A 16W Linear Amplifier
September 9, 2010 3 Comments
The Softrock Ensemble RXTX that I’m using has a 1W output, which is a little low, so I built a linear amplifier to boost the signal. It delivers 16W when powered by a 24V supply (a 45W switching adapter from an old Mac laptop), about 10W when powered from a 16V supply (from an old IBM laptop), and about 5W when running on 12V. Laptop adapters are becoming very plentiful (because they keep working when the laptop fails or gets too old to be useful), and at 16-24V and 45-90W and a small size, they are very useful for powering transmit amplifiers. They might be on the noisy side for receivers (I didn’t check), but that’s not much of an issue for transmitters.
The circuit, which uses IRF510 power MOSFET, is very conventional. The broadband input section uses a 1:4 un-un and an 18Ω resistor to drive the MOSFETs gate. At 14MHz the input capacitance of the MOSFET translates into a reactance of around 87Ω, so the 12.5Ω input source drives it easily even though quite a bit of the input power is wasted in the 18Ω resistor. The output section with another 1:4 un-un and a low-pass filter is taken from the GenesisRadio G2030. The RF sensor that activates the receive-transmit relay (and a fan to cool the heat-sink) is taken from Wayne McFee’s Miniboots amplifier. The bias circuit, T/R switching, and the fan are all powered by a 12V regulator. The unit has a a high-voltage jack (the label says 20V but it will work on anything between 15V and 28V) and a 12V jack. You can take power out of the 12V jack to power a small transceiver or accessory, or feed the amplifier with it (the diode across the regulator allows the 12V jack to feed the MOSFET).
Many of the parts came from the junk box, so I don’t always know what they are exactly. This includes, for example, the relay, the diodes in the RF sensor (they appear to be similar to common 1N4148, however), and the large ferrite toroid for the output un-un. In the picture below you can see that the input un-un has 9 turns (on a FT50-43), the output un-un has 18 turns on a large toroid (the vendor from which I bought it at a swapfest told me it’s 43 material). The inductors in the output filter are 11 and 12 turns on a T50-6.
About a year ago I started experimenting with RF power amplifiers, inspired by Rick Campbell’s articles in QST (February and March 2009). I knew from those experiments that it is really easy to blow transistors either by frying them (if they are not mounted on a heat sink that can dissipate the heat) or by biasing them so high that they are completely turned on and exceed their current rating. The fix to the over-current problem is either a current-limited supply (which I didn’t want to build), or being careful with the bias setting (which I though I was capable of). The fix for the overheating is a good heat sink. I used the heat sink from a discarded Pentium-III processor, which came with a 12V fan. I drilled another hole in the heat sink, bolted the transistor to it (using an insulating pad and insulated hardware), and bent the transistor’s pins so that they stick into the circuit board through a large hole. The Pentium-III dissipates about 30W, so I knew that the heat sink can dissipate that much, at least with the fan on. I decided to connect the fan in parallel with the T/R relay, so that it is off during receive periods and on during transmit periods. This keeps the heat sink at about 30ºC at all times. I could make the amplifier cooler and save some power by setting the bias to zero during receive periods, but I didn’t bother. If you are home brewing a power amplifier, I think that starting the mechanical and packaging design from the heat sink is a good strategy. And give CPU heatsink/fan assemblies a consideration; there must be millions of discarded ones around, and they are easy to put into use.
I started the design by prototyping the circuit on a piece of PCB bolted to the heat sink, using ugly construction (high-value resistors that serve as standoffs for electrical connections).
When the circuit worked, I rebuilt it on another piece of PCB, this time creating pads by scoring the PCB with a hobby knife. The circuit board fits inside a plastic box, with the heat sink sitting outside the box. The pins of the MOSFET pass through a hole in the box and a hole in the PCB. The fan’s wires also enter the box through a hole. The plastic box fails to shield the amplifier, but it was easier to drill than a metal box would have been. While preparing the box I did manage to destroy one MOSFET, presumably due to static electricity created by the drilling and filing.
I biased the amplifier for about 150mA drain current. The current consumption during transmit is about 1.1A at 24V, a bit higher at 16V or 12V. I was able to make contacts with the amplifier on 14MHz, and it should also work fine on 10MHz, although I didn’t try.