## 2 Materials and Methods  
### Problem Formulation: Initial Design Assumptions  
Figure 1 shows a diagram of a tube amplifier intended for replacement. The device is a typical push-pull LF system based on LF triodes, L2 and L3, and LF power tetrodes, L4 and L5. Appendix A includes a tabular list and description of elements comprising the circuit. The system is part of one of the electronic modules within the considered anti-aircraft missile launcher (Figure 2), and its task is to generate a radio fuse trigger angle setting signal depending on the velocity of the missile approaching an air target. The phase inverter system is crucial for this issue. In general, a phase inverter should be characterized by minimum output signal distortions, a wide frequency bandwidth, and high output signal symmetry, which is required to correctly control the LF power output tubes. In this case, the phase inverter stage is based on two LF triodes, L3A and L3B, placed in a single glass bulb of electron tube L3. Resistors R19-R21 determine the operating conditions of lamp L3 for a DC component stage. Elements R22-R25 and capacitors C6, C7, and C16 affect the system operating conditions for the AC component. Introducing local feedback for the AC component implements an input signal phase inverter system. Output signal symmetry is determined by the value of resistor R22. Elements C6 and C7 separate the phase inverter stage from the power stage of the DC component. The power stage design is based on lamps L4 and L5, resistor R26, and transformer TR1, which adjusts the high output impedance of the lamps to the low load impedance of this stage. Capactor C8 limits the amplifier bandwidth. Capacitor C13 is the last anode voltage power supply filter capacitor. Half of electron tube L2, together with auxiliary elements (resistors R14-R17, thermistor R32, and capacitors C4-C5), implements the voltage amplifier stage supplying an adequate level of control voltage to the phase inverter system. An undoubted advantage of this LF amplifier system is the high amplification of the output signal. Its downside is its high sensitivity to clipping. A significant limitation is the fact that input signal source voltage should not exceed the L2 tube grid polarization voltage, since exceeding this value translates to a significant increase in distortions on the amplifier output. It should be emphasized that triode L3 may age unevenly, which will lead to increased nonlinear distortions (loss of balance) over time. Another significant operational limitation is high heat emission because of the considerable impact of temperature on  
Figure 1: Diagram of the LF tube amplifier. Figure 2: Side view of an anti-aircraft missile launcher. the operation of other electronic components and adjacent functional blocks, implying a need to use cooling systems. Please also note that tube systems require additional filament circuits for lamps L2-L5. They should be filament voltage power supply systems, which demand more power. Consequently, we proposed replacing this LF tube amplifier system with an innovative system using semiconductor technology. The main tasks of the newly designed LF power amplifier include (a) generating an LF output signal with specified parameters over the entire range of assumed processing band; (b) generating a minimum signal DC component level at the output (load) regardless of instantaneous unbalanced supply voltage fluctuations; and (c) possibly ensuring the lowest distortion product ratio (DPR) level while maintaining a high signal-to-noise (SNR) ratio. Table 1 lists the assumed structural parameters of the system. The amplifier must spontaneously balance the output DC component value at a near-zero level during prolonged operation and when controlling through an LF analog signal (i.e., active operation). Furthermore, it is assumed that this functionality will be obtained without applying additional elements within the LF channel, which would separate the DC component from load in the form of LF transformers or blocking capacitors.