## 3 Results and Discussion  
The preliminary measurements performed were aimed at evaluation of the selectivity of the sensor in both configurations (voltage and transimpedance). For this purpose the setup presented in Figure 3 was configured to measure the frequency response in a step by step mode, which means that every single point of the chart was obtained by setting the sine generator to a given frequency value and measuring the output signal from the amplifier with a lock-in technique and additional averaging. Due to use of such a method the obtained frequency response does not include any information about the signal-to-noise ratio, but expresses the output amplitude level from the QTF and selectivity of the sensor (ability of filtering the unwanted signals). The obtained results are presented in Figure 4. It is clearly visible that the output amplitude from the sensor working in a voltage mode is substantially higher, while selectivity of the sensor working in this mode is slightly worse than in a transimpedance configuration. It should be noticed, however, that in practical applications aimed at very sensitive measurements additional filtering and use of a lock-in (or another equivalent) technique are inevitable, no matter which of the configurations of the sensor is used. The next set of the measurements was carried out with all the averaging capabilities switched off, in order to observe the signal-to-noise properties of the investigated circuits. At first, with the acoustical stimulation switched on, we used a spectrum analyzer, which performed a frequency sweep with narrowband measurements of the amplitude of the preamplifier output signal. All four op-amps working in both configurations were investigated in this manner and results of these measurements are presented in a form of curves in Figure 5. The SNR numbers given in Figure 5 were obtained separately. In this case at first the frequency of the acoustic stimulation was tuned to the QTF resonance (frequency at which the preamplifier output signal was reaching its maximum level)--the frequency was adjusted with the resolution better than 0.1 Hz. The output signal level from the preamplifier was then measured (signal measurement). Then the acoustic stimulation was switched off and the output signal level from the preamplifier was measured again (noise measurement). Finally the two values  
Figure 4: Frequency response of the QEPAS sensor with the preamplifier working in transimpedance and voltage configurations: amplitude (**left**), amplitude zoomed around the resonance frequency (**middle**) and phase (**right**) measured with the frequency resolution better than 0.1 Hz. were used to calculate the SNR. Such an approach was used, because if we deal with a narrowband operation, SNR is defined as a ratio of the signal level to the noise floor level (background noise level at the signal frequency). Taking into consideration that QEPAS signal is usually measured with the use of lock-in amplifiers (which limits the bandwidth to a fraction of Hz), it is definitely a narrowband operation, and that is why the above definition applies. Results of these measurements are presented in Figure 5 and in Table 2.  
The obtained results clearly show that in the case of all tested operational amplifier models the use of the QTF preamplifier operating in the voltage configuration gives better results in terms of both the output signal amplitude and signal-to-noise ratio. A comparison of the contents of Tables 1 and 2 leads to a conclusion that in the case of the voltage configuration the main factor which determines the noise properties is the current noise and definitely not the voltage noise. Such a result was predictable taking into consideration that the QTF has a relatively high impedance. For the same reason a too low input differential impedance may also reduce the signal-to-noise ratio of the voltage configuration.