Active HF switch. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Active HF switch. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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In sports KB equipment, high-frequency switches (usually according to the T-shaped attenuator scheme) made without diodes have found wide application. In the closed state, they provide good decoupling between stages, but introduce relatively large losses in the open state, especially when operating on a low-resistance load.

The figure shows a variant of a high-frequency switch assembled on transistors. It is also made according to the scheme of a T-shaped attenuator, but in comparison with the prototype on diodes, it introduces lower losses in the open state. Moreover, it effectively works as a buffer stage, since the transistors VT1 and VT3, through which the RF signal passes, are connected in a common collector circuit.

Active HF switch

If a control voltage of +2 V is applied to the base circuit of the transistor VT12, then it will enter saturation and the transistors VT1, VT3 will reliably close, since closing polarity voltages, determined by the resistive dividers R1R2 and R6R7, will be applied to their emitter junctions, respectively. In addition, the saturated transistor VT2 itself effectively shunts the signal circuit.

To achieve good decoupling, transistors VT1 and VT3 must have a low capacitance of the emitter junction, and transistor VT2 must also have low saturation resistance. When using the transistors indicated in the diagram (capacitance of the emitter junction VT1 - 0,8 pF, VT3 - 1,5 pF, saturation resistance VT2 - 2 ohms), the calculated decoupling between the output and input of the switch is approximately 9 dB at a frequency of 110 MHz. This value is obtained for the "idle" mode on the output, the connection of the load improves this parameter. For the practical implementation of such large decoupling values between the output and the input, effective interstage shielding in the switch is required.

The output impedance of the switch is about 6 ohms. which allows you to use a load with a resistance of 60 ... 75 ohms without noticeable attenuation of the RF signal. The current through the transistor VT3 is approximately 5 mA, so the maximum output voltage at a load of 60 ohms is limited to 260 mV, and at a load of 600 ohms - 2,6 V.

In the active switch, you can use domestic transistors of the KT337 (VT1), KT608 (VT2) and KT610 (VT3) series.

Author: Monficelli D.; Publication: N. Bolshakov, rf.atnn.ru

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