Broadband power amplifier. Encyclopedia of radio electronics and electrical engineering

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Field-effect transistor power amplifiers have a number of advantages over bipolar transistor amplifiers. In particular, it is easier to obtain good linearity of the amplitude-frequency characteristic and high stability of parameters in them [1].

The described amplifier (see the diagram in Fig. 1) provides an output power of about 70 W in a load with a resistance of 75 Ohms and a gain of about 40 dB in the middle of the 30 ... 2 MHz range. The frequency response is shown in fig. 1. Preliminary amplification stages are assembled on field-effect transistors VT2 and VT1. The first one works with a small positive mixing voltage at the gate, set by the divider R2R1. The load of the transistor VT1 is a broadband transformer T2. Its secondary (step-down) winding is included in the gate circuit of the transistor VT3, which operates with zero bias voltage at the gate. The secondary (lowering) winding of the broadband transformer T4 is connected through resistors R5 and R3 to the gates of the output stage transistors VT4 and VTXNUMX, which also operate with zero mixing voltage.

Fig.1 (click to enlarge)

Ris.2

The step-up winding of the output transformer T3 is connected to the antenna filter. The latter is necessary due to the fact that the harmonic coefficient of the amplifier is not better than -15 dB. The scheme of the antenna filter is shown in fig. 3. You can also use the antenna filter from the broadband amplifier described in [2].

Ris.3

Important elements of the amplifier are broadband transformers. The broadband of transformers is proportional to the ratio Lo/Ls, where Lo is the inductance of the windings, Ls is the leakage inductance. It should be taken into account that a decrease in Lo leads to a narrowing of the frequency band of uniform amplification from below, and an increase in Ls - from above. Small values ​​of Ls can be obtained with a strong connection between the windings, which is achieved by a special design of transformers [3, 4].

In the amplifiers tested by the author, broadband transformers were used, the design of which is shown in Fig. four.

Ris.4

The transformer consists of a metal frame 1, which is two copper tubes connected by a copper jumper. 9 rings of 2 sizes K10X6X3 from M1000NN ferrite are put on each tube. The rings are glued together with BF-2 glue. Two turns of wire 3 MGTF 0.65 are passed through the tubes so that its ends come out from the side of the jumper. The wire should fit snugly into the tube. Tubes with a jumper are a step-down winding, and two turns of wire are a step-up winding.

The power supply must provide a voltage of 40 V at a current of up to 3 A.

You can, for example, use the source described in the article by V. Drozdov "Single-band telegraph KB transceiver" ("Radio", 1983, N 1, pp. 17-22).

The amplifier uses MLT resistors, capacitors KD, K52-5, pass-through capacitors KTPS-1, RF chokes D1.2-40, DMZ-12. Inductors can also be made independently on segments of ferrite (600НН) rods 15 ... 20 long, 2 mm in diameter. The winding is carried out turn to turn with wire PEV-2 0,31 until the magnetic conductor is filled. The natural resonant frequencies of the inductors must be higher than the upper frequency of the operating range of the amplifier. The relay, contacts K 1.1 of which are used to control the amplifier mode, is a reed switch RES-55 (passport RS4.569.601). It is located in the exciter.

It is advisable to select transistors VT2-VT4 according to the initial drain current. For transistor VT2, it should be 30 ... 40 mA, for VT3, VT4 - 80 ... 120 mA (but it is desirable that this parameter be the same for both transistors). Transistor KP901B can be replaced by KP901A. In the output stage, you can use one KP904A transistor, but the output power of the amplifier will drop to 40 watts.

All transistors are placed on a common massive heat sink with an area of ​​about 1000 cm2, on which a circuit board made of foil-coated getinax with cutouts for transistors is fixed. Mounting is carried out in a hinged way. The foil layer is used as the common wire. The foil has been removed at the mounting posts.

The data of capacitors and filter coils are given in the table. The coils are wound on ring (size K24X13X7) magnetic cores made of M50VCh ferrite.

Filter Capacitors (in pF) and Coils (in µH)

(click to enlarge)

A properly assembled amplifier starts working immediately. Selecting the resistor R2, set the drain current of the transistor VT1 within 110 ... 140 mA. If the gain in the low-frequency ranges is high, it is necessary to turn on the resistor R3 with a lower resistance (100 ... 560 Ohm).

The amplifier does not have special protection for the output transistors. As the experiment showed, it works stably with various loads - both with tuned and with various "random" antennas, for example, a piece of wire 2,5 m long. A short circuit at the amplifier output also does not damage the output transistors due to a drop in their steepness heating characteristics.

Literature

1. Ilyin V., Yankovsky R. Field-effect transistors in the output stage of a power amplifier.- Radio, 1983, No. 2, p. 54-55.
2. Bunin S. G., Yaylenko L. P. Handbook of a shortwave radio amateur.- Kyiv: Technique. 1978, p. 118.
3. Ben Lowe. A 15 - Watt - Output Solid - State Linear Amplifier for 3,5 to 30 MHz. - QST, 1971, No. 12, p. 11-14.
4. Helde Granberg. Build This Solid-State "Titan".- QST, 1977, No. 6, p. 27-31.

Author: B. Andryushchenko (UT5TA), Kharkov; Publication: N. Bolshakov, rf.atnn.ru

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