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ISOTRON 80/40
GET READY FOR THE FALL HF SEASON
 
 
USE OF AN AMPLIFIER
 
Many amateur operators make use of a linear amplifier. This will increase your output power of your radio to as much as 1500 watts. Should you use one? What type, tube or transistorized?
 
It really comes down to a personal choice. However, possibly a few facts can help.
 
There are several manufactures making Solid State amplifiers. What are some pros and cons?
 
Most Solid State amps use Power MOSFET transistors. This is the heart of the amp. What is this thing?
 
It is an interesting device. It has a minimum of 3 connections. Source (S), Drain (D) and Gate (G).
 
The Source is normally the ground side of the device. The Drain is normally the + voltage side. Current will travel from the S to the D when biased or triggered properly. It will also turn off the current flow if needed. This is all controlled by the Gate. Current values of as much as 50 amps for a basic MOSFET can be controlled by the Gate.
 
The Gate is unique in that it has no physical connection to the D and S material. It is insulated by a thin dielectric. Therefore the input resistance is almost infinite or very high.
 
On an N type transistor, you can trigger it fully on by just touching 4 or 5 volts + to the G. Take the voltage away and the transistor will stay on for along time. Ground out the G, the device will turn off.
 
The resistance from the S to D when fully on is only a fraction of an ohm. At RF frequencies this device is at a very low impedance and needs to be matched, usually with ferrite type transformers. This makes the amplifier very broad banded and does not need tuning over the entire HF spectrum. Most amps will have a band selector only, for switching bands.
 
Even though the manufactures of Solid State amps have a SWR protecting circuit, the MOSFETS are not damaged by a high SWR.
 
What damages the MOSFETS is excessive heat. Good heat dissipation is essential to the life of the device.
 
The amplifiers are reasonably efficient, run at low voltages and have no warm up time. In theory there is nothing to wear out or deteriorate. They are light weight and smaller than the tube competitors.
 
On the down side. A transistor just does not get weaker in time like a tube. If the transistor fails, it is out. The cost can be quite high for a device. If installed without knowing the cause, it can blow the replacement very quickly.
 
Most solid state amps will require a high wattage Tuner.
 
Next article we will see how a tube amplifier will fair.
 
73,
Ralph WD0EJA
 
09-15
BILAL COMPANY
137 MANCHESTER DR.
FLORISSANT, CO. 80816 U.S.A
PH/FX: 719/687-0650
 
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