_Article_  
**Improving the Signal to Noise Ratio of QTF**  
**Preamplifiers Dedicated for QEPAS Applications**  
**[PERSON] 1, [PERSON] 1,*(r) and [PERSON] 2(r)**  
Footnote 1: [[https://github.com/journals/p](https://github.com/journals/p)]([https://github.com/journals/p](https://github.com/journals/p))  
several different op-amps tested in both configurations (voltage and transimpedance). The designed preamplifier was equipped with a socket (Figure 1), so that testing different models of the op-amp required only simple placing of the required model of the op-amp into the socket. For this reason all the tested op-amp models were in dual-in-line plastic (DIP) packages. Taking into consideration that the parasitic capacitance of the socket pins was less than 0.5 pF, while the input capacitance of the tested op-amps were at the level of a few pF and that the frequency of the circuit operation was low (tens of kHz) influence of the socket on the circuit operation was negligible. A circuit diagram of the preamplifier circuit is given in Figure 2 (for the purpose of clarity of the power supply components visible in the photo from Figure 1 were omitted. However it should be mentioned that extreme care must be taken during the design of the preamplifier power supply. Selection of the preamplifier configuration (voltage or transimpedance) was possible by a simple change of the SW switch position. Due to such a solution, the op-amp and preamplifier configuration could be changed easily and without the need for preamplifier disassembly or readjustment of the measuring setup and allowed that the measurements could be performed in identical conditions. An additional ground copper layer below the PCB was used to reduce the possible influence of external E-M noise. For the tests the following op-amp models were chosen: OP27 (Analog Devices, Norwood, MA, USA), OPA227 (Texas Instruments, Dallas, TX, USA), TL071 (Texas Instruments, Dallas, TX, USA), OP42 (Analog Devices, Norwood, MA, USA) in order to evaluate influence of input voltage and current noise, input bias current, input impedance and op-amp bandwidth on the preamplifier properties. Selected parameters of these op-amp models are given in Table 1.  
The circuit in Figure 2 can either act as a non-inverting amplifier, in which the voltage divider of the feedback loop is formed of RF = 1 M\(\Omega\), R1 = 10 k\(\Omega\) and C1 or a transimpedance amplifier, in which the RF resistor defines the current-voltage conversion ratio. In the non-inverting configuration, the capacitor C1 acts as a low impedance device for relatively high frequencies. Therefore, when an AC (alternating current) voltage is applied at the circuit's input, the voltage gain is ku = R4(RF/(R1) + 1)/(R3 + R4). However, when a DC voltage is applied the C1 capacitor acts as an infinite impedance and therefore the preamplifier acts as a voltage follower. Such a solution allowed us to minimize the output offset voltage resulting from the input bias currents causing voltage drops on R1 and RF resistors. Moreover, the undesirable low-frequency components are cut-off from the output signal. The whole circuit was subjected to further measurements performed with a spectrum analyzer (MDO3024 + MDO3 SA, Tektronix, Beaverton, OR, USA), in which the default input resistance is 50 \(\Omega\) and the input signal is applied with a coaxial 50 \(\Omega\) cable. In order to limit the maximum output current of the amplifier resulting from the 50 \(\Omega\) load we used a voltage divider (current limiter) formed of resistors R3 and R4. Every operation amplifier requires an input voltage (or current depending on the technology used) applied to inverting and non-inverting inputs. The QTF is a resonant circuit in which the DC resistance is infinite. Therefore a QTF cannot provide any bias voltage or current at the non-inverting input (in the non-inverting configuration provided by SW). For this reason the R2 resistor is present in the circuit in its non-inverting mode of operation. In our solution we have chosen R2 = 1 M\(\Omega\).