Amplitude Control of Twin-T and Phase-Shift Oscillators Based on Direct Feedback Control Technique

Transcription

1 NU Science Journal 2013; 10(2): Amplitude ontrol of Twin-T and Phase-Shift Oscillators ased on Direct Feedback ontrol Technique Anucha Kaewpoonsuk 1, Ayutthaya Sri-in 1 and atchanoo Katman 2* 1 Physics-Electronics esearch Group, Department of Physics, Faculty of Science, Naresuan University, 2 esearch enter For Academic Excellence in Applied physics, Faculty of Science, Naresuan University, 3 Program of Physics, Faculty of Science and Technology, Pibulsongkram ajabhat University Phitsanulok, 65000, Thailand * orresponding author. ratchanook@psru.ac.th ASTAT This article presents a simple circuit design for control the amplitude value of the Twin-T and the Phase-Shift cillators. The proped circuit design uses feedback control technique to control the cillator directly without multiplier circuit. The structure of the designed system consists of a target cillator, an offset eliminator, an amplitude detector, and an error integrator to generate direct current-type (D) voltages of the target cillator. oth the Twin-T and the Phase-Shift cillators can adjust amplitude value by using an external control signal. Experimental results verifying performance of the designed circuit by using direct feedback control technique agreed with expected values. Keywords: Oscillator, amplitude control, feedback control, second-order cillator, multitime variable technique. INTODUTION A sinusoidal signal cillator is a useful circuit building block in electronics, instrumentation and measurement systems. It is used in form of the excitation signal generators for some tested circuits, sensors, and transducers (Kaewpoonsuk et al., 2008; hen et al., 2011; era and hakraborty, 2009; erkratn et al., 2007). In addition, it also plays as a carrier signal generator role in control and communication systems (Margarit et al., 1999; Schuler, 2013). The traditional realization of a sinusoidal signal generator was implemented by using a triangular signal generator connected with a triangular-to-sine waveform converter (Jacob, 1993; Franco, 1998). This method provides an easy amplitude control of the generated signal. The accuracy of this approach is strongly dependent on the performance of the waveform converter circuit. Moreover, it also causes the large size topology. The sinusoidal cillator based on second-order differential equation technique can directly generate sinusoidal waveform signal. However, the amplitude control of the signal is difficult. One fundamental approach for implementing an amplitude control of the sinusoidal signal cillator is based on feedback control technique (Hou, 2005;

2 NU Science Journal 2013; 10(2) 11 Filanovsky and Fortier, 1985). It requires a multiplier for the functional circuit building block. This approach has a great disadvantage in the topology of the scheme, and it is the ct for realization. ecently, the circuit analysis of sinusoidal signal cillators based on multi-time variable technique is presented (Maneechukate et al., 2008). It has been demonstrated that the amplitude value of sinusoidal signals from three important cillators; Twin-T, Phase-shift, and Wien bridge cillators, is dependent on the external bias voltages. However, the relationship between the amplitude value and the external bias voltage is not the pitive slope and it has an offset component in the output signals. In 2013, an improved Wien bridge cillator with variable output amplitude value is introduced (Kaewpoonsuk et al., 2013). This approach is based on the feedback control technique without multiplier requirement. To complete this concept, the other two sinusoidal signal cillators, Twin-T and Phase-shift, are investigated in this paper. ONEPT AND DESIGN TEHNIQUE Twin-T and Phase-Shift Oscillators The general schemes of the Twin-T and the Phase-shift cillators are shown on the left side in figure 1. The proped cillators use the operational amplifier (Op-amp) as basic building block. According to the suitable conditions and the conventional analysis of each circuit used, both cillators provide the sinusoidal signals with cillation frequency f as f 1 1/ 2 2 1/ 6 for Twin - T cillator for Phase- Shift cillator (1) Hence, the amplitude values of both generated signals are fixed at the saturation output voltage of the Op-amps used. The Twin-T and the Phase-Shift cillators with external D voltage using the multi-time variable technique are shown on the right side in figure 1. In the case of >0, the generated signals of both circuits can be stated as equation (2) A c( 2 f t) (2) offset where sat for Twin -T cillator A (3) sat 29 for Phase-Shift cillator f 1 1/ 2 2 1/ 6 for Twin - T cillator for Phase- Shift cillator (4) and for Twin - T cillator offset (5) 29 for Phase-Shift cillator

3 12 NU Science Journal 2013; 10(2) Note that we can control the amplitude value of the sinusoidal signals with the D voltage which can be adjustable. However, there are not direct variations but are inverse relations. In addition, the signals comprise of the offset terms offset /2 2 /2 General scheme With external D voltage (a) Twin-T cillator General scheme With external D voltage Proped technique (b) Phase-Shift cillator Figure 1 Twin-T and Phase-Shift cillators The improved amplitude control system of the Twin-T and the Phase-Shift cillators based on direct feedback control technique is shown in Figure 2. It consists of a target cillator, an offset eliminator, an amplitude detector, and an error integrator. The signals and out denote the external control voltage and the sinusoidal output signal of the system, respectively. The Op-amp A 2 acts as the offset eliminator which removes the offset term from the cillator signals of each cillator. The suitable conditions such as 3= 4= 5= 6 and 4= 5=29 3=29 6 were set in offset eliminator for the Twin-T and the Phase-Shift cillators, respectively. Therefore, the output voltage signal out can be stated as out A c( t) (6) where A sat sat 29 (7)

4 NU Science Journal 2013; 10(2) 13 and 1/ 1/ 6 for Twin - T cillator for Phase- Shift cillator (8) The amplitude value of output signals out is detected by the amplitude detector which designs by using Op-amp A 3. The amplitude value of out is D voltage A as A A (9) The Op-amp A 4 is used to design the error integrator. The voltage signal A is compared with the external control signal. If A=, the error integrator holds its output voltage. Whenever the value of A and is different, this different value is integrated to produce the new value of. From routine circuit analysis, the relation of the voltage A,, and the amplitude value (A ) of the output signal out can be expressed as A A sat (10) 2( ) 2( ) If we let 12 >> 11, then equation (10) can be approximated as A A (11) From equation (11), it is clearly seen that the amplitude value of out can be directly controlled by the external voltage signal. 3 Oscillator Offset Eliminator out Oscillator 4 A Offset Eliminator out Error Integrator A Amplitude Detector A Error Integrator A D D 2 A 3 7 Amplitude Detector Figure 2 Proped amplitude control circuit of Twin-T and Phase-Shift cillators

5 14 NU Science Journal 2013; 10(2) DESIPTION OF THE DESIGNED IUITS The performance of the proped technique has been confirmed by hardware implementations on a breadboard. The figure 3(a) shows the improved Twin-T cillator with variable output amplitude using Op-amps UA741, diodes 1N4148, capacitors and resistors. The supply voltages are ±10, while the values of device components are: = 11 = 1 kω, 1 = 50 kω, 2 to 7 = 10 = 10 kω, 8 = 9 = 12 = 100 kω, = 1 = 0.1 µf, and 2 = 10 µf. Hence f = 1.59 khz can be achieved. Another cillator is improved. It is shown in figure 3(b) which is the Phase- Shift cillator with the variable output amplitude. The Phase-Shift cillator circuit uses the Op-amp UA741 like the Twin-T cillator circuit but the used values of device components are different which are: = 500 Ω, 1 = 3 = 6 = 9 = 12 = 100 kω, 2 = 50 kω, 4 = 5 = 2.2 kω, 7 = 10 kω, 8 = 500 kω, 10 = 100 Ω, 11 = 1kΩ, = 1 = 0.1 µf, and 2 = 33 µf. Hence f = 1.30 khz can be achieved. In order to test the Twin-T and the Phase-Shift cillators, the control voltages having frequency of 1 Hz for sinusoidal waveform are applied. In addition, the control amplitude value of is provided by varying external voltage in range of /2 4 A Offset Eliminator out A 2 6 out Twin-T Oscillator A A D1 8 D2 A 3 Phase-Shift Oscillator 12 A 4 A 1 10 Offset Eliminator D1 A 3 8 D Error Integrator 11 Amplitude Detector Error Integrator 11 Amplitude Detector (a) Twin-T cillator (b) Phase-Shift cillator Figure 3 Improved cillator circuits with variable output amplitude EXPEIMENTAL ESULTS AND DISUSSION The measured results of the voltage A versus are plotted for determining D transfer characteristic of both proped cillators. These results are displayed in figure 4(a) which is obvious that the relations of the output amplitude values A and the external voltages are linear. The figure 4(b) illustrates plots of non-linearity error versus. The maximum non-linearity errors of the Twin-T and the Phase- Shift cillators are about 200 m (or 4 %) and 170 m (or 3.4 %) of full scale range, respectively.

6 NU Science Journal 2013; 10(2) Twin-T cillator 5.0 Phase-Shift cillator A ( ) A ( ) ( ) Twin-T cillator (a) D transfer characteristic ( ) Phase-Shift cillator (% ) Error FS ( ) (% ) Error FS (b) Non-linearity errors ( ) Figure 4 Measured results for varying in range of 0-5 However, the accuracy can be expected if the accurate amplitude detector is improved further (Maneechukate et al., 2008). The figure 5 shows the signals (,, out, and A) of both cillator circuits. The signals are the results obtained from applying in sinusoidal waveform having frequency of 1 Hz. It can be seen that all measured waveforms agree well with the expected values. out out A A (a) Twin-T cillator (b) Phase-Shift cillator Figure 5 Measured results from applying having frequency of 1 Hz

7 16 NU Science Journal 2013; 10(2) ONLUSION The Twin-T and the Phase-Shift cillators are improved using commercially available devices and only Op-amp as active elements. The output amplitude value of both cillators can be variable. The proped technique in this article utilizes the offset eliminator, amplitude detector, and error integrator connecting with the external control voltage. From experimental results, it is evident that the proped cillators work correctly and agree very well with the expected values. Moreover, the structure of both sinusoidal cillators has been simply designed. EFEENES era, S.., and hakraborty,. (2009). A novel technique of flow measurement for a conducting liquid. IEEE Transaction on Instrumentation and Measurement, 58(8), hen, D., Yang, W. and Pan, M. (2011). Design of impedance measuring circuits based on phase-sensitive demodulation technique. IEEE Transactions on Instrumentation and Measurement, 60(4), Filanovsky, I. M. and Fortier G. J. (1985, August). Fast amplitude control in Twin-T bridge cillators. Electronics Letters, 21(18), Franco, S. (1998). Design with operational amplifiers and analog integrated circuits. McGraw-Hill, Hou, A. S. (2005, February). Oscillation amplitude control for the bandpass- type sinusoidal cillator over a wide frequency range. International Journal of Electronics, 92(2), Jacob, J. M. (1993). Applications & design analog integrated circuits. egents/prentice Hall, Kaewpoonsuk, A., Maneechukate, T., Sri-In, A., erkratn, A. & Julsereewong, A. (2013, March). Improved Wien bridge cillator with variable output amplitude. II Express Letters, 7(3), Kaewpoonsuk, A., Petchmaneelumka, W., erkratn, A., Tammaruckwattana, A. & iewruja,. (2008, August). A novel resolver-to-d converter based on OTA-based inverse-sine function circuit. Paper presented at the SIE Annual onference 2008, Tokyo. Margarit, M. A., Tham, J. L., Meyer,. G. and Deen, M. J. (1999, June). A lownoise, low-power O with automatic amplitude control for wireless applications. IEEE Journal of solid-state circuits, 34(6), Maneechukate, T., Keeyaporn, J., Wardkein, P. and Keeyaporn, P. (2008, October). Wide-band amplitude control of the second-order cillator circuit. AEU-International Journal of electronics and communications, 62(9),

8 NU Science Journal 2013; 10(2) 17 erkratn, A., Pulkham, J., hitsakul, K., Sangworasil, M. and Kaewpoonsuk, A. (2007, October). High current low frequency eddy current imaging system. Paper presented at the onference on ontrol, Automation and Systems 2007, Seoul. Schuler,. A. (2013). Electronics principles & applications. McGraw-Hill,