Designing and Building Transistor Linear Power Amplifiers
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In Part 1 of this series, I described an experi mental method for designing a linear amplifier starting with a blank sheet of paper, some basic test equipment and an assortment of can didate transistors. In December 2006 QST I described a single b...
In Part 1 of this series, I described an experi mental method for designing a linear amplifier starting with a blank sheet of paper, some basic test equipment and an assortment of can didate 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.
How Much Power do We Need?
Once the signal has been moved around and processed at the 0 dBm power level and is at the desired output frequency, most applications will require more power. If 0.25 W is enough, then the amplifier described in Experimental Methods in RF Design, Figure 2.93 and included in the December 2006 QST article is highly recommended. 2 Many on-air contacts have been made at that power level over remarkable distances when band conditions enhance the transmitted signal and noise and interference are low at the receiver.
I’ve played that game, and every contact entered into the log is cause for a little celebration. But the bands are not always kind and high levels of noise and interference are common. You may clearly hear the station on the other end of the contact but he is probably running at least 20 times (13 dB) more power, even in a low power (QRP) contest. One more stage of amplification added to the 0.25 W amplifier can overcome that 13 dB difference. The 5 to 10 W output level is a common standard for portable radios with many commercially available examples. 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 the data sheet. The operating conditions were obtained experimentally by vary ing the supply voltages while watching the output waveforms.
The values of the π attenuators between stages were selected experimentally for best gain and distor tion distribution among the three amplifier stages. The single-section low-pass filter on the output of the driver transistor made a sig nificant reduction in high-order intermodu lation products, and seemed to improve the symmetry of the intermodulation distortion products as well.
Tweaking it into Submission Figure 4 is the single-tone output spectrum of the exciter driving the amplifier in Figure 3 with the two-tone output spectrum shown in Figure 5. Excellent linearity was obtained at a PEP output level of several watts. Since the amplifier is experimental and the parts are inexpensive, I adjusted the collector supply voltage on the output stage up and down and observed the impact on AM and two-tone waveforms without worrying much about burning out the device. I also varied the base bias, and changed the drive level with a step attenuator.
As expected, increased collector voltage made a big improvement in the linearity of strong signals. Increased base bias improved the linearity of small signals. Since 100% modulated AM, SSB and two-tone outputs vary from some peak volt age all the way to zero, both collector supply voltage and base bias determine the linearity of the output. Each can also be used to destroy the device. Too much collector voltage will burn out the transistor directly (remember that the voltage at the collector will generally swing to significantly higher than twice the supply voltage, even in a linear amplifier).
Too much base bias will either destroy the transistor quickly as it conducts too much collector current and overheats, or slowly as the base-emitter junction warms up and the device goes into thermal runaway. I enjoyed exploring these options in the design phase of this amplifier, but have not burned out a device since selecting the component values and supply voltages shown in Figure 3. Give Me Power The collector power supply for the output stage is a common circuit, with a big capaci tor instead of the expected three terminal regulator.
That gives me about 18 V open- circuit, and about 16 V at maximum output. The big capacitor is split in two. The little box on the floor holds 2200 μF while another 3500 μF is in the box with the speaker, vari able bias supply, TR relay and 12 V three terminal regulator. The regulator supplies regulated voltage to the receiver, transmitter and other amplifier stages. The big capacitor provides the low impedance at audio needed in a SSB linear amplifier.
By splitting it in two all of the components in the power supply and regula tor circuitry are physically and electrically close to a big reservoir capacitor.
Keeping power supply lines clean is particularly important around receivers. It is a simple power supply for an inexpensive transistor, and any efficiency I would have gained by using an expensive 13.8 V linear RF power transistor in one of Granberg’s wonderfully engineered circuits is more than offset by eliminating the series regulator. For more power, I’ll experiment with operating the transistor closer to its breakdown limits. I don’t mind burning out a few output transis tors during these experiments, because the transistor is easy to change and costs less than a cup of coffee.
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