# Front-End Amplifiers for Tuning Forks in Quartz Enhanced PhotoAcoustic Spectroscopy
## 4 Results and Discussion  
The performances of the four amplifiers were evaluated and compared by acquiring the QEPAS spectral scans of the water absorption features, as shown in Figure 4. The QEPAS signal was recorded while varying the laser current, i.e., the laser wavenumber. In Figure 5, the normalized QEPAS spectral scans obtained by using the four different architecture amplifiers are reported. Normalization was performed to allow for an easy comparison of the SNRs. Measurements were performed by switching the amplifiers and maintaining the experimental conditions, namely the gas pressure, flow and concentration, as well as the optical alignment and lock-in integration time being fixed. For each spectral scan a current modulation depth of 24 mA was used, thereby maximizing the two QEPAS peaks signals. Figure 4: Absorption cross-section of standard air at 760 Torr. In each spectral scan, the two water vapor absorption peaks are clearly distinguishable with a small overlap between the negative lobes of the 2f-waveforms. The range between 1296.1 and 1296.25 cm\({}^{-1}\) is free from any absorption feature as also shown by a HITRAN simulation in Figure 4. Hence, this range is suitable for the estimation of the 1\(\sigma\) noise level for the amplifiers under test. Starting from these values, the SNR of the two peaks was calculated for each employed amplifier as the performance parameter to be compared. The noise levels and SNRs for both water vapor peaks are summarized in Table 1 for the four amplifier architectures. By using a single-ended transimpedance amplifier (Figure 5a), the measured noise is 0.0172, with an SNR\({}_{\rm ST}\) of the strongest water vapor peak of 58. With the differential output transimpedance amplifier (Figure 5b), the QEPAS noise measured is 0.0130 with an SNR\({}_{\rm DT}\) of 77 for the same water vapor absorption line, 1.3 times higher than the value obtained with the single-ended architecture. \begin{table}
\begin{tabular}{c c c c} \hline
**Amplifier Architecture** & **Noise (\(\sigma\))** & **SNR @ 1296.48 cm\({}^{-1}\)** & **SNR @ 1296.71 cm\({}^{-1}\)** \\ \hline
**Single-Ended Transimpedance** & 0.0172 & 58 & 26 \\ \hline
**Differential Output** & 0.0130 & 77 & 34 \\
**Transimpedance** & 0.0107 & 93 & 41 \\ \hline
**Differential Input Voltage** & 0.00951 & 105 & 49 \\ \hline \end{tabular}
\end{table} 
Table 1: Quartz-enhanced photoacoustic spectroscopy (QEPAS) 1\(\sigma\) noise and signal-to-noise ratio calculated for water vapor absorption features falling at 1296.48 and 1296.71 cm\({}^{-1}\) when implementing the four different amplifier architectures. Figure 5: Normalized 2f-QEPAS signals of a mixture containing 2% of water vapor acquired with (**a**) single-ended transimpedance amplifier, (**b**) differential output transimpedance amplifier, (**c**) differential input voltage amplifier and (**d**) fully differential voltage amplifier. In the insets the enlarged views of the noise fluctuations are shown. This result confirms that the SNR can be enhanced when a differential design is selected. The noise level measured by using a differential input voltage amplifier is 0.0107 (Figure 5c). The calculated SNR\({}_{\text{DIV}}\) is 93, 1.6 times greater than SNR\({}_{\text{ST}}\) and 1.22 times greater than SNR\({}_{\text{DT}}\), suggesting that a voltage amplifier architecture improves the QTF front-end electronics performance. This result is confirmed by the measurements made by using the fully differential voltage amplifier architecture. For a QEPAS noise of 0.00951 (Figure 5d), the achieved SNR\({}_{\text{FDV}}\) for the peak at 1296.48 cm\({}^{-1}\) is 105. Combined with a differential output configuration, the voltage amplifier architecture leads to an overall SNR enhancement of nearly double, with respect to the most used single-ended transimpedance amplifier This demonstrates that a QTF can be more efficiently schematized as a voltage generator rather than a current generator.