JBOT – An Easy QRP linear amplifier
JBOT stands for Just a Bunch of Transistors. It is a simple, stable and easy to build 5 watts linear amplifier build out of a bunch of ordinary low power NPN transistors.
Why another linear amp?For many of us, getting hold of high power RF transistors is impossible. They are also expensive and easily blown. In the last decade, hams have pressed the power MoSFETs into use as RF devices. This has been a mixed success. The power FETs (mostly the IRF series for International Rectifiers) have very high input and output capacitance, they need higher drain voltage and they are very non-linear devices. As result, IRF device based RF power amplifiers have remained beyond easy replication for the average homebrewer. Often, the RF power amplifiers are run flat-out without any attempt at stabilizing the performance, This has to lead to a lot of grief. The BITX’s linear amplifier painfully illustrates both these issues. The IRF510 takes a lot to get it going. Many of those who scratch build it on their own often found themselves struggling with stability and insufficient power output. The inspiration from this power amplifier came from watching a colleague assemble an incredibly fast computer by adding together a number of ordinary hard disks and old mother boards. By stringing them all together, he was able to extract an amazing performance. When I asked him what kind of architecture this was, his reply was “It is JBOD (Just a bunch of disks)”. Paralleling transistors is not new to hams. I can recall an article by Doug (sk W1FB) in a 1970s QST where he had paralleled up six 2N3904s to generate a single watt of power. Harry’s small linear amplifier pumped up to 23dBm out of general purpose NPN transistors. In this design, the attempt is to apply these principles to higher power output from the 2N2218 variety of transistors to get 5 watts of clean, stable and linear output from a 1milliwatt signal.
What is JBOT?JBOT stands for Just a Bunch of Transistors. This linear replaces one big RF power device with a bunch of smaller, ordinary transistors.
- Each transistor handles less power, simple clip-on heat sinks are enough.
- Each output transistor has its own emitter degeneration, making the configuration very stable.
- Any general purpose NPN transistor rated for more than 500 mA current with metal can will work.
- The drivers are biased for higher than usual current to prevent saturation and good linearity.