## 2 Materials and Methods
### Circuit Design  
The proposed device is a symmetric, three-stage LF amplifier system (Figure 3a). A diagram of the designed amplifier is shown in Figure 3b. Appendix A includes a tabular list and description of the elements comprising the circuit. The first (input) stage consists of operational amplifiers OP1A and OP1B and resistance elements from R3 to R11. The OP1A amplifier is a voltage amplifier. Its voltage gain in the negative feedback loop is determined through resistors R6 to R7. Amplifier OP1B acts as a voltage follower that reduces the first-stage output impedance. The control voltage from signal source \(1/2D\) is directed to the non-inverting input of the amplifier OP1A through the separating capacitor C5. The latter has two functions. First, it protects the OP1A amplifier channel against the unwanted appearance of a DC component, which may appear at the output of source \(1/2\dot{D}\) (by definition, the amplifier is not a DC component amplifier). Second, it separates the source of signal \(1/2\dot{D}\) from the side of amplifier OP1A through the application of a system of resistors R9-R11 and POT1, which compensates for the output DC component of the OP1A and OP1B amplifiers. The ceramic capacitor C6 protects OP1A against excitation, limiting the OP1 bandwidth. \begin{table}
\begin{tabular}{l c} \hline
**Parameter** & **Value** \\ \hline amplifier bandwidth & from 20 Hz to 20 kHz \\ output signal power & \(\geq\)10 W \\ distortion product ratio & \(<\)1\% \\ DC level of output signal & \(<\)2.5 mV \\ height \(\times\) length \(\times\) width (incl. mounting platform) & 120 \(\times\) 210 \(\times\) 300 mm \\ mass of the amplifier module (incl. frames and covers) & \(<\)5 kg \\ \hline \end{tabular}
\end{table} 
Table 1: Assumed parameter values for the LF amplifier under design. The second stage (LF voltage follower) is based on bipolar transistors T1 and T2. Elements R12, POT2, and R13 form a voltage divider that polarizes the bases of transistors T1 and T2 while simultaneously determining the DC operating conditions. Resistors R14 and R15 separate the low impedance of the polarization divider from bases of transistors T1 and T2 while improving system parameters in terms of low frequencies. Elements C7 and C8 act as capacitors coupling the first amplifier stage with the second one for the AC component while simultaneously separating these stages in terms of their DC component. The third (end) stage was constructed using T3 and T4 transistors that employ MOSFET technology. Resistor R0 has a load function within the system. The current of collectors T1 and T2 flowing through R16 and R19 polarizes end-stage transistor gates, thus impacting the opening voltage for gates T3 and T4, which leads to determining the operating conditions for this stage of the DC component. The amplifier's operation is protected by three independent DC power circuits. The first stage is supplied with stabilized balanced voltage of \(\pm\)15 V. The second stage is supplied with V1 unbalanced voltage, which, after passing symmetrical low-pass filters of  
Figure 3: (**a**) Block scheme and (**b**) diagram of the designed LF semiconductor amplifier. the \(\pi\) type (elements R1, C1, C2, and R2, C3, C4), forms a balanced power supply system. Similarly, the third stage is supplied with a balanced system that supplies unbalanced V2 voltage to symmetrical low-pass filters of the \(\pi\) type (elements R26, C11, C12, and R27, C9, C10). The application of independent DC power of the voltage follower stage (T1 and T2) and power followers T3 and T4 (the so-called power output stage) is a particular and crucial solution within the amplifier. Separating the stages due to power supply is necessary to generate and obtain minimum unbalanced voltage of the amplifier at a near-zero level, with load resistance and associated with voltage self-balancing of the amplifier at its output under conditions of continuous and uninterrupted operation.