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APPLICATION NOTE TS912 : DRIVING A PIEZOELECTRIC CELL WITH A RAIL TO RAIL OP-AMP
by F. MARICOURT
INTRODUCTION The TS912 is an input/output rail to rail dual CMOS operational amplifier. It is able to operate with low supply voltage (2.7V) and to drive low output loads (600). As an illustration of these features, the following note is describing the TS912 behaviour when driving a piezoelectric cell. APPLICATION CIRCUIT The TS912 is used in a single supply push-pull configuration as displayed below in figure1. The closed-loop gain value is 20dB.A small serial resistor Rout is placed in order to remove HF oscillations. The piezoelectric speaker used as load has the following characteristics :
CHARACTERIZATION The dynamic behaviour is displayed through the two following figures :
.Fr .Out
21dB
equency response at VCC = 3V (figure 2). This application is particularly stable within the range of the piezo (the gain is dropping by only 2.5dB).
put swing versus supply voltage : the pushpull configuration is theorically allowing a doubled output voltage amplitude versus VCC : the outputs on each side of the cell are rail to rail and phase opposite.
Figure 2 : Frequency Response
.Fr .Capaci .I
equency range: 500Hz-20kHz tance: 140nF at 120Hz
20dB
GAIN
0 deg
-10 deg 19dB PHASE 18dB -30 deg -20 deg
nput voltage: 30vp-p max.
Figure 1 : Electrical Schematic
100K
17dB -40 deg
10K 1/2 TS912 Vin Rout Vcc
START 500,000Hz
STOP 20 000,000Hz
Figure 3 shows that this goal is highly dependant of the frequency ; for example the total output swing available at 2kHz is recovered as followed :
piezo cell Vcc/2 10K 1/2 TS912 100K
.V
CC =3V VCC = 4V VCC = 5V VCC = 8V
Out.vol.=3.4Vpp 6.6Vpp 8.7Vpp 14.6Vpp
57% of 6vpp theor. 82% 87% 91%
AN857/0299
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APPLICATION NOTE
As a typical feature of any audio application,one may consider Total Harmonic Distorsion (THD) which is here considered with power supply as a parametric value (Figure 4). Figure 3 : Output Swing versus Supply Voltage The piezo is then correctly driven with low voltage supply down to 4V
Figure 4 : Distorsion + Noise versus Frequency (Vo = 1Vrms)
20 18
2
MAXIMUM OUTPUT SWING (V)
16 14
THD + noise (%)
1.5
Vcc = 5V
12 10 8 6 4 2 0 3 3.5 4 4.5 5 6 8 10 f = 1kHz f = 2kHz
1 Vcc = 3V
Vcc = 4V
f = 5kHz
0.5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
FREQUENCY (kHz)
SUPPLY VOLTAGE (V)
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. (c) The ST logo is a trademark of STMicroelectronics (c) 1999 STMicroelectronics - Printed in Italy - All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - France - Germany - Italy - Japan - Korea - Malaysia - Malta - Mexico - Morocco The Netherlands - Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A. (c) http://www.st.com
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