September 04, 2020
AD822ARZ is a true single-supply operational amplifier with rail-to-rail output, extremely low input current and low frequency noise, and is suitable for simultaneous operation with high impedance signal sources. AD822 has a single power supply capability of 5V, which makes it a good choice for this design example.
R6 provides matching load for piezoelectric sensor. R5 and D1 protect IC1-1 from possible high voltage spikes from the piezoelectric sensor. IC1-1 provides primary gain (approximately 1+R7/R8 in the operating frequency range) and partial gain frequency characteristics. R8 and C6 suppress the infrasonic frequency (cut-off frequency is 1/2πR8C6) and linearize the frequency response of the sensor. The combination of R7 and C5 can suppress frequencies beyond the operating frequency range (the cutoff frequency is 1/2πR7C5). R10-C9 and R13-C11 are additional low-pass filters. The second-order filter, which is placed around IC1-2 and can suppress infrasound frequencies, is the main high-pass filter. The output of the preamplifier is the open collector of Q2. The load of Q2 (Rg: 1.5kΩ) is placed at the receiver.
The power supply of the preamplifier involves the following factors: The voltage divider R2, R9 and C7 provides the offset voltage of half the power supply voltage (R2=R9) for the two parts of IC1 through R6 and R11. Assuming that the power supply voltage is 5V, the maximum value of the quiescent current Iq of the AD822 is 1.6mA. Studies have shown that the collector current Ic (no signal) of the output transistor should be several times Iq. R14 is used to set Iq, the calculation method is as follows: R14EB)/Iq=(5V/2-0.68V)/1.6mA=1.14kΩ (0.68V is the typical value of the emitter base voltage VEB of this type of transistor in the amplification mode ). Setting the value of R14 to 560Ω, the value of the collector current Ic of the output transistor is (Vs/2-VEB)/R14. When the hFE of BC847C is not lower than 420, the base current of Q2 is so small that it can be ignored. Therefore, Ic=(5V/2-0.68V)/560Ω=3.25mA. The maximum current (no signal) through the collector load is: Imax=Ic+Iq=3.25mA+1.6mA=4.85mA.
Studies have shown that since high noise will be reflected in the load resistance, general regulator ICs are not suitable at this time. The simulation process did not show this phenomenon. The simple Q1 circuit becomes the best solution. The output voltage Vs at the emitter of Q1 is:
(Vext is the external DC voltage (15V), VEB=0.68V.)
This value is an approximation. In fact, the value of Vs is smaller because the above equation does not take into account Q1's base and voltage divider current. It is recommended to set the minimum value of the R3-R4 voltage divider current to more than 10 times the base current of Q1. The AC component of the collector of Q1 is filtered out by C4. The value of C4 should be selected according to the lowest operating frequency of the preamplifier. The corner frequency 1/2πR3C4 should be at least 10 times lower than the low end of the preamplifier passband.
The R1-C3 network is an optional configuration; in the case of using the R1-C3 network, if a wider high-frequency passband is required, the value of C3 should be set lower. C9 is also an optional configuration, which affects the high frequency range. If you need to use C9, you can reduce the value of the decoupling capacitor C1 to 10μF. The preamplifier shown in the figure provides a gain of 26dB (Rg=1.5kΩ), and the frequency of its passband is about 8Hz～36Hz. The maximum value of AC output voltage is about 5Vp-p (±2.4V when the static operating point is about 7.8VDC). The current consumption of the actual device is 4.8mA. So far, the solution has been reliably and continuously running on 28 devices with a line length of 150m.