|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Simple high quality UMZCH. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers The analysis of letters from radio amateurs who responded to the article [1] made it possible to come to the following conclusions. Firstly (and this is natural), everyone is in favor of creating simple 3H power amplifiers (UMZCH); secondly, the simpler the amplifier circuit, the less trained radio amateurs undertake to assemble it; thirdly, even experienced designers often ignore the known installation rules, which leads to failures when repeating UMZCH on a modern element base. Based on the foregoing, the UMZCH was developed (see Fig. 1) based on the amplifiers described in [1, 2].
Its main features are the use of an op-amp in a low-signal mode (as in the amplifier described in [1]), which expands the frequency band of signals reproduced without exceeding the slew rate of the output voltage of the op-amp [3]; transistors of the output stage - in the OE circuit, and the final stage - with a divided load in the circuits of emitters and collectors. The latter, in addition to the obvious design advantage - the possibility of placing all four transistors on a common heat sink, provides certain advantages compared to the output stage, in which the transistors are connected according to the OK circuit [2]. Main technical characteristics of UMZCH:
Op-amp DA1 is powered through transistors VT1 and VT2, which reduce the supply voltage to the required values. Quiescent currents of transistors create voltage drops across resistors R8 and R9, sufficient to provide the necessary bias voltage at the bases of transistors VT3, VT4 and VT5, VT6. At the same time, the bias voltages for the transistors of the final stage are chosen such (0,35 ... 0,4 V) so that they remain reliably closed when the supply voltage rises by 10 ... 15% and overheats by 60 ... 80 ° C. They are removed from the resistors R12, R13, which simultaneously stabilize the operation mode of the transistors of the final stage and create local OOS for current. The ratio between the resistances of the resistors R11 and R4 of the OOS circuit is selected from the condition of obtaining a nominal input voltage of 0,8 V. The inclusion of the external correction and balancing circuits of the op-amp is not shown in the diagram for simplicity (this will be discussed in the section on setting up the amplifier). Low-pass filter R3C2 and high-pass filter C3R10 with cutoff frequencies in the region of 60 kHz prevent the operation of relatively low-frequency transistors VT3-VT6 at higher frequencies in order to avoid their breakdown. Capacitors C4, C5 correct the PFC of the pre-terminal and final stages, preventing their self-excitation in case of unsuccessful installation. Coil L1 increases the stability of the UMZCH with a significant capacitive load. UMZCH is powered by an unstabilized rectifier. It can be common for both channels of the stereo amplifier, however, in this case, the capacitance of the filter capacitors C8 and C9 must be doubled, and the diameter of the wire of the secondary winding of the transformer T1 must be 1,5 times. Fuses are included in the power circuits of each of the amplifiers. The design of the UMZCH may be different, however, some design features on which the success of its repetition depends must be taken into account. A drawing of a printed circuit board and the placement of parts for one UMZCH channel are shown in fig. 2
The length of the leads of the parts should be no more than 7 ... 10 mm (for ease of installation, the leads of the op-amp DA1 are shortened to about 15 mm). In UMZCH, it is necessary to use ceramic capacitors with a rated voltage of at least 50 V. The board can be fixed on the heat sink of the transistors of the final stage using racks 15 ... 20 mm high or in close proximity to it, using some kind of detachable connector, for example MPN-22 (sockets and pins of the connector are included at points 1-5). In the latter case, the resistance of resistors R12 and R13 should be selected equal to 43 ... 47 Ohm, and on the socket of the connector with transistors VT5, VT6 connected to it, install resistors of the same resistance R12 'and R13 ' (this will prevent the failure of transistors if contact is lost in the connector). The length of the conductors between the board and the final stage transistors must be no more than 100 mm. In addition to what is indicated in the diagram, in the UMZCH it is possible to use the OU K140UD6B, K140UD7A, K544UD1A, however, the harmonic coefficient at frequencies above 5 kHz will increase in this case to approximately 0,3%. The transistors of the final stage are placed on a heat sink, bent from a plate with dimensions of 70x35x3 mm (excluding a foot with a hole with a diameter of 2,2 mm) made of aluminum alloy, which is fixed to the board with one M2X8 screw with a nut to prevent breakage of the transistor leads during accidental mechanical influences. The transistors of the terminal stage can be placed both on a heat sink common to each UMZCH channel, and on a heat sink common to both channels. In the first case, they are fixed on the heat sink and the latter is isolated from the UMZCH case, in the second case, transistors are isolated, and the heat sink can be a structural element of the amplifier case. To reduce the thermal resistance of the transistor case - heat sink, it is necessary to use heat-conducting paste. When using separate (for each channel) heat sinks, transistors in a plastic case can be used, which, due to the small area of metal bases, can overheat if the gaskets are poorly made or there is loose thermal contact with the heat sink and an excessive amount of paste in the gap. It is advisable to install transistors in a metal case on a heat sink common for both channels. The heat sink area per transistor must be at least 500 cm2. Of great importance is the installation of the UMZCH, the connection of its channels to the power source. The power wires (+22 V, -22 V and common) should be as short as possible (they should be laid separately for each channel) and of a sufficiently large cross section (at a maximum power of 42 W - at least 1,5 mm2). Speakers, as well as the emitter and collector circuits of the final stage transistors, must be connected to the UMZCH board with wires of the same cross section. UMZCH is adjusted with the terminal stage turned off. If a detachable connector is used to connect parts of the UMZCH, it is convenient to use a technological socket to which only the power wires and the output of the 3H signal generator are connected. When directly connecting the terminal transistors to the UMZCH board, it is enough to remove the jumpers from the solder from the printed conductors of the circuits of their bases and temporarily solder the latter to the emitter terminals. To balance the op-amp DA1 (if necessary), the board provides holes for tuned and fixed resistors or wire jumpers for connecting the microcircuit pins in accordance with the balancing scheme for a particular type. For example, to balance the OU K544UD2, its terminals 1 and 8 are connected through a 62 kΩ resistor to the output of the engine and one of the terminals of the resistive element of a tuned resistor with a resistance of 22 kΩ. The free terminal of this resistor is connected with a wire jumper to terminal 7 of the op-amp, and through a resistor with a resistance of 75 kΩ "- to terminal 5 (in Fig. 2, these elements are shown by dashed lines). When using the K544UD1 op-amp, its terminal 1 through a resistor with a resistance of 4.3 kΩ is connected to the terminals a tuned resistor with a resistance of 1,5 kΩ. Its free terminal is connected to terminal 8 of the op-amp through a resistor with a resistance of 5,1 kΩ, and to terminal 7 - with a wire jumper. To balance the OU K140UD6 and K140UD7, resistors of the same ratings are used, but the free terminal of the trimmer through a constant resistor with pin 5, and a jumper with pin 4 of the op amp. However, balancing may not be necessary, so these parts are installed only if necessary. The adjustment begins with the fact that the input of the amplifier is short-circuited, an oscilloscope is connected to the output, turned on in the maximum sensitivity mode, and power is supplied for a short time. If there is no alternating voltage at the output, i.e., there is no self-excitation, the operating mode of transistors VT3, VT4 and op-amp DA1 is measured in direct current. The supply voltages of the op-amp should lie within +13,5...14 and -13,5...14 V and be approximately the same (deviation is permissible within 0,2...0,3 V). The voltage drops across resistors R12 and R13 should be equal to 0,35 ... 0,4 V. If they differ significantly (more than 10%) from the specified value, it is necessary to select resistors R8, R9, making sure that they the new resistances remained the same. Resistors are replaced with the UMZCH power off. The approximate resistance of the resistors for the K544UD2A OU is indicated in the diagram. When using the K544UD1A and K140UD6 op-amps, their initial resistance should be 680 ohms, and when using K140UD7 - 560 ohms. Having picked up the resistors R8, R9, they measure the constant voltage at the UMZCH output and, if it exceeds 20 ... 30 mV, balance the op-amp DA1. Then the bases of transistors VT5, VT6 are connected to the emitters VT3, VT4 and, after briefly turning on the power, they make sure that the UMZCH does not self-excite in this form. The AC noise and hum voltage with the short-circuited input must not exceed 1 mV. Next, a resistor with a resistance of 16 ohms with a power dissipation of 10 ... 15 W is connected to the UMZCH output, the UMZCH input is opened, a generator tuned to a frequency of 1 kHz is connected to it and, gradually increasing its signal until a voltage of 13,5 ... 14 V, check the symmetry of the limitation of the positive and negative half-waves of the sinusoid. The minimum (within the specified limits) constant voltage at the output of the amplifier is achieved, if necessary, by final balancing of the op-amp DA1. After that, you can begin to measure the main characteristics of the UMZCH by loading it with a rated load - a resistor with a resistance of 4 or 8 ohms. In more detail, the features of establishing UMZCH of this type are described in [XNUMX]. However, it should be taken into account that an attempt to establish, and even more accurately evaluate the parameters of the UMZCH, assembled without observing the above installation rules, without installing it in the place intended for it and without powering it from its own power supply, will not only not give the desired result, but it can also lead to failure of the output stage transistors. The adjustment of the UMZCH and the measurement of its characteristics should be started only after the completion of its design. The simplicity of the amplifier is only apparent. It should not be forgotten that transistors with maximum generation frequencies of 1 ... connections and loads of sufficient magnitude. The insignificant inductance of the emitter circuit wire, the parallel arrangement of the base and collector circuits over a considerable length of wires can cause self-excitation at high frequencies, which is extremely dangerous for the transistors of the terminal and pre-terminal stages. (However, this is true not only for the described device, but also for the UMZCH assembled according to any other scheme.) UMZCH characteristics are measured by well-known methods using appropriate measuring equipment. To measure individual parameters, the values of which are beyond the capabilities of serial measuring instruments (for example, small non-linear distortions), one can use the methods published in the journal "Radio" (see, for example, [4]). When measuring the harmonic coefficient and the relative level of noise and interference, one should be aware of possible interference from the power supply, television and radio transmitters, televisions and other radio devices due to poor shielding of the connecting wires, the UMZCH input and sensitive measuring instruments, as well as in the absence of their connection ungrounded enclosures with each other. Sometimes it is enough to rearrange the plug of the power cable of one of the devices or UMZCH in the outlet to get the wrong result. By the way, you should not use the method known from the old amateur radio practice to check the UMZCH by touching your finger to its input circuit. This can lead to such a level of high-frequency interference that the output transistors will fail. The considered scheme can be taken as a basis for creating UMZCH with different output power. To do this, you just need to change a number of elements of the UMZCH and the power supply. Some recommendations in this regard can be gleaned from the table.
When building an UMZCH with an output power of approximately 25 W, some of the elements can be excluded (see Fig. 3).
As you can see, instead of a resistor in the circuit of the non-inverting input of the op-amp DA1, connected to a common wire, a divider of resistors R1-R3 is used here, which made it possible to abandon the middle output of the secondary winding of the network transformer T1. This allows the use of transformers with a secondary winding voltage of 24...28 V and protects the speaker system from failure in the event of a breakdown of one of the final stage transistors. UMZCH according to the scheme in Fig. 3 can be mounted on the same PCB (see fig. 2). In this case, the holes for the terminals of the resistors R2, R5-R7 are left free, the resistors R8 and R9 are soldered directly into the power circuits of the op-amp DA1, for which wire jumpers are installed in the holes for the outputs of the emitters and collectors of transistors VT1, VT2. With an output power of less than 25 W, transistors of the KT805 and KT837 series with any letter indices can be used in the final stage. Establishment of UMZCH according to the scheme of fig. 3 does not differ from that described above. References:
Author: E.Gumelya
Machine for thinning flowers in gardens
02.05.2024 Advanced Infrared Microscope
02.05.2024 Air trap for insects
01.05.2024
▪ Graphene based infinite energy generator ▪ Rare earths from the ocean floor
▪ section of the site for the radio amateur-designer. Article selection ▪ article Pediatric surgery. Crib ▪ article by Elsholtz Patren. Legends, cultivation, methods of application ▪ article Wheel ointment. Simple recipes and tips ▪ article Extraordinary package. Focus Secret
Comments on the article: Schulbertino Lorenti This is not a very simple amplifier! Do not be tempted by its simplicity and high parameters. In the original version, this amplifier is not very stable !!!! For inexplicable reasons, the output transistors fly out. I don't recommend repeating...
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua | |||||||||||||||||||||
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page