|
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 Transistor power amplifier of the radio station of the first category. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / RF power amplifiers The widespread opinion that it is impossible or very difficult to build a broadband transistorized power amplifier for a first category radio station repels most shortwavers from this undertaking. The task set by the author was to show the possibility of building, describing the circuit and setting methods for a highly reliable transistor power amplifier providing an output power of at least 150 W with the output transistors in various unfavorable conditions using the example of an operating and in operation linear power amplifier. conditions, such as: operation for an unmatched load, cable breakage or short circuit in the antenna-feeder system, switching errors of band filters, overheating of the amplifier transistor cooling radiator, and more. When building an amplifier, preference was given to bipolar transistors for a number of reasons:
The main characteristics of the broadband power amplifier: - operating frequency range - 1,8 ... 30,0 MHz; Fig.1. Power amplifier circuit diagram The signal from the transceiver goes to the L-shaped link of the electronically controlled attenuator of the power amplifier protection circuit in case of mismatch with the load. The attenuator is built on powerful pin diodes VD5 and VD6. The control circuit is assembled on transistors VT1 - VT4, VT6. A characteristic feature of this circuit is to maintain a constant value of the total current flowing through the diodes VD5 and VD6. In the working state of the amplifier, the transistor VT2 of the attenuator control circuit is open, and VT3 is closed. A current of about 5 mA flows through the open pin diode VD120. The voltage drop across the resistor R9 is the blocking voltage for the second pin diode VD6. The maximum attenuation of the radio signal power in the series circuit C5, VD5, C9 of the L-shaped attenuator is 0dB. If there is a mismatch between the power amplifier and the load, the voltage generated by the reflectometer is fed through the diode VD15 of the "OR" circuit to the base of the transistor VT6 of the differential amplifier. There is a redistribution of the current flowing through the diodes VD5 and VD6, as a result of which the loss of the radio signal along the circuit C5, VD5, C9 increases up to 30 dB. The parallel circuit C7, VD6, R8 and C10 of the L-shaped attenuator serves to stabilize the input impedance of the power amplifier and ensures the transceiver's load resistance is constant. So, with a fully open pin diode VD6, the active component of the resistance of the circuit C7, VD6, R8 and C10 is 50 ohms. In this case, the resistor R8 dissipates all the power of the signal at the input of the amplifier. With the help of resistor R1, the threshold switching voltage of the electronically controlled attenuator is adjusted. The H1 LED is an indicator of the mismatch between the power amplifier and the load. The glow of the LED is pulsed. The glow frequency is 25 - 30 Hz, determined by the time constant of the discharge of the capacitor C12 through the resistor R17 and the input resistance of the transistor VT6. The push-pull power amplifier is made on transistors VT11 and VT12 of the KT957A type. The autonomous bias voltage of each power amplifier transistor is set using two stabilizers assembled on transistors VT7, VT9 and VT8, VT10. The presence of autonomous sources of the initial bias voltage of the output transistors operating in mode B makes it possible to eliminate the spread in the gain factors of the transistors and obtain a linear amplitude characteristic of the power amplifier. The adjustment of the initial bias voltage of the transistors is carried out by variable resistors R18 and R19. Stabilizers simultaneously carry out temperature stabilization of the quiescent current of the output transistors of the power amplifier. As temperature sensors, transistors VT7 and VT8 of the KT904A type are used, placed next to the KT957A transistors. The balancing transformer T1 with a transformation ratio of 4:1 matches the unbalanced 50-ohm input of the power amplifier with the input resistances of transistors VT11 and VT12, the active component of which is 1,3 ... 1,8 ohms. Transformer T2 provides power to the collector circuits of transistors VT11 and VT12, balancing the voltage shape on the collectors of transistors in order to reduce the level of even harmonics in the collector circuit, as well as creating a frequency-dependent negative feedback. Balancing transformer T3 with a transformation ratio of 1:3 provides a transition from the low output resistance of transistors to a single-ended output with a resistance of 50 ohms. Correction circuits R20, C20 and R21, C21 provide matching of the input impedances of the amplifier and a decrease in the gain at low frequencies. The circuit formed by the secondary winding of the transformer T 1 and capacitor C15; a circuit consisting of resistors R26 and R27 and a circuit formed by the secondary winding of the transformer T2 and capacitor C27; also, the circuit formed by the primary winding of the transformer TK and capacitor C36 provide an increase in the amplitude-frequency characteristic of the amplifier at high frequencies (20 - 30 MHz). The frequency response correction circuits of the power amplifier make it possible to obtain a power gain unevenness of less than 2 dB in the frequency range from 1,8 to 30 MHz. Diodes VD11, VD13 and VD12, VD14 are used to protect transistors VT11 and VT12 from overvoltage in the collector circuit. The reflectometer of the power amplifier protection circuit includes: a reflected wave sensor made on a current transformer T4, capacitors C43, C44 and a rectifier based on a VD17 diode; DC amplifier on transistors VT13, VT14 and the "OR" circuit on diodes VD15 and VD16. The variable resistor R37 sets the required threshold for the operation of the SWR protection circuit. The differential amplifier of the electronically controlled attenuator is powered by an unstabilized voltage of +18 V. The bias circuits of the output transistors of the power amplifier and the UPT of the reflectometer are powered by a voltage stabilizer made on the DA1 chip and the regulating transistor VT5. The maximum current consumption but value +12 V - no more than 0,5 A. The output voltage of the stabilizer is adjusted by resistor R15. The power supply of the collector circuit of the power amplifier consists of a full-wave rectifier assembled according to a bridge circuit on diodes VD7 ... VD10 and a compensation stabilizer on transistors VT15, VT16, VT17 and a DA2 chip, which has protection against overcurrent and K3. To obtain a current in the load up to 13 A, a parallel connection of two regulating transistors VT15 and VT16 of the 2T827A type with equalizing resistances in the emitter circuits was used. The amount of voltage drop across one of these resistors serves as the control voltage for the overcurrent protection circuit. The output voltage of the stabilizer is adjusted by a variable resistor R38. The voltage drop across the resistor R46 is used to control the current of the power amplifier with a RL1 microammeter with a scale of no more than 200 μA. LED H2 is used to indicate the overload mode of the power amplifier collector circuit power amplifier. LED H2 serves for the amplifier circuit, power. LED H2 goes out if the load current exceeds the threshold value. In order to increase the reliability of the amplifier, fuses for a current of 12 A and 25 A are included in the +0,5 V and +15 V circuits, respectively. To filter the harmonic components of the radio signal at the output of the power amplifier, six band low-pass filters of the 5th order (Fig. 2) with a Chebyshep characteristic are installed, having a maximum reflection coefficient in the passband of 10%, which corresponds to SWR < 1,2 and power loss - 0,2 .50 dB. Input and output load resistances XNUMX Ohm. The table shows the values of the filter elements and their cutoff frequencies (fcp).
Reactive power of filter capacitors - 200 VAr. It is permissible to connect identical capacitors in parallel with a smaller unit value of reactive power, but the total is not less than 200 VAr. Table 1
Here: d - winding length. D - outer diameter of the coil Diameter and type of wire PEV-2 1,2. For ranges 1.8; 3.5 and 7,0 MHz coils are solid wound. The coils are fixed with BF2 glue. The power amplifier is assembled on two printed circuit boards mounted on radiators for cooling the transistors of the amplifier. On the first printed circuit board, the power amplifier itself, an L-shaped attenuator, a protection circuit, and bias voltage stabilizers are assembled. The printed circuit board is mounted on a radiator, on which transistors VT11, VT12, VT7, VT8 and pin diodes VD5, VD6 are placed. The dimensions of the radiator are 120x250x60 mm. The height of the ribs is 45mm, the distance between them is 15mm. +12 V and +25 V voltage stabilizers are assembled on the second printed circuit board. The printed circuit board, regulating transistors VT5, VT15, VT16, diodes VD7 - VD10 and the DA2 microcircuit are installed on the second cooling radiator of the power amplifier. The dimensions of this radiator are 120x200x60 mm. The height of the ribs and the distance between them are the same as for the first radiator. Regulating transistors and rectifier diodes are installed on the radiator on electrically insulating spacers made of aluminum with an anodic oxidized insulating coating. Cooling radiators are the supporting structural elements of the power amplifier. So, the first radiator with output transistors, RF and IF connectors of the power amplifier is the rear wall of the chassis, and the second radiator acts as a side wall. Inside the chassis housing there are low-pass band filters with a biscuit range switch, electrolytic capacitors C3 and C39 and a power transformer with an overall power of at least 350 W (not shown in the electrical diagram). The following types of radio elements are used in the power amplifier: fixed resistors C2 - 33N, MLT, C5-1 b MB; variable resistors - SP3 or SP5; capacitors C5 - C10, C32, C34, C33, C35-KM-4, the rest - KM-5, KM-6 KT-3, K 10-17; electrolytic capacitors K50-6, K50-18; chokes L1, L2, L3, L4, L5, L10 - DM0,6 or similar. Inductors L6 - L9 are wound on an annular magnetic core made of 1000 NM material of size K18x8x5 and contain 7 turns of PEL-2 0,8 wire. Transformer T1 is made of three glued Sh-shaped closed magnetic cores of the M2000 HM brand, size Sh5x5. The primary winding contains 4 turns of MPO 0,35 wire, passed inside soldered brass rectangular frames, tightly inserted into the windows of the W-shaped magnetic circuit. Rectangular frames, interconnected on one side by a jumper, form a three-dimensional turn of the secondary winding of the transformer T1. Transformer T2 is made on a ring magnetic circuit of the brand 1000 NM, size K32 x 20 x 6. The transformer contains 7 turns of twisting from 8 wires of the brand PUL-2 0,8 with a pitch of one twist per centimeter. Four strand wires form the primary winding, the other four form the secondary winding of the transformer. The connection coil is made with MPO 0,35 wire, passed through the magnetic circuit. Transformer T3 is made similarly to transformer T1 from four glued W-shaped closed magnetic circuits of the M2000 NM brand, size Sh7x7. The primary winding of the transformer is a volume coil, the secondary is made of three turns of MPO 0,35 wire, threaded inside the volume coil. The current transformer T4 of the reflected wave sensor is made on an annular magnetic circuit of the M20V42 brand, size K20x10x5. The primary winding is a mounting wire passed through the magnetic circuit, the secondary winding contains 20 turns of PELSHO 0,15 wire. Setting up the power amplifier is done in the following order. First, all incoming devices are configured: stabilizers, reflectometer, differential amplifier, and others, then a comprehensive adjustment of the amplifier as a whole is carried out. Instruments are required for tuning: an avometer, an oscilloscope with an operating frequency band of up to 50 MHz, a spectrum analyzer or a measuring receiver with a frequency range of up to 80 - 100 MHz, an SWR meter, a load non-inductive resistor for power up to 100 - 200 W, a standard signal generator G4- 118 or all-band transceiver with at least 20 watts output power. The establishment of a power amplifier begins with an autonomous check of the operation of rectifiers and voltage stabilizers +12 V and +25 V. Variable resistors R15 and R38 set the voltage values required by the circuit. The +12 V voltage regulator is tested by connecting a 15 ohm load resistance to the emitter of the VT5 transistor, while the change in the output voltage of the regulator should be no more than 0,1 V, and the output ripple should not exceed 50 mV. Checking the operation of the +25 V voltage regulator, determining the current protection threshold is carried out when a load resistance of 1,5 - 4 Ohm is connected. The load is carried out in the form of a coil with taps made of nichrome wire with a diameter of 1 mm, wound on a ceramic frame with a step of 2 - 3 mm. The test of the stabilizer is carried out by placing the described load in a three-liter jar of cold water. The current value is controlled by an ammeter with a scale of at least 15 A. The stabilizer must operate stably at load currents up to 13 A. The threshold current value at which the output voltage of the stabilizer drops to 2 ... 3 V must be no more than 14 ... 14,5, XNUMX A. The current protection threshold (1e) can be adjusted by selecting resistors R41 and R42. The value of Ia can be determined by the formula Ia=1,4/R41=1,4/R42 The decrease in the output voltage of the stabilizer + 25V at the maximum load current should be no more than 1 V, and the magnitude of the ripple should not be more than 400mV. By choosing the value of the resistor R30, you can set the maximum temperature for heating the crystal of the DA2 chip installed "on a single heatsink of the power amplifier. When the heatsink temperature is above + 90 ° C, the thermal protection of the DA2 chip is activated, dropping the voltage at the output of the stabilizer to zero. The bias voltage stabilizers are adjusted with the bases of the output transistors VT11, VT12 disabled. During the tuning process, the possibility of adjusting the output voltage within 0,5 ... 0,65 V is checked at a maximum load current of up to 0,2 A. The adjustment of the stabilizers is completed by setting the minimum value of the output voltage. The adjustment of the reflected wave sensor is traditional and has been repeatedly described in the literature. UPT on transistors VT13, VT14 provides the formation of a voltage on the VT13 collector, equal to + (0-0,7) V in the absence and + (10 - 11,5) V in the presence of a load mismatch. Resistor R37 sets the threshold for the operation of the protection circuit according to the load SWR value of more than 3. The operation of the differential amplifier, which is the control circuit of the L-shaped attenuator on pin diodes, is checked when a constant voltage Uk, varying from 6 to 0V, is applied to the input of the "OR" circuit (socket XS12). At Uk \u0d 2 V, the voltage on the VT17 collector should be +3 V, and on the VT0 collector - 9 V. The voltage drop across the resistor R10 must be at least 7 V. At Uk \u1d 2B, by adjusting the resistor R 3, the output transistors VT1, VT2 are switched differential amplifier and the glow of LED H3. The interval for changing Uk, at which the switching of transistors VT0,7 and VT1, should be no more than 51V. Checking the correct operation of the pin-diode attenuator is carried out when the RF signal of the GSS or transceiver is sent to the XS1 input and the RF signal is measured by an oscilloscope on a load resistor with a resistance of 0 ohms, which is switched on instead of the primary winding of the transformer TXNUMX. With Uk=XNUMX V, the RF voltage across the load resistor must be the same as at the input XS1 over the entire operating frequency range of the amplifier and the power of the GSS or transceiver is not more than 20 watts. With Uk=10 V and all other conditions being equal, the RF voltage across the load resistor should be 30 or more times less than at the XS1 input. Before setting up the power amplifier, the feedback circuit, consisting of R26, C25, R27, C26 and the connection coil on the balancing transformer T2, must be open. The tuning of the power amplifier should be carried out with a permanently switched on load with a resistance of 50 ohms, which can be performed as described in [2]. In order to protect high-power transistors, it is recommended to install a 5A fuse the first time you turn on the power amplifier. The initial current of transistors VT11, VT12 of the power amplifier is set first by resistor R18 to a value of 150 ... 200 mA, then by resistor R19 the total current of the collector circuit of the amplifier is increased to 300 ... 400 mA. The correctness of turning on the communication loop is checked for the stability of the power amplifier to RF excitation when a signal with a power of not more than 1 - 0,5 W is applied to the XS1,0 input. When the amplifier is excited, which manifests itself in a sharp increase in the collector current with a smooth increase in the input signal, the ends of the connection coil of the transformer T2 are reversed. With complex tuning of the amplifier, it is desirable to use the GSS G4-118 as a signal generator, the maximum output power of which is 3 W, and the operating frequency range is 0,1 ... 30 MHz. Applying an amplitude-modulated GSS signal to the input of the power amplifier with a modulation depth of at least 50% and an amplitude of not more than 10 V, variable resistors R18 and R 19 achieve a symmetrical shape of the signal envelope observed on the screen of an oscilloscope connected to the load. During this adjustment, it is necessary to control the initial current of the collector circuit of the power amplifier, which should not exceed 300 ... 400 mA. Capacitors C15, C27 and C36 achieve an increase in the frequency response of the power amplifier at frequencies of 25 ... 30 MHz. The control of the power level of the harmonic components of the output signal of the amplifier, the presence of high-frequency or low-frequency parasitic modulation is carried out using a spectrum analyzer or a measuring receiver. If parasitic modulation occurs in the amplifier, it is necessary to increase the blocking capacitors in the collector and base circuits of the output transistors VT11 and VT12. The final check of the power amplifier operation is carried out together with the transceiver connected to the RA input, when measuring the voltage at the load on all amateur bands. Since the output power of the amplifier in this case will reach 200 W, and the tests can be lengthy, forced airflow to the radiator of the power amplifier is required, which is mandatory for long-term operation. Preliminary tuning of the amplifier protection system is carried out by adjusting the resistor R37 until the LED H1 glows at an output power level of not more than 30 - 40 W and an amplifier load resistance of 200 Ohms (SWR-4). By disconnecting the load or closing it, the operation of the amplifier protection system is checked. With the correct operation of the protection system, its final fine-tuning is carried out at the rated power of the amplifier. It should be noted that by reducing the output power of the amplifier, it is possible to achieve its normal operation for a strongly mismatched load. The measurement of the main parameters of a tuned power amplifier is carried out with the included low-pass band filters, the setting of which consists in checking the compliance of the cutoff frequencies with the values given in Table 1. Literature 1. Zavrazhnov Yu. et al. Powerful high-frequency transistors. - M.: Radio and communication, 1985. Author: V. Usov, Novosibirsk; Publication: N. Bolshakov, rf.atnn.ru
Artificial leather for touch emulation
15.04.2024 Petgugu Global cat litter
15.04.2024 The attractiveness of caring men
14.04.2024
▪ Flexible battery recharged by human sweat ▪ NXP Unveils Ultra-Compact, High-Precision MEMS Frequency Synthesizer ▪ Efficient organic solar panels
▪ section of the site Stories from the life of radio amateurs. Selection of articles ▪ article Noble crook. Popular expression ▪ article What conditions will an astronaut find on Venus? Detailed answer ▪ article Dentist. Standard instruction on labor protection
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