# Bar-Wave Calibration of Acoustic Emission Sensors
## Appendix D FET Buffer  
Since [PERSON] et al. [19] designed an input buffer circuit with an FET for increasing the sensitivity of a piezoelectric sensing element, such an FET buffer has been used in many other AE sensors. While the internal wirings of commercial sensors have not been disclosed, the circuit is expected to be similar to Figure A9. This drawing is based on [PERSON]'s input buffer circuit, in which he used 2 SK715, a load resistance of 750 \(\Omega\) and power supply of 18 V. Here, the actual load resistance of 83 \(\Omega\) (nominal value = 75 \(\Omega\)) and 9 V supply voltage were used. This circuit differs from that suggested by DWC for use with their B1080-LD (results were reported in Section 3.2.2). The DWC circuit uses an inductor (3.3 mH used) in lieu of the road resistor in Figure A9 and adds a 75-\(\Omega\) termination after the dc-blocking capacitor (1 \(\upmu\)F). The gain of the FET buffer is expected to change with the load resistance. By using 2 SK168 with load resistance values of 83 \(\Omega\) or 531 \(\Omega\) (16.1 dB difference), their effects on the output were evaluated. KRN (#12060) sensor was used and placed face-to-face with Olympus V192, driven by monopolar pulse of \(-\)130 V peak. With the load resistance of 83 \(\Omega\), the peak output of the FET buffer circuit (Figure A9) was 0.433 V, while it was 2.23 V with the load resistance of 531 \(\Omega\); an increase of 14.3 dB. An approximate proportionality is observed. (Note that 2 SK168 is obsolete. A better choice is 2 SK3557, which has the same drain current limit of 50 mA as 2 SK715.) Consequently, the use of an FET buffer with a particular power supply can result in different gain for the FET buffer. While the type of FET and internal circuit can also change the net gain, it is concluded that the increased sensitivity levels with FET-buffered KRN sensors, (observed in Section 3.2.1), are due to the gain of the FET buffer, as noted in Appendix B. Effects of an FET buffer and the input resistance, \(\text{R}_{\text{in}}\), were further examined. The same test set-up as in the preceding paragraph was used. First, with \(\text{R}_{\text{in}}=1\) M\(\Omega\), the sensor output was connected to an oscilloscope through an FET buffer (R = 83 \(\Omega\)). Its output FFT spectrum is given as the red curve in Figure A10. Next, the sensor output was connected directly to the oscilloscope input (\(\text{R}_{\text{in}}=1\) M\(\Omega\) in parallel to 14 pF) using a 10 cm tinned wire. This minimized the stray capacitance (around 3 pF) and produced the green curve. Note the increased levels below 100 kHz for the red curve, indicating the low-frequency enhancement of 8 dB due to the FET buffer circuit. (The gain of the FET buffer was \(-\)1.4 dB and not adjusted.) In the next series, \(\text{R}_{\text{in}}\) of 10 k\(\Omega\) was used. Using another FET buffer, the obtained FFT spectrum is given as the blue curve with its peak nearing that of the red curve. However, a sharp drop-off occurred below 400 kHz. Next, the direct connection to the oscilloscope input with 10 k\(\Omega\) in parallel, giving the purple curve. The shape of this curve is similar to that of the green curve above 500 kHz, but low-frequency drop-off is present. Finally, a coaxial cable was used for connecting the sensor to the oscilloscope with a 10 k\(\Omega\) termination. This is given as purple dash curve, showing approximately 6 dB below the purple curve above 500 kHz, decreasing as the frequency is reduced and merging to the blue and purple curves below 50 kHz. The low-frequency output reduction is due to the small sensor capacitance (40 pF) and \(\text{R}_{\text{in}}\) of 10 k\(\Omega\), which together form a high-pass filter acting on the voltage generated in the sensor. This effect is especially significant below 150 kHz. Thus, this type of conical element sensors requires at least 100 k\(\Omega\) input resistance.