Linear power amplifier at 144 MHz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Linear power amplifier at 144 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / RF power amplifiers

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A power amplifier is described below, which, with good linearity, allows you to get an output power of 2,5 watts when working on SSB and 3 watts - by telegraph or telephone with FM.

Schematic diagram of the amplifier is shown in the figure. It is assembled on two transistors. The use of the KT610A (T1) transistor in the first stage made it possible, due to the large power gain (14-17 dB at a frequency of 145 MHz), to obtain an RF voltage sufficient to excite the second stage. The KT904A (T2) transistor provides, with good linearity of the characteristic, the required output power (amplification at a frequency of 145 MHz - 10-12 dB). Transistor T2 has no overload protection, so the amplifier must not be operated without a load.

144 MHz Linear Power Amplifier

The first stage operates in class A mode. To stabilize the operating point, resistors R3 and R4 are used, connected to two separate outputs of the emitter. The signal source resistance (50 ohms) is matched to the input resistance of transistor T1 using the C1C2L1 circuit.

With the help of the C7C8L3 chain, the output resistance of the first stage is transformed to the input resistance of the stage on transistor T2. This transistor operates in class AB mode. To stabilize the operating point, a resistor R6 is included in the emitter circuit. In order to eliminate RF feedback, it is shunted with two capacitors C9 and C10 (the inclusion of two capacitors reduces their parasitic inductance). In this cascade, thermal compensation is also used using a silicon diode D1. Mounting the diode directly to the transistor or to its heatsink provides fairly effective thermal feedback.

The next LC chain is a two-stage resistance transformer. Part of it - L5C13C15. - is a step-up transformer, and the L6 coil is a step-down transformer to the required output resistance (50 ohms).

To ensure the stability of the amplifier, multiple decoupling of the power circuits is used. This measure prevents the amplifier from self-excitation.

Construction and details.

The amplifier is mounted on a foil board. Cooling both transistors is not difficult, since their cases are electrically isolated and can be directly connected to a massive heatsink.

Coil data are shown in the table. All of them are frameless (winding diameter - 5,8 mm) and wound with silver-plated wire with a diameter of 0,8 mm. Inductor Dr1 contains 2,5 turns of PELSHO 0,2 wire wound on a Manifer-340 ferrite rod. The inductance of the inductor should not be too large - this can lead to self-excitation of the amplifier.

Table 1

designation according to the scheme	Number turns	Length winding , mm
L1 L2 L3 L4 L5 L6	4,5 9,5 1,5 9 3 4	6,5 13,5 3 6,5 14 6,5

All bypass capacitors are ceramic. Trimmer capacitors C1, C2, C7, C8 - ceramic, C13, C15 - with an air dielectric.

Customization.

To set up the amplifier, you need power supplies with current limiters, a milliammeter for measuring collector currents, a 144-145 MHz frequency signal generator with a power of at least 10 mW, a power measuring device (for example, a high-frequency voltmeter and a load resistor).

When first turned on, the output stage is supplied with a voltage of 12 V. The current limiter of the power supply for the first stage must be set to 150 mA, for the second - to 30 mA. First, the transistor modes are set to direct current. The collector current of transistor T1 should be equal to 100-120 mA, transistor T2 - 7-10 mA.

Check the amplifier for the absence of self-excitation. If there is no self-excitation, a signal can be applied to the input of the amplifier and, starting from the output, by repeatedly adjusting the capacitors, adjust its circuits to the maximum output power. At this point in the setup, the second stage current limiter should be set to 250 mA. The amplifier setting should be repeated several times, each time gradually increasing the amplitude of the input signal and the supply voltage of the second stage.

Results. As already mentioned, one of the main requirements for an SSB power amplifier is the linearity of its characteristics. As a criterion of linearity, the intermodulation coefficient is usually used. The meaning of this parameter is as follows. If two signals of different frequencies f1 and f2 are applied to the input of the amplifier, then due to the nonlinearity of the transmission characteristic at the output, in addition to these signals, signals of combination frequencies will also appear: 2f1-f2, 2f2-f1, etc. The ratio of the level of combination components, for example, 2f1-f2 to the signal level f2 is called the intermodulation coefficient.

To assess the linearity of the characteristics of this amplifier, measurements of the mutual modulation coefficient were carried out when two signals of the same amplitude were applied to its input. With an output power of 2,5 W, the intermodulation coefficient was -28 dB, which is quite acceptable in amateur radio practice.

Author: M. Knitzsch (DM2GBO); Publication: N. Bolshakov, rf.atnn.ru

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