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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING UMZCH with deep environmental protection. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers It is known that negative feedback (NFB) not only linearizes the amplification process of the audio signal, but also ensures its functional stability and damping of the reactive component of the load. The effectiveness of the OOS depends on its depth, i.e., intra-loop amplification, minimization of the still inevitable stage-by-stage delay of the amplified signal, elimination of spurious connections. To fulfill these conditions, it is not enough just to use high-frequency transistors and high-speed op amps; it is important, under the control of the main linearizing function of the OOS, to rationalize the very construction of the UMZCH. As publications in the journal "Radio" have shown, many designers associate the use of deep OOS with the tendency of UMZCH to self-excitation, the appearance of dynamic intermodulation distortions and advocate the need to limit the depth of OOS within the reproducible frequency range [1, 2, 3]. At the same time, little attention is paid to the control of obvious differences between the output and input signals of the UMZCH, as well as to the assessment of the frequency dependence of the intra-loop gain. Namely, these, easily controlled indicators, allow us to establish the true causes of gain distortion and select technical solutions that allow them to be eliminated. Passion for limiting the depth of the OOS without taking measures to improve the stability of the UMZCH leads to a delay in the action of the OOS at higher sound frequencies, and therefore to the appearance of dynamic intermodulation distortions. The underestimation of the ability of deep OOS to eliminate step-type distortions makes some designers embark on the path of reasoning about the so-called switching distortions and recommendations for using the amplification mode with a high quiescent current [4]. From my point of view, despite the very contradictory estimates of the OOS, it is very difficult to build a high-quality amplifier without deep OOS in the entire range of reproducible audio frequencies. It was not only my own design experience that allowed me to make such a conclusion, but also a long-term analysis of the results of objective control of the parameters of many UMZCH exhibited at three All-Union amateur radio exhibitions, as well as sent to the Radio magazine. In all cases, the control of the distortions introduced by the amplifiers was carried out using the method of selecting the signal of distortions and interference by directly subtracting the input voltage of the tested UMZCH from the output [5]. The possibility of objective and, most importantly, operational quality control of the UMZCH amplification of real audio signals provided by this method allows you to build a high-quality amplifier, overcoming the fear of deep OOS and the so-called transistor sound. When choosing a circuit diagram, offered to the attention of UMZCH readers with deep OOS, several variants of amplifiers were tested using the so-called "current mirror". However, their widely advertised advantages did not justify the material costs required for their implementation. Much hope was placed on simpler amplifiers with two differential stages. However, they found a hard-to-remove tendency to self-excitation due to the asymmetry of the matching circuits of the pre-terminal and final amplifiers. Hybrid UMZCH were also tested with various ways of matching and powering the OS. As a result of the experiments, the choice was made on the UMZCH, the scheme of which is shown in Fig. 1.
The amplifier is simple in design and provides fairly good parameters, primarily due to the introduction of deep feedback. Of particular note is its high linearity at higher audio frequencies, a low level of quiescent current, the ability to work without a special loudspeaker protection device from the direct current component, and maintaining performance when the supply voltage decreases. Rated output power UMZCH at a load of 8 ohms - 16 W, at a load of 4 ohms - 24 W; reproducible frequency range - 20...20 000 Hz; harmonic coefficient, measured by the defect signal selector, at a frequency of 1 kHz - 0,005%, at a frequency of 20 kHz - 0,008% at the maximum level of the output signal. The UMZCH pre-terminal amplifier is a two-stage one with a high-resistance inverting input. The non-inverting input is used to balance the supply voltage, the source of which is not galvanically connected to the common wire. Transistors VT1, VT2 of the first stage of the pre-terminal amplifier are connected according to the scheme of a composite emitter follower. The base of the transistor VT3, blocked by the capacitance of the capacitor C3, is connected to the resistive circuit R6R7R8. The transistor VT4 operating in the second stage is connected according to the scheme with OE. Together with a current source on transistors VT5, VT6, it provides a more linear amplification of the maximum levels of the audio signal. The current source also performs the functions of a current mode stabilizer of the terminal amplifier. The differentiating circuit C5R2C6, connected between the input and output circuits of the amplifier, prevents its self-excitation and, using the capacitor C8, allows you to shift the frequency cutoff of the frequency response beyond the reproducible audio frequency range. The final stage of the amplifier is built on complementary pairs of transistors connected according to a common collector circuit. To stabilize the current mode and damping switching processes, a transistor shunt VT7, VT8 is included at the input of the final amplifier UMZCH, controlled by voltage on the bases of the transistors of the output stage VT11, VT12. This method of stabilization [6] ensures the operability of the UMZCH with a threefold decrease in its supply voltage. The UMZCH is powered by an autonomous rectifier connected to a separate winding of the mains transformer. All parts of the amplifier and rectifier are mounted on two plates of fiberglass, between which the heat sinks of the output transistors VT11, VT12 and oxide capacitors C 11, C 12 are clamped. . Hanging installation. Coil L1 is wound on resistor R15 and contains 30 turns of PEL wire 0,8. The proposed version of the UMZCH design makes it possible to weaken the mutual influence of its circuits on each other and makes it convenient to place it in a stereo complex or an active speaker. Establishing UMZCH was reduced to setting (using resistor R12 or R13) a quiescent current within 15 ... 25 mA. The first test of the UMZCH performance was carried out, as usual, when a limiting resistor R16 and a milliammeter RA1 were connected to the break in the power circuit. To control UMZCH distortions, a compensation selector with a defect-signal preamplifier was used, the circuit of which is shown in Fig. 2. Moreover, not only the sinusoidal signal was controlled, but also the real sound signal during the operation of the UMZCH with the AU. The selector itself is a resistive circuit R1 - R4, to which the UMZCH input signal is supplied through the capacitor C1 (from control point A), and through the divider R5 - R7 - an antiphase output signal (from control point B). Next, the signals are balanced by adjusting resistors R6 and R5 and compensation for the delay of the output signal by capacitor C2 is achieved. From the selector output (connection point of the resistors R2, R3), the processed difference signal (the so-called defect signal) through the capacitor C3 is fed to the preamplifier on transistors VT1, VT2 and then to the oscilloscope or millivoltmeter. To estimate the magnitude of the defect signal, we used the scale calibration of the oscilloscope screen or the scale of a milliammeter. To do this, by pressing the SB1 button, the voltage applied to the preamplifier was reduced to 0,005 of the UMZCH input signal, and then the defect signal was compared with it. The method of working with the selector is described in more detail in [5]. To estimate the depth of the OOS at 1000 and 20 Hz using the SB000 switch, the preamplifier should be connected to the control point B of the UMZCH and sinusoidal signals of the corresponding frequencies should be fed to the input of the latter. The selector is mounted on a fiberglass plate, fixed for the duration of the UMZCH tests near its control points. Author: I. Akulichev
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