Because the clarity of signals at every stage is critical much care is required at power supply lines. It may cause unnecessary coupling or feedback.
That is why power supply filtered at every stage is used in RF amplifiers.
In C-25/1 different kinds of power supply line decoupling filter arrangements are shown.
The most important thing to be taken care of in coupling stages is maximum signal coupling at minimum leak.
In solid state circuits SWR SWR matching (explained in upcoming chapters) is very important. Whatever be the input-output impedance difference or the input signal unbalancing required, toroids are unavoidable for satisfactory coupling.
In C-25/2 there are a few circuits using toroid coils and transformers.
In C-25/3 the circuit details of a broadband RF amplifier is given with necessary descriptions. Recommended toroid is Amidon make. Along with a chart showing the details of components recommended for different Ham bands are given.
In solid state circuits SWR SWR matching (explained in upcoming chapters) is very important. Whatever be the input-output impedance difference or the input signal unbalancing required, toroids are unavoidable for satisfactory coupling.
In C-25/2 there are a few circuits using toroid coils and transformers. 
In C-25/3 the circuit details of a broadband RF amplifier is given with necessary descriptions. Recommended toroid is Amidon make. Along with a chart showing the details of components recommended for different Ham bands are given.
In figure C-25/3, is a method of switching unique filters for different bands. Final transistor breakdown can be avoided to some extend by adding a zener diode of required wattage between the collector and the ground.
Zener diodes with a cutoff rating of 10% below the actual emitter - collector cut off voltage is recommended here.
In most makes of final transistors a protection circuit is already added inside it.
Details of Transistor, 2SC 1945 | ||||
Parameter | Measurement | Parameter | Measurement | |
VCBO | 80V | Po | 14 – 16W | |
VEBO | 5V | VCC | 12V | |
VCEO | 40V | f | 27MHz | |
IC | 6A | |||
Every RF transistor is built for exclusive purposes and a random selection of RF transistors in RF stages are not advised. Every transistor manufacturer provides along with a charts showing all its ratings including its peculiarities, ambient temperature, collector dissipation, current gain, Collector-Emitter breakdown voltage, base impedance, maximum frequency of operation, handling power, load capacity, gain, package etc. Also model circuits, where this particular transistor fits also is provided. A home brewer should necessarily go through it before mounting it on the board.
In data books only very important details will be available. To get an overall idea please check the chart and see the meanings of symbols shown in data books.
The Mitsubishi make high gain (-14db) RF transistor 2SC1945 is ideal for low-power transmitters. In 12V and at 0.5W input it provides upto 12 W in the out put without overload. in class AB mode. Since heatsink is connected to the emitter, no chassis separator is required for this transistor.The packing of this transistor is like the regulator IC 7812. At 27 Mhz, and on 16V a power output of 16 Watts is possible from this transistor. It is usable in all frequencies below 27 Mhz.
A test circuit of 27 Mhz for 2SC1945 and important functional properties are given in fig. C-25/4. In Ham Bands change the value of components accordingly.
CCB | Collector to Base Capacitance | hfe | DC forward current gain | |
Fr | Transition Frequency | PO | Output power | |
IC | Emitter Current | Si P | Silicon PNP | |
VCC | Collector Supply Current | Ge N | Germanium NPN | |
PC | Collector Dissipation | f | Frequency | |
V(BR)EBO | Emitter to base breakdown voltage | Tj | Junction temperature | |
IEBO | Emitter cut off current | PEP | Peak Emission Power |
The Mitsubishi make high gain (-14db) RF transistor 2SC1945 is ideal for low-power transmitters. In 12V and at 0.5W input it provides upto 12 W in the out put without overload. in class AB mode. Since heatsink is connected to the emitter, no chassis separator is required for this transistor.The packing of this transistor is like the regulator IC 7812. At 27 Mhz, and on 16V a power output of 16 Watts is possible from this transistor. It is usable in all frequencies below 27 Mhz.
A test circuit of 27 Mhz for 2SC1945 and important functional properties are given in fig. C-25/4. In Ham Bands change the value of components accordingly.
C-25/4 Coil Details (All Air core) | |||
Coil | Diameter (mm) | Turns | Pitch (mm) |
L1 | 8 | 6 | 1 |
L2 | 8 | 7 | 1 |
L3 | RFC | - | - |
L4 | 8 | 5 | 1 |
L5 | 8 | 7 | 1 |
L6 | 8 | 8 | 1 |
According to manufacturers advice, instead of the 100 PF capacitor close to L5, use a parallel configuration of 10PF, 30PF, 100PF, 330PF, 200PF, 10micro F, and in parallel to 0.01 micro Farad and 2200PF connect 100PF and 10PF capacitors.
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