Low noise amplifier at 144 MHz. Scheme, description

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

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Encyclopedia of radio electronics and electrical engineering / radio reception

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The sensitivity of a VHF receiver is largely determined by the noise figure of the first stage of high frequency amplification. And the lower its noise figure, the higher, of course, the sensitivity of the receiver can be. Recently, in professional and amateur VHF receivers, low-noise field-effect transistors of low and even high power, in particular, the so-called field-effect transistors with a vertical gate structure, have begun to be used. The main purpose of these transistors is to work in the final stages of HF and VHF transmitters of equipment, as well as low-frequency power amplifiers.

But it turns out they can be used to great effect in the input circuits of a highly sensitive VHF receiver. The low noise figure and wide dynamic range give reason to consider them the best element base for such devices at present.

Schematic diagram of an RF amateur receiver amplifier at 144 MHz. which uses a high power FET with a vertical gate structure, is shown in the figure. Capacitors C 1, C2, C8 and C9 are designed to tune the input and output resonant circuits to a frequency of 144 MHz and ensure optimal matching of the input and output of the amplifier with the connected devices.

Low noise amplifier at 144 MHz

A feature of the RF amplifier on a high power field effect transistor is the purpose of biasing the gate of this transistor. For the transistor to operate in the channel enrichment mode, it is necessary to apply an opening voltage to the gate relative to the source, which can be regulated within a wide range due to a sufficient spread in the parameters of individual transistors.

In addition, this voltage must be stable. Therefore, the mixing voltage is stabilized by a diode parametric stabilizer, consisting of a resistor R4 connected in series and four diodes connected in the forward direction. This voltage from the tuning resistor R3 through the filter R2C3 and the resistor R1 is applied to the gate of the transistor. With a tuned resistor R3, you can adjust the drain current over a wide range. For the specified transistor, it should be equal to 40 mA.

The transistor must be mounted on a small plate heat sink. For stable operation of the amplifier when the temperature changes, it is recommended to install the stabilizer diodes on the same heat sink, ensuring good thermal contact. Keep the gate and drain wires as short as possible to avoid self-excitation.

The amplifier has a noise figure of about 2,4 dB at 24 V, but as stated in the original article, the performance of the amplifier remains almost unchanged when the supply voltage is reduced to 12 V.

When repeating the amplifier, you can use field-effect transistors of the KP901 or KP902 series and KD521A diodes.

Author: Radio-Amater; Publication: N. Bolshakov, rf.atnn.ru

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