Designing and Building Transistor Linear Power Amplifiers

Putting Power in the Antenna Figure 2 is the block diagram of an experimental single-band 36 dB gain 5 W linear amplifier that may be easily constructed using whatever output device is available. Two noteworthy differences between Figure 2 and other commonly published circuits are the use of a resistive attenuator and low-pass filter between the driver and final stage, and the floating ground at the final amplifier device. These two features make it easy to experiment with different final amplifier transistors without mechanical headaches or oscillations. Figure 3 is the schematic of a 7 MHz version of the amplifier.1 It was optimized to use common, inexpensive ($0.79) switching power supply transistors. Since the 2N5739 is not designed as an RF device, there are no suggested RF operating conditions in In Part 1 of this series, I described an experimental method for designing a linear amplifier starting with a blank sheet of paper, some basic test equipment and an assortment of candidate transistors. In December 2006 QST I described a single band SSB exciter with 0 dBm output.1 An output level of 0 dBm (1 mW) is very common for signal interconnections between 50 Ω blocks in radio systems. At this power level, the SSB exciter output may be connected directly to an antenna for very low power experiments, it may be amplified to any desired output level or it may be converted to a different frequency using a mixer and VFO. It could also be connected to a simple RF clipper followed by a filter to obtain higher average-to-peak ratio SSB, or it might even be converted back down to the audio range with a second oscillator for a number of interesting analog signal processing applications. Several of these applications are illustrated in Figure 1.