## 3 Results and Discussion
### Real-World Device Tests  
The physically constructed LF amplifier was tested under laboratory conditions. The tests were carried out using an AudioPrecision AP x525 multifunctional measuring and testing system, which is designed to assess the operation of signal processing equipment in the LF domain. Figures 9 and 10 present the measurements of basic device parameters. The results are presented for individual pairs of power transistors, which allows us to demonstrate the universality of the solution and its resistance to changing operating or production conditions. \begin{table}
\begin{tabular}{c c c c c} \hline
**Parameter of \(\mathbf{U_{DC}}\)** & **140/9140** & **140/9240** & **240/9140** & **240/9240** \\ \hline mean value (V) & \(1.50\times 10^{-11}\) & \(3.06\times 10^{-12}\) & \(1.62\times 10^{-11}\) & \(7.80\times 10^{-12}\) \\ standard deviation (V) & \(9.74\times 10^{-12}\) & \(6.55\times 10^{-12}\) & \(6.98\times 10^{-12}\) & \(6.57\times 10^{-12}\) \\ \hline \end{tabular}
\end{table} 
Table 2: Numerical data for Figure 8.  
Figure 8: Voltage \(U_{DC}\) determined by simulation at load R0 with input capacitors C7 and C8 shorted to ground for: (**a**) 140/9140; (**b**) 140/9240; (**c**) 240/9140; and (**d**) 240/9240 transistor pair. Figure 7: Numerically simulated: (**a**) amplitude; and (**b**) phase characteristic of the LF amplifier (without the pre-processing stage). Figure 9 shows the amplitude-phase characteristics of the amplifier for the four configurations of transistor pairs used--IRFP140/IRFP9140 and IRFP240/IRFP9240--for an actual circuit with the preamplifier stage omitted. The circuit shows minimal gain differences (approximately 0.15 dB) for individual pairs of power transistors (Figure 9a). This is due to their non-uniform internal resistances, which translates directly into differences in the resting currents of the MOSFETs (in the case of components from different manufacturers or production batches, the matched pairs may show statistically insignificant gain differences). The circuit inverts the phase of the input signal (Figure 9b) and exhibits favorable phase characteristics, while in the case in question, an influence of internal parasitic capacitances of BJT and MOSFET transistors was observed. This becomes apparent in the faster phase drops for the extreme upper frequencies of the frequency response. Figure 10 shows the following: the frequency response of the amplifier for an output power of 1 W at load resistance R0 (Figure 10a) and the distortion contributed by the amplifier for all harmonic frequencies (Figure 10b). The lowest values of the DPR coefficient (below 0.1% in the whole passband under study) were recorded for the pairs 240/9140 and 240/9240. The measurements of voltage \(U_{\mathrm{DC}}\) on a load resistance of 8 \(\Omega\) for the tested pairs of IRFP transistors are presented in Figures 11-15. To allow these results to be correlated to other publications, we selected a typical value for load resistance R0, which is commonly used in analogue audio applications. It should be noted that for the intended application of the circuit, the value of R0 is different, but for obvious reasons, it will not be disclosed here. Figure 10: Measured: (**a**) RMS and (**b**) DPR of the LF amplifier. Figure 9: Measured: (**a**) amplitude and (**b**) phase characteristic of the LF amplifier. The tests were designed to verify (a) the ability of the amplifier to achieve the assumed values of the signal parameters (cf. Table 1) and (b) the amplifier's self-balancing capability. The measurement of \(U_{DC}\) was first performed with input capacitors C7 and C8 shorted to ground, i.e., with no \(1/2\dot{D}\) signal on the BJT transistors. Figure 11 presents sample 100 s waveforms of the voltage measured at R0--for each pair, its values are within \(\pm 100\) uV.