Power amplifier without power transformer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / RF power amplifiers

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This article is a further development of the idea of ​​transformerless power supply [1].

In all the diagrams below, the numbering of elements that perform the same purpose is preserved from diagram to diagram. Additional new elements of the schemes are consecutively numbered. If there is no next element number, this means that it was in the previous scheme (and this number simply does not exist on this one). 1.Low frequency amplifier

The ULF circuit (Fig. 1) is known as a transformer. Its peculiarity is in the absence of a power transformer. The anodes of the lamps are powered from the 220 V network according to the voltage doubling scheme and Ua-k \u620d 220 V. The glow of the lamps is from the 6 V network through the current-limiting capacitor C1. As Tr2, Tr5, you can use power transformers from old tube radios with a midpoint in the secondary winding (as a rule, kenotrons of the 4Ts5S, XNUMXTsZS, etc. type were installed in them). The network winding of these transformers is used as a high output when working in line for subscribers, the filament winding is used as a low-resistance output.

Fig.1 (click to enlarge)

In amateur conditions, a power transformer from tube radios without a midpoint on the secondary winding (for example, from Records) can be used as an output transformer, but for this you need to turn on the network and step-up windings in series, and the connection point will be average.

As an input transformer, in amateur conditions, an output transformer from tube amplifiers of old radios with a push-pull output stage (two 6P14P lamps, two 6P6S, etc.) can be used.

This amplifier provides at Рin=20...30 W at the output Рout=120...130 W. Capacitors C4, C5 limit the anode current of the lamps, in proportion to their capacitance, for example, if C4 \u5d C20 \u400d XNUMX microfarads each, then the anode current of the lamps is limited to XNUMX mA.

It makes no sense to use C4, C5 of a larger capacity, because. the anode current of two lamps does not exceed 350 mA. In addition, the larger the capacitance of these capacitors, the greater the current surge when you first turn on the 220 V network and breakdown of the diodes is possible. As diodes, D226 or the like, connected in pairs in parallel, can be used. 2. KB broadband power amplifier

The circuitry of the amplifier (Fig. 2) is practically no different from the ULF, only the transformers are made on ferrite rings. Moreover, up to frequencies of 7 MHz, rings of 2000НН can be successfully used, but better - 400 ... 600НН; when operating up to 28 MHz - 50 HF, while providing a minimum blockage of the frequency response in the HF bands. There must be good insulation between the primary and secondary windings. The windings contain 12...15 turns each.

Fig.2 (click to enlarge)

Output transformer - size K40x25x25 or close to it. Input transformer - K16x8x6 or close to it. Sizes can be provided by a set of several rings. At Рin=30 W, the lamp anode current was 250 mA at Uа-к=620V. 3. KB power amplifier with common cathode

As you know, the circuit for switching on lamps with a common cathode requires a full set of supply voltages: anode, screen grid, control grid, incandescent (Fig. 3).

The usual network doubling circuit (220V) provides a source for powering the anode-screen circuits of lamps (+620V +310V). To power the incandescent lamps, capacitor C6 is used, which limits the incandescent current.

Fig.3 (click to enlarge)

The negative voltage source is assembled on Tp1, V9 ... V12, C20. As Tr1, a small-sized transformer is used, because control grid consumption is very low.

I want to draw attention to the fact that such circuits have two "common wires". One is for the DC circuit, this is the negative plate of the capacitor C5, designated 0V. Relative to this point, it is necessary to make measurements in direct current. Moreover, during these measurements, safety precautions must be observed, because. such targets do not have galvanic isolation from the network. For example, to measure the anode and screen voltage, you need to connect the "-" of the voltmeter to the 0V point, and the "+" of the voltmeter to pin 3 of V5 or V6. This is the tension on the screen grids. If pin 6 is V5 or V6, this will be the anode voltage.

To measure "-" on the control grid, you need to change the polarity of the voltmeter, i.e. "+" the voltmeter to the point 0V, and "-" - to leg 2 V5 or V6 and resistor R1 set the quiescent current of the lamps in the TX mode - transmission (no input signal). In the receive mode (RX) on the control grids - the maximum "-" and the lamps are closed, the current through them is zero. The lamp mode is set by the resistor R1 in the carrier mode according to the RA1 device. By moving R1 towards the contact of the relay P2, reduce the "-" on the control grids until there is a linear increase in the readings of PA1. As soon as the linear growth has stopped, R1 is slightly returned back and fixed with varnish.

The second common wire is the amplifier housing - this is the common wire for the RF signal. And all RF voltage measurements; if necessary, they are made relative to the body. Most elements of the amplifier are non-critical and can vary significantly in value. For example, capacitances C1, C2, C7, C8, C19, C1b can vary within 1000 PF ... 10000 pF. The main thing is that they withstand the voltage of the circuit, i.e. C1, C2 - at least 250 V, C8 - at least 1000 V (it can be dialed from two for 500 V), C7 - at least 500 V, C19 - at least 250 V, C16 - any. C 14 - 80...200 pF.

Only one element is critical - C9. It must have a significant voltage margin - at least 1000 V, and most importantly, its capacitance should not be more than 3000 pF. C9 is the "highlight" of the circuit that ensures safety with transformerless power. In the event of a break in the common ground, the current between the case and the common ground does not reach a value that affects the human body, because limited by capacitance C9 < 3000 pF at the level of 250 ... 300 μA in the most unfavorable case. Another feature is that instead of a choke, a resistor R5 is used in the control grid. As experience has shown, the use of a resistor will significantly increase the resistance of the cascade to self-excitation.

Also, the issue of using the contours L7, L8, L9, L10, L11, L12 has been quite successfully resolved. They are used reversely, i.e. when receiving (RX), they are input narrow-band with adjustment of the C18 input, and when transmitting (TX), they match the low output impedance of the transceiver (usually 50 ... 75 Ohms) with the high input impedance of a tube amplifier according to a common cathode circuit.

When transmitting (TX), C 17 is connected in parallel with C18, but since the capacitance C17 is small (2pF), it almost does not affect the tuning of the circuits L7, L8, L9, L10, L11, L12, similarly, Csv is connected in parallel with C12 and also does not affect the tuning of the circuit. Csv is made in the form of one or two turns around the mounting wire connecting C10 to C12. This piece of mounting wire is made of a high-voltage wire, or of a coaxial cable, from which the outer braid is removed, and the turns are wound over a thick nylon filler. Such a coupling capacitor can withstand large reactive voltages and currents and can be used in more powerful amplifiers. After a low capacitance (Csv) - and low voltages, so P1 is not very critical to the gap between the contacts.

This antenna switching scheme from RX to TX with reversible use of the elements of the P-loop and the input "narrow-band" loop allows you to make "cold" tuning to the correspondent - at maximum volume, with knobs C12, C13, C18, without radiation of the "carrier" on the air, which significantly reduces mutual interference and tuning at the frequency of the DXs. Instead of L7, L8, L9, L10, L11, L12, you can get by with just two coils: one is tuned in the HF bands - at 28 MHz at least C18, the other at 7,0 MHz with a minimum of C18, but the maximum capacity of C18 should be up to 500 pF (to cover the remaining ranges).

The taps for the coils L7, L8, L9, L10, L11, L12 are made from approximately 1 / XNUMX turns (from the grounded end), but it is better to choose on each range for the maximum RF voltage on the lamp control grids.

Coils are made on any frames with cores (and even without them). The main thing is that they need to be adjusted to the maximum volume of the received stations (in the absence of devices), you may have to slightly change the capacitances connected in parallel to them.

Tubes V5, V6 are switched on for power addition in the range of 28 MHz; L5 and L6 are tuned for maximum output power at 28 MHz by shifting and expanding the turns. It must be remembered that L5, L6, L4 are under anode voltage and all precautions must be observed.

L4 to reduce the dimensions of the U-circuit and the convenience of mechanical fastening, it is made on a toroidal ring made of textolite, getinax, fluoroplastic, etc., it is mounted directly on the biscuit. The taps on L4 are selected experimentally, depending on the input impedance of the antenna.

L5, L6 - frameless, they are wound on a frame with a diameter of 15 mm and contain 1 turns of wire PEV-1,5 25 mm, winding length - XNUMX mm.

L4 - 60 turns, winding - turn to turn, taps - approximately from 4, 18, 32 turns, the first 4 turns - with 1 mm wire, the rest - 0,6 mm.

Inductor L3 is wound on any insulating material and contains approximately 160 turns of wire 0,25 ... 0,27 mm, some of the turns are wound turn to turn, the rest are in bulk. The winding turn to turn is connected to cL4 ("hot" end L3).

Coils L7, L8, L9, L10, L11, L12 - on a frame of at least 6 mm with an SCR-1 core.
L7 - 10 turns of PEL 0,51, branch from the 3rd from the bottom;
L8 - 12 turns of PEL 0,51, branch from the 4rd from the bottom;
L9 - 16 turns of PEL 0,25, branch from the 5rd from the bottom;
L10 - 25 turns of PEL 0,25, branch from the 8rd from the bottom;
L11 - 35 turns of PEL 0.25, branch from the 10rd from the bottom;
L12 - 45 turns of PEL 0,25, branch from the 12rd from the bottom;

C21 -10pF; C22-15pF; C23 - 68 pF; C24 - 120 pF; C25 - 200 pF; C26-430pF.

P1, P2 can be connected both according to the scheme of Fig. 9, and in parallel, one relay with several groups of contacts can be used, for example, RES-22, RES-4, etc. The relay type also depends on Ucontrol. coming from the transceiver. XNUMX. Hybrid power amplifier

Hybrid amplifiers are known to many radio amateurs. In Fig.4. some details of the connection of these amplifiers with a transformerless power supply are presented.

On the transistor VI 4 and the resistor R7, a voltage regulator for screen grids of lamps is assembled. Resistors R4 and R6 are current-limiting (a kind of protection) at the extreme positions of R7, as well as in emergency situations. R5 creates a leakage current from the base-emitter junction for the normal operation of the voltage regulator. Resistor R1 sets a negative voltage on the control grids of the lamps, when receiving (RX), the lamps are blocked by the maximum voltage (negative). R2 is protection against "pumping" the amplifier and creates a partial automatic displacement on the control grids of the lamps.

R8, R9, R10, R11 - load for the transceiver. These resistors determine the input impedance of the amplifier.

The circuit in Fig. 4 has a common DC wire isolated from the case. It is the negative plate of the capacitor C5 (indicated by the point 0V). Relative to this point, you need to make all measurements for direct current in the circuit.

Fig.4 (click to enlarge)

The tuning methods and methods are reduced to the correct choice of the initial current through V 13, which must be no less than the initial current (at the beginning of the rectilinear section of the V13 characteristic). The same current through the lamps must be set by resistors R1, R7. Good results are obtained when using 6P45S lamps.

C14 must be high voltage, like C9.

I want to warn radio amateurs against the mistake that many make when repeating such schemes. Many, by controlling the anode current of the lamps, are trying to get the maximum possible current. This is wrong, because such circuits are capable of providing large anode currents, but the output power does not correspond to them (currents). So, through one GU-50 (according to this scheme), I managed to get a current of up to 450 mA (Uak \u620d 200 V), but there was no output power of XNUMX W, which significantly reduced the service life (cathode emission was quickly lost), caused TVI, those. the circuit worked as a DC amplifier.

Given the above, it is necessary to "squeeze out" not the maximum possible anode currents (they are only indirectly related to the output power), but the maximum RF voltage on the equivalent, or on the antenna according to the output indicator. With an increase in the RF voltage, it is also necessary to use only a straight section and not lead into the "saturation" zone. The lamps are switched on for power addition, the parameters of the P-circuit are typical (described in the previous section). You can use bipolar KT904 instead of KP907. The emitter is turned on instead of the source, the collector is turned on instead of the drain. The necessary bias to the base is supplied through a powerful 500m resistor, a shift of a 3,3 k potentiometer connected between the "-" of the rectifier and the lower terminal of R7, which is accordingly disconnected from the "-" of the rectifier. This potentiometer sets the initial current of the cascade. Between the potentiometer slider and the "-" of the rectifier, a blocking capacitor is turned on for a small (<100V) voltage, 5. Amplifier on GU74B

The diagram in Fig. 5 shows a power amplifier on a GU74B lamp, which needs 1200V at the anode. This voltage is obtained by adding the voltages of the two sources. The first one is assembled according to the voltage doubling scheme without a transformer from a 220 V network and produces two voltages (relative to the 0V point): +310 V and +620 V. These voltages are quite enough to power the screen grids of most lamps with high anode voltage.

Fig.5 (click to enlarge)

The second source (it can be conditionally called "voltage boost") is assembled on a transformer (TC-270). In order to obtain a total voltage of 1200 V, there must be approximately 400 V AC on the secondary winding of the transformer. After rectification by diodes V10 ... V17 and filtering by capacitors C27, C28, the constant voltage is obtained by about 1/3 more - in total with the first (+620 V), the voltage necessary for the operation of the lamp is reached. Since these sources work on the addition of voltages and powers, the power consumption is distributed approximately in proportion to their voltages, which means that you can safely use a transformer with an overall power of at least half that of a conventional transformer circuit. The negative voltage source is assembled on the diode V9 and capacitor C20. Since the circuit is half-wave, the capacitance C20 must be large enough - 200 microfarads.

Instead of a choke in the control grid, a resistor R5 is used, which makes the cascade more resistant to self-excitation.

Serial power supply of the lamp through the elements of the P-circuit is applied. This has its drawbacks - the elements of the P-circuit are under high voltage, and its advantages - with a series power supply, the efficiency in the HF bands is somewhat higher, and the requirements for the L3 inductor for dielectric strength are somewhat lower, because. it stands after the elements of the P-contour (L5, L4).

The P-circuit can also be made according to a typical parallel power supply scheme.

Somewhat increased requirements for capacitors C12, C13 - they must have a sufficient gap between the plates. C12 with the rotor plates wound up must have a gap of at least 1,5 mm. C10, C11 must withstand large reactive powers at a voltage of at least 2,5 kV. Capacitor C9 provides safety precautions, and its capacitance should not be more than 3000 pF. C4, C5, C27, C28 - 180 uF x 350 V each.

The power amplifier is put into operation in the following sequence.

1. S1 turns on (all others must be turned off). The lamp blower motor starts working, the whole circuit is turned on at a reduced voltage through the capacitors C, C '. They prevent the inrush of current to charge capacitors C4, C5, C27, C28.

2. After a few seconds, S1 turns on - it supplies full voltage to the circuit, while the maximum negative voltage appears on the control grid of the lamp and the full filament voltage - the lamp is warming up.

3. After a few minutes, when the heat has warmed up the lamp, the VK2 toggle switch turns on. If there are no emergency modes in the circuit, VK1 is turned on. When working on the air, switching from reception to transmission is carried out by relay P1.

Turning off the amplifier is carried out in the reverse order.

The mode setting is carried out by the resistor R1. The linear increase in power is controlled by the output indicator PA1. If the increase in power has stopped or is going too slowly (saturation zone), R1 needs to be returned a little back and fixed.

S2, S1, S1', BK1, BK2 must have switching levers made of insulating material. In addition, it is advisable to install them on an insulating decorative lining (isolated from the body) made of thick plexiglass, textolite, etc.

L4 is mounted directly on S2 in order to reduce the size and ease of attachment. It is desirable to perform it on a toroidal ring made of fluoroplast, getinax, etc.

Circuits L7, L8, L9, L10, L11, L12 are the same as in section 3.

If your transceiver does not "rock" this amplifier, do not be upset - you can install another amplification stage in it according to the diagram in Fig. 6. These are lamps of the type 6P15P, 6P18P, 6P9 (or any other triode lamp of sufficient power), switched on by a triode.

Ris.6

The glow is taken from the TS-270 (-6,3 V). The common wire is connected to the 0V point - this is the "-" of the capacitor C5. The anode voltage is taken from "+" C4 (+620 V). Negative voltage is taken with R1 (fig.5a) connected in parallel. The input-output of the cascade is connected to the break point (marked "x" in Fig. 5) of the capacitor C14. The contour data are the same as in section 3.

L1, L2 are wound on ferrite with a thicker wire - 0,37 ... 0,4 mm, 25 ... 30 turns.

Using this circuitry, you can get small-sized amplifiers (desktop with a source) with good energy.

Literature

1. V. Kulagin. Power amplifier KV "Retro". RL, 8/95, p.26.

Author: V.Kulagin. (RA6LFQ), Volgodonsk; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section RF power amplifiers.

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