Protection of microwave transistors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Protection of microwave transistors. 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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Probably many people know the unpleasant feeling of loss when a scarce or very expensive transistor fails during tuning or experiments.

In order to protect you from a premature heart attack and save your family budget, I suggest using simple devices during experiments that will protect you from such situations. It is about current protection of transistors.

To protect the transistors, resistors are included in the collector power circuit, which limit the current during overloads. The degree of protection of the transistor is directly proportional to the value of the resistance of the resistor. With an increase in resistance, there is a loss of power in the cascade, linearity deteriorates, power is wasted on the resistor uselessly. Therefore, a compromise resistance value is chosen that is acceptable for a particular case.

The situation is even worse in microwave technology, where the capacitance of pn junctions depends on the voltage between the electrodes of the transistor. With pulse modulation types (SSB, CW), the cascade current varies from 0 to the maximum value. In this case, the voltage across the collector will change due to the drop across the resistor. This causes a change in the tuning of the resonant circuits, the amplified signal "crumbles", self-excitation of the cascade appears. From this it follows that to power the collector circuits of transistors, it is necessary to use a more stable voltage, and to carry out current protection using a threshold current limiter.

Microwave transistor protection
Ris.1

We will consider the operation of this device using the example of a 1296 MHz TX mixer circuit (Fig. 1), when the voltage at the terminals of the resistor R2 reaches a value at which the VD1 diode opens, the transistor VT1 opens, VT2 closes, the supply voltage drops. The use of a protection circuit made it possible to obtain a more linear mode of operation of the mixer and protect the transistor when pumping through the 144 MHz input. In a two-stage 1296 MHz amplifier, a protection circuit is used that removes the supply voltage from both stages simultaneously when the specified current value of one of the amplifier transistors is exceeded (Fig. 2).

Fig.2 (click to enlarge)

The shutdown current is stepwise regulated by the number of diodes and smoothly by selection of the resistance of resistors R2, R4. Some transistors (for example, KT979, KT946), due to their design features, have an initial quiescent current. In the receive mode and when working on other ranges, it is necessary to remove the supply voltage from the cascade. This operation is performed by the device, the scheme of which is shown in Fig. 3.

Microwave transistor protection
Ris.3

In addition to voltage switching, the circuit provides current protection of the cascade.

Author: V. Petrushnko (RB5EC), Dnepropetrovsk; Publication: N. Bolshakov, rf.atnn.ru

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