CONVERTING SOLID STATE CLASS C AMPLFIERS TO LINEAR OPERATION

Commentary by NR6CA, Methodology by W3KM

There are any number of amplifiers that are Class C. These have been designed and optimized for use with CW, FM or possibly even some of the single carrier digital modes of communications. So what if you want to use one of them for SSB use? Well a Class C amplifier simply can not be used for SSB and similar modes since extremely high levels of distortion will result. You need to use a linear amplifier that operates in Class A, Class AB₁, AB₂, or Class B. Of the different classes of linear operation the first one, Class A is the most linear and Class B is the least linear. What that means is an SSB signal would sound the best from a Class A amplifier and the worst out of a Class B linear. AB₁ and AB₂ fall in between. Conversely, Class A is the least efficient and Class B the most efficient of the typical linear modes.

Not every Class C amplifier can be converted to a high quality linear amplifier. But here is one way to do a simple conversion and stand the best chance of success. The input voltage can also be 12 VDC if that is the voltage required for your solid state amplifier. The nominal +.6 VDC would be tied into the base of the transistor through an appropriate RF choke and RF bypassing capacitors. R3 can be adjusted to set the quiescent (no RF input idling) current of the transistor. If you have two transistors in the output circuit, each one should have a separate bias circuit so you would need two of everything shown below. Adjust each transistor to the same idle current with no RF applied.