# Front-End Amplifiers for Tuning Forks in Quartz Enhanced PhotoAcoustic Spectroscopy
## 2 Front-End Electronics Architecture  
This investigation on representing a quartz tuning fork as a current or a voltage source aims to enhance the performance of the front-end electronics in terms of the collected signal. The SNR can be further improved by designing an amplifier input/output architecture that lowers the signal noise. The most common amplifier architecture employed in QEPAS sensors has a single-ended input configuration, where the voltage signal is referred to ground, as shown in Figure 1a. The noise affecting the QTF signal is a combination of the common-mode and differential-mode noise. This last type of noise is related to the characteristics of the sensitive element (e.g., thermal noise, shot noise) [24]. Once each noise source is characterized, a differential-mode contribution can be lowered by an analog filter. The common-mode noise is mainly related to electromagnetic interferences [25; 26] and their contribution can be lowered by using a differential input amplifier (Figure 1b) [27]. In a typical QEPAS set-up, the amplified QTF signal is further demodulated by a lock-in amplifier. An improvement of the SNR can be achieved by employing a differential demodulation of the QTF signal. In this case, an amplifier architecture with a differential output configuration is required. The two outputs of the amplifier are fed into the lock-in amplifier for differential demodulation, as shown in Figure 1c. By combining the advantages provided by a differential input and a differential output configuration, a fully differential amplifier, as depicted in Figure 1d, is expected to yield the highest SNR. Among the eight possible amplifier architectures obtained by alternating between a transimpedance and voltage amplifier, combined with the four input/output configurations shown in Figure 1, the performances of the four amplifier architectures were investigated and compared: a single-ended transimpedance amplifier, a differential output transimpedance amplifier, a differential input voltage amplifier, and a fully differential voltage amplifier. Circuit diagrams of the tested QTF amplifiers are depicted in Figure 2.  
Figure 1: Schematic of the quartz tuning fork (QTF)/front-end amplifier/lock-in connection for four input/output amplifier configurations: (**a**) single-ended, (**b**) differential input, (**c**) differential output and (**d**) fully differential. Figure 2: Simplified circuit diagrams of the tested QTF signal amplifiers with: (**a**) voltage amplifier input, (**b**) transimpedance amplifier input. In both cases, the amplifier consists of three stages. The first one contains an input circuit working in a voltage (Figure 2a) or transimpedance (Figure 2b) mode. The use of an instrumental amplifier (AD623) [28] in the voltage input amplifier (Figure 2a) allows for the implementation of a fully differential input, in which the QTF is placed between the inverting and non-inverting inputs of the amplifier and none of these inputs are tied to the local signal ground of the signal amplifier, (the 2.2 M\(\mathrm{Ohm}\) resistors are used only in order to supply the minimum necessary bias current to both inputs). The separation of both inputs from the ground of the measurement apparatus set-up reduces the influence of the noise (such as electromagnetic parasitic interferences from the environment) that may be induced on the ground plane in the case of a non-differential configuration. The transimpedance input (Figure 2b) is implemented in a standard way. The remaining two stages shown in Figure 2a,b are identical in both amplifiers. The second stage is a programmable gain amplifier (PGA) based on an AD603 voltage gain amplifier (VGA) [29], in which the gain was adjustable with a potentiometer. A PGA was used instead of a fixed gain amplifier to easily adjust the gain, depending on the level of photoacoustic signal obtained in the experiments. The third stage consists of two fixed gain amplifiers which produce the output signals in counter-phase.