Chapter 22 - SSB Lenier Stages

 

 There are enough disciplines to observe even while designing a RF linear amplifier that takes the SSB signal, coming from the mixer, upto the Antenna. First decide the type of active components you propose to use. Valves are outstanding in a few features. Valves are more durable except that they are fragile, handle more power, give better harmonic suppression and are not easily prone to damages due to wrong circuits or other wrong wiring configurations. At the same time it has its own drawbacks too. They are bulky, require high voltages, bulky cabinets, expensive and heavy power supplies. Other minus factors are much heat more power consumption. Their transconductance ability at high frequencies also are poor. It should be remembered that many commercial transceivers like FT 101 uses valve in its final stage. One important thing to remember is that valves are not manufactured these days and replacements are not as easy as it was before where junk/flee markets were loaded with discarded valve equipments. These minus points are the merits of a transistor. But only careless ness is enough to spoil a transistor, which is much more expensive than a valve. 

According to a reception test done on SSB signals using VU and Dx stations, it was found that a power variations from 5W to 15 W, 15 W to 40 W and 40W to 150W makes no difference in a Receiver 'S' Meter. Another experiment proved that a normal environmental conditions, 75% of the usual contacts are possible with just 15W of SSB.  Another observation was that even at 100% power all contacts are not possible, except with the support of rotator type directional antennas, antenna tuners and antenna towers. Another important observation was that most of the stations spend unnecessary out put power, with the difference that a SWL found it difficult to resolve it with his BFO. 

Even if a transmitter pushes into the atmosphere the full power allotted to it by international authorities concerned, at 'Black outs' and when solar cycle effect is negative, we may not be able to get to at least one receiver and that at eye sight. Those little techniques scribed in commercial equipments itself may not be as supporting as we expect. Taking into account all these plus and minus factors I suggest that a home brewer can compromise with a 25 W SSB equipment working on 12 V Batt.  Steady power supply solves half the distortion, image, interstage related issues that might pop up in an assembled equipment. 

In fig: C-22/1 the diagram of a 500W valve power amplifier (single stage) is given. 

One peculiarity of this circuit is that this cannot be switched on without connecting both input (15 W) and the output. Another thing to be observed is that even if the 'ft' of the active component used is 10 times more than the frequency of operation, it will play high gain at lower frequencies and comparatively low gain at higher frequency ranges. If pre driver stages are in broadband style, this can be adjusted to some extend regulating the input signal strength accordingly. 

Assembling a single band transmitter is always easy. Accessories collected should necessarily be fitting to the band/bands we work. A sight compromise we make might give you big troubles in the future. In effect, any study on RF linear amplifiers are detailed study on tuned circuits. A tuned circuit at the output and input of all the active stages are usual in linear amplifier stages. One important thing to note here is that the tuned circuit should be linked to the active component only at a point where the two impedances match perfectly. In grounded emitter/base configurations the impedance could be comparatively low and so tappings might be required in the connecting coils.  In C-22/2, a series of impedance matching circuitries are shown. 

When we fix the bandwidth of RF amplifiers, do not forget that at higher bandwidth the selectivity will be reduced highly. 

If triode valves are used in RF Amplifier stages noise factor will be low; but pentode valves are better for higher gain. Because the inter electrode capacitance in triode valves result in positive feed back and self oscillation, they are used as low gain grounded grid amplifiers.  To rectify the problems due to inter-electrode capacitance, one solution is a signal that is equal in amplitude but opposite in phase is fed from the output stage back to the input or to the cathode/emitter. In transistors this technique might only help to reduce feedback related problems. This is called neutralization. Unless there are neutralization circuits, there could be signal distortion in transistors and self-oscillaltion in valves. 

Chapter 21                                                     Chapter 23