Power amplifier on IRF630 for HF radio station. Field-effect transistor radio receivers Kv high-voltage mind on field effect transistors

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1 33 POWERFUL AMPLIFIER ON 4 FIELD TRANSISTORS output power with minimal losses when summing the output signals. To obtain high values ​​of output power, it is possible to connect two or more field-effect transistors MRF150 from Motorola in parallel. This switching method is practically not used for bipolar transistors due to their low input impedance. In a common-source circuit, high-power FETs have approximately a factor of higher input resistance than a bipolar transistor of comparable power in a common-emitter circuit. The value of the output impedance depends on the supply voltage and output power level. The number of transistors connected in parallel is limited by physical factors rather than electrical ones. The total inductance of the transistor leads is the most significant factor limiting the maximum operating frequency. The influence of the inductance of the outputs increases with a decrease in the supply voltage and an increase in the output power. Since the minimum distance between transistors is limited by the size of their packages, a practical improvement is to reduce the size of the transistors. For more high frequencies the inductance of transistor leads can be used as part of a distributed circuit, but this severely limits the operating frequency range. Such circuits are widely used in microwave devices based on bipolar transistors. When connecting high-power MOSFETs in parallel, another factor must be taken into account. important aspect. If the unity gain frequency (f) of the transistor is high enough, then the amplifier can turn into an oscillator, the resonant system of which will be formed by the inductances of the gate leads and the drain-source capacitances of the transistors. Positive Feedback is carried out through the passage capacity of the drain-gate. The resulting phase shift of 360 occurs at frequencies typically above the operating range of the amplifier. Thus, the oscillations that have arisen may be absent at the output of the RA, but have a significant amplitude at the drains of the transistors. Generation can be eliminated by reducing to the minimum possible values ​​of the inductance in the gate circuit, which consists of the inductances of the outputs of the isolating capacitors C7...C10 (Fig. 1) and the outputs of the transistor gates. The use of low-resistance non-inductive resistors R15 ... R18 does not reduce the gain in the operating frequency range and allows you to achieve better stability of the RA. Description of the principle electrical circuit Figure 1 shows a complete power amplifier circuit for field effect transistors. The supply voltage can be V and depends on the linearity requirements of the device. The bias voltage is set for each transistor separately, so there is no need to select transistors according to the cutoff voltage value. The power gain of MOS transistors is significantly higher. one

4 36 AUGUST Pic. 6 ry Curie. On the other hand, it is rather difficult to find magnetic circuits with low µ i and large cross-sectional areas. To achieve the minimum required inductance for a frequency of 2 MHz, two line transformers are connected in series. Both have a resistance ratio of 9:1. It is possible to apply a parallel connection of the secondary windings of the transformer, while doubling the number of turns in each winding. C11 must be designed to carry large amounts of reactive current through it. Structurally, C11 is fixed directly across the turn of the primary winding of the transformer. Application parallel connection ceramic or mica capacitors with lower capacitance values ​​are not recommended. Design features Due to the proximity of four MOSFETs, it was not possible to provide effective grounding at high frequency, as a result of which the gain decreases by 1.0...1.5 dB at a frequency of 30 MHz (Fig. 4). You can improve the situation by connecting a conductive strip to all grounded terminals of the transistors. Another method is to place petals under the screws holding the transistors, which are soldered to the nearest ground point. In this case, the radiator is used as a high-frequency ground. Although the value of the 3rd order intermodulation distortion factor is not very high (Fig. 4), for the 5th order intermodulation products, this factor is better than -30 dB at all frequencies. The rejection of 9th and higher intermodulation products can also be expected to be -50 to -60 dB. It is also seen that the intermodulation coefficient remains constant with decreasing output power, in contrast to the PA circuits made on bipolar transistors, where an increase in intermodulation distortion is observed. The content of the harmonic components in the spectrum of the output signal of the amplifier depends very much, as in other similar balanced devices, on the balancing of the shoulders of the push-pull stage. Worst of all is the situation in low frequencies, where the suppression of the second harmonic is dB. Suppression of the 3rd harmonic component of the output signal at a carrier frequency of 6.0...8.0 MHz is 12 dB. In this case, it is necessary to use signal harmonic filters, the description and design of which can be found in the literature. The amplifier remains stable with a 3:1 output mismatch, as well as with a decrease in the supply voltage. In common-source MOSFETs, the feedback gain is several times higher than that of common-emitter bipolar transistors. As a result, a properly designed MOSFET amplifier is more stable, especially under varying load conditions. Particular attention should be paid to the design of the heatsink, which must ensure efficient heat removal from the transistors. With an output power of W, it is necessary to use cooling radiators made of a material with high thermal conductivity, such as copper. It is possible to use a combined radiator, which has copper inserts in the places where transistors are attached, and the rest is made of aluminum alloy. The fastening points of transistors must have a smooth (polished) surface, which is desirable to be lubricated with a heat-conducting grease. Figures 5 and 6 show the printed circuit boards of the amplifier. Adapted from Motorola RF Application Reports.

6 38 AUGUST resistance to create partial automatic displacement. The operating frequency at which generator lamps can operate reliably should not exceed the value specified in the handbook as the limit, as this leads to the following undesirable phenomena. 1. The temperature regime of the lamp is violated due to an increase in high-frequency losses at the electrodes, the bulb, and the electrode leads. Overheating of the mesh and glass-to-metal junctions can lead to the formation of local mechanical tensions, microcracks, which causes a loss of vacuum and failure of the lamp. The total amount of heat released in the glass-to-metal junctions and at the electrode terminals is proportional to the frequency to the power of 2.5 and the instantaneous value of the square of the potential difference between the anode and the grid. 2. The output parameters of the lamps (power and efficiency) are reduced due to an increase in the angle of passage of electrons. 3. The risk of self-excitation of lamps increases due to an increase in intra-lamp connections. Required operating temperature generator lamps high power and some types of generator lamps of medium power is achieved using one of three types of forced cooling air, water and evaporative. Air cooling is the easiest to operate and allows you to reduce the anode temperature to 250 C. When using generator lamps with this type of cooling, the following recommendations must be observed. The cooling air must be dry and clean. If water or oil settles on the glass in the air duct, it may damage the lamp. The amount of air supplied for cooling must not be less than the norm given in the manual for each type of lamp. The air flow for cooling the glass bulb of the lamp and the leg must be directed in such a way that the temperature of the glass does not exceed 150 C anywhere and there are no zones with sharp temperature drops on the glass surface. When supplying air for cooling from fans located in the immediate vicinity of the lamps, special measures should be taken to protect them from vibrations, for example, air ducts should be connected through flexible connections, soft rubber or silk hoses, etc. Water cooling lamps in some cases allows you to slightly increase the power dissipated by the anode, since this type of cooling can reduce the temperature of the anode to 120 C. Powerful water-cooled generator lamps are immersed in a tank with running cooling water. Water consumption per 1 kW of power removed from the anode surface depends on the power of the lamp, its design and the design of the tank and varies within l / min. When using water-cooled generator lamps, the following rules must be applied. Cooling water must be clean and free from mineral impurities. It is recommended to cool the anodes with distilled water. Water with a hardness exceeding 0.17 g/l and having a resistance of less than 4 kΩ/cm3 should not be used. For uniform cooling of the anodes, the water flow washing the anode must be directed from the bottom up. In this case, it is necessary that the density of the water flow around the entire working surface of the anode be uniform and no air cushion is formed. The inflow and removal of water from the grounded section of the pipeline to the cooled parts of the lamp, which are energized with respect to earth, must be carried out through pipelines of insulating material of the required length so that the water column placed in them has a sufficiently high resistance and leakage current was minimal. The length of the insulated pipeline is usually chosen depending on the resistivity of water at the rate of 0.3 ... 0.6 m per 1 kV of voltage. The amount of water supplied for cooling must be sufficient and in accordance with the norms indicated in the manual for each type of lamp. To avoid intense scale formation, the outlet water temperature should not exceed 70 C. Evaporative cooling differs from water cooling in that the heat released by the anode goes mainly to water evaporation. This type of cooling is more economical, since the conversion of water into the vapor phase requires more heat than heating it from normal temperature to boiling. To increase the cooling surface and improve its wettability with water, the anode radiator of an evaporatively cooled lamp has conical teeth. In the depressions between the teeth, the anode surface temperature has the highest value, and the water that has got there turns into steam bubbles, which are ejected from the depression, giving way to water, etc. This type of cooling makes it possible to remove up to 500 W of power from 1 cm 2 of the anode surface. With a further increase in power, a vapor film is formed and heat transfer deteriorates. Other requirements for the operation of generator lamps with evaporative cooling are similar to the requirements for the operation of generator lamps with water cooling. In addition to the above features of the use of generator lamps, the following recommendations for the operation of generator lamps must also be observed. 1. Radio devices in which generator lamps are used must provide special protection devices for generator lamps in case of emergency conditions of the equipment (lack of cooling, significant excess of permissible currents, etc.). It should be provided that in the absence of at least one of the types of cooling, the supply voltages will be disconnected and it would be impossible to turn them on. The cooling system should use hydraulic contacts that respond not to pressure changes, but to changes in coolant flow. In the circuits of the anode and grids of powerful generator lamps, devices must be provided that turn off the supply voltage of the electrodes when the maximum current values ​​\u200b\u200bare exceeded by 2.5 ... 3 times or limit the discharge current. The following devices can be used as such devices: - high-speed relays (operating time no more than 100 ms) that cause the corresponding power source to turn off or break the primary winding of the supply transformer (for industrial-type installations with a power of no more than kW); - shunting of lamps during breakdown by gas-discharge or other devices with a small internal resistance; - inclusion in the anode circuit of a limiting resistance that reduces the discharge current.

7 39 To prevent the destruction of a powerful generator lamp (with a power of more than 15 kW) when a discharge occurs in it, in the case of using a power supply with a capacitive filter, it is necessary to install high-speed electronic protection in parallel with the anode circuit. In order to avoid overloading the control and screening grids, the protection circuit must provide for the simultaneous removal of the excitation voltage and the supply voltage of the screening grid when the anode voltage is turned off. It is also necessary to provide for changes in the modes of the lamps of the preliminary stages after the operation of the protection of the output stage. 2. Turning the generator lamp into operation and applying voltage to the electrodes must be carried out in the following sequence: - after connecting all the electrodes, all types of cooling of the lamp and equipment elements are turned on; - the heating voltage is turned on, while it is necessary to control that the starting current does not exceed the value specified in the reference book, or does not exceed more than one and a half times the nominal value (for generator lamps of medium and high power); - turn on the voltage that locks the lamp; - the voltage of the anode and the screening grid of the lamp is turned on (smoothly or in steps in accordance with the operating instructions), while turning on the voltage of the screening grid earlier than the anode is strictly prohibited; - alternating voltages are switched on (excitation or modulation), and direct voltages are brought to nominal values. Turn off the lamp in reverse order. In order to ensure that, when the excitation is removed, the constant voltages do not exceed the maximum permissible values, it is recommended to first reduce them if necessary. Forced cooling of all types for general lamps should stop only minutes after the filament voltage is turned off, unless another time is indicated in the technical documentation for a particular type of lamp. It is forbidden to turn on the high voltage of the anode and the screen grid when the filament voltage is turned on, as this can disable the lamp due to breakdown and destruction of the cathode. 3. To improve the vacuum and restore the electrical strength of generator lamps, in some cases, special training is used, which must be carried out when the lamp is first turned on and during long breaks (up to 3 months) in operation, as well as periodically (1 time in 3 months) when storage, if indicated in the passport or label on the lamp. Training is usually carried out in a device in which the lamp works. The lamp is installed in the circuit, and filament and bias voltages are applied to it in the usual sequence. In this mode, the lamp is kept for 30 minutes. Then, voltages are applied to the remaining electrodes, equal to approximately half of their nominal value, on the basis that the power dissipated on the anode and other electrodes is 0.4 ... 0.5 power in the nominal mode. After the expiration of minutes (depending on the dimensions of the internal fittings of the lamp), the voltage of the anode and other electrodes is gradually or in steps brought to the nominal value (with a minute exposure at each step) and maintained for at least 30 minutes. When breakdowns occur, the anode voltage decreases until they stop and is maintained in this mode for min, after which it rises again. Such training is carried out until breakdowns disappear at full operating anode voltage. To protect the lamp from damage as a result of breakdowns during training, a resistance is usually included in the anode circuit of the lamp, several times higher than the usual limiting resistance. 4. The working position of generator lamps, as a rule, should be vertical, and for generator lamps of medium and higher power, this rule is mandatory. 5. In cases where the lamp is connected to the generator circuit when working with lamps in the VHF and HF bands, it is necessary to establish a reliable and uniform electrical contact along the perimeter of the outer part of the electrodes and maintain alignment that excludes radial stress and bending forces in the leads and fastening elements of the lamps. In addition, it is necessary to use such an anode circuit design that would exclude the occurrence of an increased concentration of high-frequency field lines in one place in the cylinder dielectric, since local overheating that appears in these cases can cause it to soften and puncture (vacuum violation). Poor contact with the leads due to overheating of the glass-to-metal junctions can lead to the same result. Fastening generator lamps of medium and high power in the equipment should be carried out only by the anode flange, tank or radiator. It is forbidden to use the rest of the lamp leads for this purpose, since their designs, as a rule, are not designed for impact. heavy loads. 6. The design of the elements in direct contact with the terminals of the lamp should be carried out in such a way as to ensure reliable electrical and thermal contacts. 7. When operating generator lamps, this is especially true powerful lamps, it should be remembered that the mode in which the filament voltage is applied to the lamp without current selection is more difficult for the cathode compared to the normal operating mode. Therefore, during breaks in the operation of the equipment from 30 minutes to 2 hours, it is recommended to reduce the filament voltage by % of the nominal value. For longer breaks in operation, the generator lamp should be put into operation gradually, i.e. run a training cycle. 8. If it is necessary to use generator lamps designed for continuous operation in a pulsed mode, one can proceed from the following considerations: in the range of pulse durations from 0.1 μs to 1 ms, the electrical operation of the lamps should be recalculated based on the inadmissibility of exceeding the average powers dissipated by electrodes. With a pulse duration of more than 1 ms, the recalculation can only be performed taking into account thermal heating during the passage of the pulse. An increase in constant voltages on the electrodes of generator lamps intended for operation in a continuous mode, relative to the operation of the values ​​in the case of their use in the mode with a pulsed grid modulation, is not allowed. 9. When using pulse generator and modulating lamps, it is strictly forbidden to use them in pulsed modes exceeding those indicated in the reference book as limiting ones, for example, reducing the duty cycle or increasing the pulse duration at the maximum anode current.

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The appearance of the amplifier is shown in fig.

His diagram is in Fig.

The amplified signal applied to the XW1 connector enters through an attenuator from resistors R1-R3 and a transformer T1 to the gates of field-effect transistors VT1 and VT2. The scheme used provides good symmetry of the gate signals. With the help of a tuning resistor R7, a constant bias is set on the gates of the transistors, which provides a quiescent current in the circuit of their drains (in the absence of an alternating voltage at the gates) of about 80 ... 100 mA. The total quiescent current, which can be measured by connecting the ammeter to the break in the power wire marked in the diagram with a cross, is twice as much - 160 ... 200 mA. At maximum output power, the current here increases to approximately 4 A.

The resistive attenuator serves to better match the amplifier with the signal source and dampen the excess power of this signal. The values ​​of the resistors R1-R3 indicated in the diagram are optimal when working from the Kajman transceiver used by the QRP author with an output power of 2 watts. In other cases, these resistors may have to be re-selected. Transformer T1 is wound with double-folded insulated copper wire with a diameter of 0.55 mm on a ring ferrite magnetic circuit FT-82-43. Its windings contain 11 turns.

The amplifier uses an original unit for summing the output signals of the arms of a push-pull amplifier, assembled on a T2 transformer, which also serves to match the amplifier with a 50-ohm load. Isolating capacitors C6-C9 do not allow the DC component of the transistor drain current to pass into the transformer windings.

This saves its magnetic circuit from unwanted bias, which can result in increased non-linear distortion of the output signal, insufficient power, and an increased level of harmonics at the output. The design and number of turns of the windings of the transformer T2 are the same as those of T1. But its magnetic circuit is glued from two ferrite rings FT-114-43, and the diameter of the winding wire is 1 mm.

It is impossible to get rid of the DC component of the current flowing in the windings of the inductors L1, L2, L4, L5. The danger of saturation is eliminated here in a different way - by using open rod rather than closed ring magnetic circuits. Chokes L1 and L2 have 25 turns of wire 1 mm in diameter, wound on a ferrite rod with a diameter of 8 mm, and chokes L4 and L5 - 20 turns of the same wire on a rod 5 mm in diameter. The author, unfortunately, does not report the magnetic permeability of the ferrite rods, saying only that it must be high.

Coil L3 is wound on an annular magnetic circuit T68-2 made of carbonyl iron. It contains 19 turns of wire with a diameter of 0.9 mm.

The printed circuit board of the amplifier is shown in fig.

foil on her reverse side saved completely. With several wire jumpers passed through specially drilled holes, it is connected to a common printed conductor on the front side. Windows are made for the cases of field-effect transistors in the board, and the transistors themselves are mounted on heat sinks. Transistors must be selected with a parameter spread of no more than 10%. If this cannot be done, the wire jumpers shown in the printed circuit board figure in the source circuits of the transistors must be replaced with resistors with a resistance of 0.22 ohms and a power of 2 watts. When a sinusoidal signal of 9 volts was applied to the input of the amplifier, a power of 55 watts was obtained at its load of 50 ohms.

According to the radio magazine

(article updated on 02/07/2016)

UT5UUV Andrey Moshensky.

Amplifier "Gene"

Transistor Power Amplifier

with transformerless power supply

from the network 220 (230) V.

The idea of ​​creating a powerful, lightweight and cheap high-power amplifier has been relevant since the dawn of radio communications. Many beautiful tube and transistor designs have been developed over the last century.

But there are still disputes about the superiority of solid-state or high-power electronic-vacuum amplifying equipment ...

In the era of switching power supplies, the issue of weight and size parameters of secondary power supplies is not so acute, but, having actually eliminated it, using an industrial network voltage rectifier, you still get a win.

It seems tempting to use modern high-voltage switching transistors in the power amplifier of a radio station, using hundreds of volts of direct current to power it.

Your attention is invited to the design of a power amplifier for the "lower" HF bands with a power of at least 200 watts with a transformerless supply, built according to a push-pull circuit on high-voltage field-effect transistors. The main advantage over analogues is weight and size indicators, low cost components, stability in work.

The main idea is the use of active elements - transistors with a drain-source cutoff voltage of 800V (600V) designed to work in pulsed secondary power supplies. Field-effect transistors IRFPE30, IRFPE40, IRFPE50 manufactured by International Rectifier were chosen as amplifying elements. Price of products 2 (two) dollars. USA. Slightly lose to them in terms of cutoff frequency, providing operation only in the range of 160m, 2SK1692 manufactured by Toshiba. Fans of amplifiers based on bipolar transistors can experiment with 600-800 volt BU2508, MJE13009 and other similar ones.

The method for calculating power amplifiers and ShPTL is given in the handbook of the shortwave radio amateur S.G. Bunina L.P. Yaylenko. 1984

Winding data of transformers are given below. The input ShPTL TR1 is made on a K16-K20 ring core made of M1000-2000NM(NN) ferrite. Number of turns 5 turns in 3 wires. The output ShPTL TR2 is made on a K32-K40 ring core made of M1000-2000NM(NN) ferrite. Number of turns 6 turns in 5 wires. The wire for winding is recommended by MGTF-035.

It is possible to make an output SHPTL in the form of binoculars, which will have a good effect on operation in the “upper” part of the HF range, although the transistors shown do not function there due to the rise and fall of the current. Such a transformer can be made of 2 columns of 10 (!) K16 rings made of M1000-2000 material. All windings according to the scheme are one turn.

The measurement data of the parameters of transformers are given in the tables. The input ShPTLs are loaded on input resistors (the author has 5.6 Ohms instead of the calculated ones), connected in parallel with the gate-source capacitance, plus the capacitance due to the Miller effect. Transistors IRFPE50. The output ShPTLs were loaded from the drain side to a non-inductive 820 Ohm resistor. Vector analyzer АА-200 manufactured by RigExpert. The overestimated SWR can be explained by insufficiently dense laying of the turns of the transformers on the magnetic circuit, a noticeable discrepancy between the wave resistance of the MGTF-0.35 line required in each particular case. However, there are no problems on 160, 80 and 40 meters.

Fig 1. Electrical circuit diagram of the amplifier.

Power supply bridge rectifier 1000V 6A, loaded on a capacitor 470.0 to 400V.

Do not forget about safety standards, the quality of radiators and mica gaskets.

Fig 2. Schematic diagram of a direct current source.

Fig 3. Photo of the amplifier with the cover removed.

Table 1. Parameters of ShPTL TR1, made on the K16 ring.

Table 2. Parameters of ShPTL TR2 made on K40 ring.

Fig 4. Output ShPTL on the K40 ring.

Table 3 Parameters of ShPTL TR2, "binocular" design.

Fig 5. The output ShPTL of the "binocular" design.

With the parallel connection of transistors and the recalculation of ShPTL, the power can be significantly increased. For example, for 4 pcs. IRFPE50 (2 in the arm), SHPTL output 1:1:1 and power supply 310V on the drains, the output power of 1kW is easily obtained. With such a configuration, the efficiency of the SHPTL is especially high; the method for performing the SHPTL has been repeatedly described.

The author's version of the amplifier on two IRFPE50, shown in the photos above in the text, works great on the bands of 160 and 80 m. The power is 200 watts at a load of 50 ohms with an input power of about 1 watt. Switching and bypass circuits are not shown and depend on your wishes. Please pay attention to the absence of output filters in the description, the operation of the amplifier without which is unacceptable.

Andrey Moshensky

Addendum (02/07/2016):
Dear readers! By popular demand, with the permission of the Author and the editors, I also post a photo of the new design of the Jin amplifier.

Class A power amplifiers are rarely used. Basically, these are amplifiers of HF radio receivers with a large overload capacity. A practical diagram of such an amplifier is shown in Fig. 1. The input L1C1 circuit and the output L2C2 circuit are usually synchronously tuned and tuned to the frequency of the input signal.

Equivalent resistance Re of the output circuit Re=P2p2/(RL+Rн"), where р=Sqr(L2/C2), Rн" - load resistance introduced into the oscillatory circuit; RL - active loss resistance; P2 - circuit inclusion coefficient. The value of Rn "=Rn / n22, where n2 is the transformation ratio.

The quality factor of the output circuit when it is fully turned on Q=ReRi/(Re+Ri)2pfoL2 decreases due to the shunting effect of the output resistance of the transistor Ri. For powerful MIS transistors, Ri is small and usually does not exceed tens of kilo-ohms. Therefore, to increase Q2, an incomplete inclusion of the circuit is used.

The bandwidth of the output circuit is 2Df2=fo2/Q2, and the resonance frequency is fo2=l/2pSqr(L2C2). In the HF band, such an amplifier can provide up to several tens of Ki. An important indicator of the amplifier is the noise level. The noise properties of powerful MIS transistors are considered in the works.

Figure 2 shows a practical circuit of the PA on a powerful MIS transistor KP901A. Since the task of obtaining a small L2C2 frequency band was not set, the circuit is connected directly to the drain circuit and is shunted by the load Rн=50 Ohm. In class A, the amplifier had Ku=5(Ku=SRn) and Kp>20 at f=30 MHz. When switching to the nonlinear mode, the output power reached 10 W.

A two-stage PA (Fig. 3) is made on transistors KP902A and KP901A. The first stage operates in class A, the second in class B. To ensure class B, it is enough to exclude the divider from the gate value of the second transistor. The amplifier uses a broadband communication circuit between stages. At a frequency of 30 MHz, the amplifier provided Pout = 10 W with Ki> 15 and Kp> 100.

The narrow-band amplifier in Fig. 4 is designed to operate in the frequency range 144 ... 146 MHz. It provides a power gain of 12 dB, a noise level of 2.4 dB and an intermodulation distortion level of no more than 30 dB.

A resonant amplifier based on a powerful MIS transistor 2NS235B (Fig. 5) at a frequency of 700 MHz provides Pout = 17 W with an efficiency of 40 ... 45%.

The amplifier in Fig. 6 contains a neutralization circuit that reduces the level of backtalk to a level of -50 dB. At a frequency of 50 MHz, the amplifier has an increase in power of 18 dB, a noise level of 2.4 dB and an output power of up to 1 watt.

In the patented circuit in Fig. 7 (US Pat. No. 3.919563) at a frequency of 70 MHz, a real efficiency of 90% is achieved with an output power of 5 W at a frequency of 70 MHz. The quality factor of the output circuit is equal to 3.

Figure 8 shows a diagram of a three-stage PA based on domestic powerful MIS transistors KP905B, KP907B and KP909B.

The amplifier delivers 30W of power at 300MHz. The first two stages use resonant U-shaped matching circuits, and the output stage uses an L-shaped circuit at the input and a U-shaped circuit at the output. The dependences of efficiency and Pout on Uc and Pout and Kp on Pin, obtained experimentally and by calculation, are shown in Fig. 9.

When using PA in AM radio transmitters (with amplitude modulation), there are difficulties associated with ensuring the linearity of the modulation characteristic, i.e., the dependence of Pout on the amplitude of the input signal. They are aggravated when using sharply non-linear modes of operation, such as class C. Figure 10 shows a diagram of a HF radio transmitter with amplitude modulation. Transmitter power 10.8 W when using a powerful UMOS transistor VMP4. Modulation is carried out by changing the bias voltage at the gate.

To reduce the non-linearity of the modulation characteristic (curve 1 in Fig. 11), the transmitter uses envelope feedback. To do this, the output AM voltage is rectified and the resulting low-frequency signal is used to create an OOS. Modulation response 2 in Fig. 10 illustrates a significant improvement in linearity.

Figure 12 shows circuit diagram key PA with an output rated power of 10 W and an operating frequency of 2.7 MHz. The amplifier is made on transistors KP902, KP904. The efficiency of the amplifier at rated output power is 72%, the power gain is about 33 dB. The amplifier is excited by logic element K133LB, supply voltage 27 V, crest factor of the output stage drain voltage is 2.9. With an appropriate rearrangement of the communication circuits, an amplifier with given parameters worked in the range of 1.6 ... 8.1 MHz.

To provide a given power at higher frequencies, it is necessary to increase the exciter power.

Structurally, both PAs were assembled on printed circuit boards using standard 100x150x20 mm radiators, which is explained by the standard dimensions of the PA unit in radio transmitters. Inductance coils in communication circuits are cylindrical on ferrite rods of the VCh-30 brand with a diameter of 16. The quality factor of the inductance coils is Q=150.

Standard chokes with an inductance of 600 μH were used as blocking chokes in the power supply circuits of the drain of transistors of a one-watt amplifier and the preliminary stage of a 10-watt amplifier. The power inductor in the drain circuit of the KP904 transistor is on a ferrite ring, its inductance is 100 μH.

Figure 13 shows a schematic diagram of a key PA with a rated output power Pout = 100 W, designed for use in unattended HF radio transmitters. The amplifier contains a pre-amplification stage, reverse on two KP907 transistors. At the VTI input, a matching U-shaped circuit С1L1С2СЗ is included.

The final stage is assembled with six KP904A transistors. This number of transistors was chosen for reasons of increasing efficiency. Instead of KP904B transistors, you can also turn on six KP909 transistors or three more powerful KP913. The optimal key mode of the drain circuit is provided by a forming circuit containing elements C14, C15, C16, L7.

The amplifier has a total efficiency = 62%. In this case, the electronic efficiency of the output stage is about 70%. The bridge circuit for switching on the transistors of the preliminary stage was used to maintain the efficiency of the amplifier (albeit with degraded parameters) in case of failure of the output transistor. For the same purpose, individual fuses are included in the sources of powerful transistors, the purpose of which is to turn off the faulty transistor. If, as a result of its breakdown, a mode occurs in the transistor line that is close to the mode short circuit, this renders the amplifier unusable.

Parallel connection of powerful MIS PT does not create additional difficulties in the calculation and tuning of the PA. The decrease in the efficiency of the amplifier compared to an amplifier of similar design (see Fig. 12) is mainly due to the use of power transistors in a 100-W amplifier. With a decrease in the output power level to 50 W, the efficiency of the amplifier increases to 85%, and the electronic efficiency to 90%. The values ​​of the parameters of the elements shown in Fig. 13 correspond to a frequency of 2.9 MHz.

The peak voltage factor at the drains of the KP904 transistors is 2.8, and the transistors themselves operate in a mode close to optimal. The crest factor of the drain voltage in the cascades on KP907 transistors is P = 2.1. The transistor operates in the key mode, however, the optimal mode is not ensured, since the optimal key mode for these transistors at Uc=27 V and cutoff angle φ=90° would be dangerous due to a significant crest factor at which the drain voltage could exceed the maximum allowable voltage equal to 60 V for the KP907 transistor.

Figure 14, a shows the experimental and calculated curves illustrating the dependences of the efficiency, Pout and he on the drain current cutoff angle. The figure shows a good approximation of the calculated data to the experimental ones. It should be noted that the range of possible cutoff angles is rather narrow. An increase in the cutoff angles is prevented by a rapid increase in the crest factor of the drain voltage, and a decrease is prevented by an increase in the required excitation voltage, which quite soon begins to exceed Uz add together with the bias voltage Uz. Of course, with a decrease in the level Pvt, the range possible changes drain current cutoff angles widens.

The amplifier is made on printed circuit board. As a heat sink, a radiator with dimensions of 130X130X50 mm is used. In the power supply circuits of KP907 transistors, standard DM-01 chokes with an inductance of 280 μH are used. Addition bridge chokes are wound on ferrite rings VK-30 dia.=26. The inductor in the power supply circuit of the output stage is wound on a ferrite ring VCh-30 dia. = 30. The inductor in the connection circuit of the output stage with the load is air, wound with silver-plated wire, diameter = 2.5, coil diameter 30 mm, L = 80 nH.

The temperature dependences of the output power Pout and the efficiency of the key PA with an output power of 100 W are shown in Fig. 14b. It can be seen from the above dependences that in the range of -60...+60°C, the PA input power changes by no more than ±10%. The temperature also has a slight effect on the efficiency, which varies by ±5% in the indicated range. In this case, there is a drop in output power and efficiency with increasing temperature, associated with a decrease in slope 5 with increasing temperature. In the usual temperature range of -60 ... +60 ° C, the change in he and Pout is insignificant, and this is achieved without any special measures for thermal stabilization of the CM. The latter is also an advantage of powerful MIS transistors.

Literature:

CIRCUIT DESIGN OF DEVICES ON POWERFUL FIELD TRANSISTORS. DIRECTORY. Edited by V.P. DYAKONOV