors [4-6]. x Use of a floating resistor, which is not convenient to further fabricate in IC [6]. x Absence of electronic controllability of magnitude and hysteresis of output signal by electronically [5-6]. x Operation in voltage-mode [4-6]. Presently, there is a growing interest in synthesizing currentmode circuits because of more their potential advantages such as larger dynamic range, higher signal bandwidth, greater linearity, simpler circuitry, and lower power consumption [710]. In the point of view, the current–mode technique is ideally suited to this purpose more so than the voltage-mode type. Recently, the Schmitt triggers based on current-mode technique and high-performance active building block such as CDTA [11], CCCDTA [12] have been introduced. However, they use one or more of the passive elements which is not convenient to further fabricate in IC or practical implementation. In 2004, the current through transconductance amplifier (CTTA) [13] presented by Biolek and Biolková, seems to be a versatile component in the realization of a class of analog signal processing circuits. It is really current-mode element whose input and output signals are currents. In addition, its output current gain can be electronically adjusted.
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The aim of this paper is to propose a novel current mode Schmitt trigger, emphasizing on the use of the MO-CTTA. The features of the proposed Schmitt trigger are that: the circuit description is very simple, its construction consists of only single MO-CTTA: the hysteresis and amplitude of the output current can be tuned independently/electronically. It can be applied in an automatic control system via a microprocessor. The performances of proposed circuit proved by PSPICE simulations and experimental results are also shown, which are in correspondence with the theoretical analysis.
(a)
x x g m 2VZ (b) Figure 1. The MO-CTTA (a) symbol (b) equivalent circuit.
II. CIRCUIT CONFIGURATION A. Basic Concept of MO-CTTA Since the proposed circuit based on MO-CTTA, a brief review of the MO-CTTA is given in this section. Generally, the MO-CTTA properties are similar to the conventional CTTA, except that transconductance g m1 at x terminal and transconductance g m 2 at x terminal can be controlled by the bias currents; I B1 and I B 2 , respectively. The relationship of voltage and current of MO-CTTA is shown as followed
Vp
Vn
0, I p
In
I z , I x
g m1Vz , I x
g m 2Vz ,
(1)
I B1 IB2 , gm 2 . (2) 2VT 2VT VT is the thermal voltage. The symbol and the equivalent circuit of the MO-CTTA are illustrated in Figs. 1(a) and (b), respectively.
where
556
g m1
A practical implementation of MO-CTTA by employing the for I in d I x , when Vz 2VT I B1 (7) I out ® commercially available ICs can be achieved as shown in Fig. 2, I B1 for I in t I x , when Vz 2VT . ¯ In this work, we employ AD844 as CCII and LM13600N as OTAs, where p-terminal of MO-CTTA is grounded. It consists Additionally, the threshold level can be found to be of three principal blocks: a second-generation current conveyor ITH I B 2 , (8) (AD844) as input stage: negative transconductance amplifier ITL I B 2 . (9) and positive transconductance amplifiers (LM13600N) as We found in Eqs. (8) and (9) that, the upper and lower output stage. The corresponding transconductances can be threshold currents can be linearly adjusted by I B 2 . From Eqs. electronically adjusted by input bias currents of the LM13600N. (8) and (9), the magnitude of output current of the Schmitt trigger can be expressed by for I in d ITL , when Vz 2VT I B1 (10) I out ® for I in t ITH , when Vz 2VT . ¯ I B1
The transfer characteristic of the Schmitt trigger is displayed in Fig. 4. I B1
IB2
I out
IB2 I in
Figure 2. Implementation of CTTA based on commercially available ICs.
B. Principle of Current-mode Schmitt trigger The current-mode Schmitt trigger is shown in Fig. 3. By using the MO-CTTA properties, the following output current can be obtained § Vz · if I in d I x ° I B1 tanh ¨ ¸ ° © 2VT ¹ (3) I out ® § Vz · ° I if I I tanh . t ¨ ¸ in x ° B1 © 2VT ¹ ¯
I B1
Current (μA)
Figure 4. Transfer characteristic of proposed Schmitt trigger.
Figure 5. The transient response of the Schmitt trigger.
Figure 3. Circuit diagram of current-mode Schmitt trigger.
If n -terminal is connected as a positive feedback where zterminal is floated, the output voltage at z -terminal can be found to be Vz VCC
if
I in d I x ,
(4)
Vz VEE
if
I in t I x ,
(5) Figure 6. Output current deviations for different temperature variations.
where VCC and VEE are the positive and negative voltage supplies, respectively. From Eqs. (4) and (5), if Vz 2VT it can be approximately reduced to § V · tanh ¨ z ¸ 1 . (6) © 2VT ¹ It is clearly seen that, the MO-CTTA operates in the saturationmode. From Eq. (3), we can receive the output current as
III. SIMULATION AND EXPERIMENTAL RESULTS To prove and verify the performances of the proposed Schmitt trigger, the PSPICE simulation program was used. The MO-CTTA was implemented using the commercially available ICs as depicted in Fig. 2. The circuit was biased with ±5V supply voltages. Fig. 5 demonstrates the simulation result in time-domain of the Schmitt trigger in Fig. 3 at its output when a triangular input signal is applied.
557
.010
Iout (A)
Amplitude Deviation (%)
.008
.006
.004
.002
(a) 0.000
IB2=20
IB2=50
IB2=80
IB1=100A
120 50
100
150
80
200
Iout (A)
0
o
Temperature ( C)
Figure 7. The amplitude deviation of the output current where temperature is varied. IB1=20
IB1=60
IB1=100
-40 -80
IB2=30A
-120 -100 -80
120 80 40 0
-60
-40
-20
0 20 Iin (A)
40
60
80
100
(b) Figure 9. DC transfer characteristics of the current-mode Schmitt trigger for (a) different values of IB1 (b) different values of IB2.
-40 -80 -120
40 0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Time (ms)
(a)
(a) Iin 120 80 40 0 -40 -80 -120
(b) Figure 8. The output current of the current-mode Schmitt trigger for (a) different values of IB1 (b) different values of IB2.
Fig. 6 shows the output current relative to temperature variations for 27qC , 50qC and 100qC . It is clearly observed that the output current is almost not dependent on the temperature variations due to operating in saturation-mode the MO-CTTA used in the circuit, as explained earlier. Fig. 7 depicts amplitude deviation values relative to variations of the temperature; the maximum deviation of the amplitude of output current signal is approximately 0.008% . The results for electronic controls of output and threshold level are shown in Figs. 8-9, which are the transient responses and the DC transfer characteristic for different values of I B1 and I B 2 , respectively. They are seen that the output amplitude and threshold level can be independently/electrically adjusted. The output responses for different input frequencies are also shown in Fig. 10. It is confirmed that the proposed circuit can operate when frequency is up to megahertz range without disturbing magnitude of the output current. Fig. 11 depicts the plot of the simulated and theoretical threshold currents versus the input bias current; I B 2 . It is seen that the simulation results are in accordance with the theoretical analysis as shown in Eqs. (8)(9).
0
2
Iout
4
6
IB1=100A, IB2=30A
8 10 12 14 16 18 20 22 24 26 28 30 Time (μs)
(b)
(c) Figure 10. The transient responses of output currents for various input frequencies at (a) 10kHz (b) 100kHz (c) 1MHz.
In addition, to insist that the proposed circuit can operate practically, the experimental results of the proposed Schmitt trigger, when IB2=20PA and 90PA and respectively using V/I and I/V converters as input and output stages, are displayed in Figs. 12 (a) and (b), respectively. Figs. 13(a) and (b) demonstrate the experimental results of output relative to input signal when IB1=50PA and 150PA. From Figs. 12-13, they are
558
confirmed that the proposed circuit can be independently/electronically tuned the magnitude and hysteresis of the output signal by IB1 and IB2, respectively. These results can be achieved by using an input voltage converted to current by an AD844-based V/I converter, where the output current is converted to output voltage by employing a resistor of 5k: to be easily measured by an oscilloscope. 150
work is to find applications of the proposed Schmitt trigger such as square-wave generator, pulse-width modulator, monostable multivibrator and etc.
ITHITH- from Eq. (9)
100
ITH+ from Eq. (8) ITH+
ITH (PA)
50
0
(a)
-50
-100
-150 0
20
40
60
80
100
IB2 (PA)
Figure 11. The upper and lower threshold currents where IB2 is varied.
(b) Figure 13. The experimental results of the current-mode Schmitt trigger where (a) IB1=50μA (b) IB1=150μA. [1] [2]
[3]
(a)
[4]
[5] [6] [7] [8]
(b) Figure 12. The experimental results of the current-mode Schmitt trigger where
[9]
(a) IB2=20μA (b) IB2=90μA. [10]
IV. CONCLUSION A novel current-mode Schmitt trigger employing only single MO-CTTA has been presented. The proposed configuration is very simple, where the hysteresis and magnitude can be independently/electronically controlled. Furthermore, the highest frequency is restricted at up to several megahertz range. All of simulation and experimental results confirm the theoretical analysis. The maximum power consumption is approximately 249mW at ±1.5V supply voltages. Our future
[11]
[12]
[13]
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