FI87029C - Broadband active RF circuit - Google Patents

Description

87029

BROADBAND ACTIVE RF CIRCUIT

The invention relates to a broadband, active radio frequency circuit whose gain is controllable.

In cable television networks, a simple transistor stage is often used as amplifiers. The amplifier 5 must then be broadband, i.e. it operates in the range of several octaves of frequency. In a network operating on a frequency of several MHz, the shortest and most interfering signal paths are required.

Networks also require different large gains, in which case several transistor stages and / or attenuators are connected in series. According to the state of the art, it is solved to change the gain by connecting, for example, circuits bypassing transistor stages, in which case, however, the mechanical switches placed on the signal path are unreliable 15 and may cause interference. The use of relays is undesirable because they are less reliable at high frequencies due to the mechanical components they contain and because their adaptation to electronic solutions poses problems. Alternatively, the gain can be changed by connecting adjustable amplifiers to the circuit, but such a transistor stage has the problem of a variable impedance level which must be compensated by arranging matching circuits on the input and output side of the transistor stage.

The object of the invention is thus to provide a simple and reliable circuit which can advantageously be used in a cable television network. According to the invention, this object is solved by supplying the operating voltage of the circuit via switching elements which, if desired, change the polarity of the operating voltage of the circuit, according to which the active member of the circuit acts as either an amplifier or a voltage follower without changing the RF signal input or output terminals.

According to the invention, the inputs of the radio frequency or RF signal do not change when the mode of operation of the active circuit is changed. The switching elements only affect the DC circuits, so that the elements do not interfere with the broadband RF signal.

The circuit according to the invention is simple in structure and reliable when preferably only one transistor stage is used, which can be controlled as either an amplifier or a voltage follower. Thanks to this solution, there is no need for separate circuits bypassing the amplifier or attenuators connected in series.

The invention will now be described in more detail, by way of an exemplary embodiment, with reference to the accompanying drawing, in which a figure shows a switchable transistor stage according to the invention.

The transistor stage T1 in the figure is used as a two-stage stage, on the one hand as a broadband amplifier stage and on the other hand as a voltage follower. The operating mode is selected by the contacts SI - S4 of the switch operating simultaneously, which change the polarity of the operating voltage of the circuit. The switches can be implemented in other ways known to a person skilled in the art, e.g. semiconductor switches. In both modes, factors affecting the quality of the RF signal, such as distortion characteristics and noise figure, and impedance level, remain essentially the same. In voltage follower operation, the throughput attenuation is as small as possible.

25 In amplifier winding, the contacts SI to S4 of the switching element are in position 1. In this case, the NPN transistor stage acts as a normal feedback common emitter stage. The gain is determined by the feedback elements LC and RC when the PIN diode pair D1-D2 is controlled to conduct at a forward DC current of 30 +12 V from the contact S4. According to the practice of the art, the capacitances and inductances of the circuit have the RF signal path from the input RF IN to the output RF OUT separated from the DC circuits of the transistor.

3 87029 In the voltage follower operation, where the switch contacts are in position 2, the direction of the collector current of the transistor T1 is changed to the opposite, i.e. from the emitter to the collector. The value of the collector current is limited by approx. 20 mA by means of a resistance of 1.5 kohm connected via contact S3. The PIN diodes in the emitter circuit are now in the inhibit state and no direct current is flowing in the circuit, causing a sufficiently large impedance from the emitter to ground. This voltage follows and works differently from the usual solution, where here the collector voltage RF OUT follows the carrier voltage RF IN.

The other circuit elements shown in the figure of the exemplary embodiment will not be explained in more detail here, since their operation is known per se and will be obvious to a person skilled in the art.

The circuit according to the invention can advantageously be used in a cable television network. In the tests performed, the circuit has shown that it works in accordance with the set goals.

Claims (6)

1. Broadband active RF circuit, the amplification of which can be controlled, characterized in that the drive voltage of the circuit is supplied through coupling means (SI - S4), with which the polarity of the circuit voltage of the circuit is switched as desired, according to which an active member (T1) in the circuit functions either as an amplifier or as a voltage follower without changing the input or output coils of the RF signal (RF IN, RF OUT). 2. A circuit according to claim 1, characterized in that the active member (T1) is broadband. 3. A circuit according to claim 1 or 2, characterized in that the active member (T1) is a transistor. 4. A circuit as claimed in claim 3, characterized in that, as a voltage follower, the collector voltage of the transistor (T1) follows the base voltage. 5. A circuit according to any one of claims 1-4, characterized in that the input and output impedance levels of the circuit are the same in both operating states. Use of a circuit according to any of the preceding claims as an amplifier connected in a cable television network.