An Si5351-Driven QRP Amplifier

This work follows directly on from my initial thoughts on an Si5351-Based Transmitter. Check out that post for background. 

This week I finally got around to improving the transmitter/amplifier I started work on back in March. With a shipment from KitsAndParts, I replaced the J110s that had been part of this amplifier and replaced with BS170s. The differences between the parts are striking – the J110 is a general-purpose JFET (which is by definition a depletion-mode FET) with a rated power disipation of a few hundred milliwatts. The BS170 is a fast-switching enhancement-mode MOSFET with a rated disipation of 800 mW.

Here’s how the schematic looks now. Pretty similar to the last time, with a couple key changes:

CW Transmitter BS170

I stripped the heat sinks that had been on the J110s off and threw them on the BS170s. These puppies still do get hot, and even with the ability to dissipate almost a Watt, I think they’d not be pleased too much heat. The heat sinks, plus some thermal goop from Microcenter, are easing my mind a bit.

As built, the amplifier had an output impedance of about 10 ohms at 7 MHz, as measured by the method suggested by W2AEW in this Youtube Video. To bring that up to the standard 50 Ohms, I built a little L-match, using the values suggested by this L-Match calculator, about .43 uH in series with the amp and a little over 1 nH shunted to ground. Because I don’t know the voltage ratings on most of my miscellaneous caps, I put for 4.7nF caps in series to form the fun capacitance. The inductance is 12 turns on a T37-6 (I did some experimenting to find the ideal number of turns). Re-measuring the output impedance (again with the W2AEW method) showed an output impedance of around 47 ohms, which is close enough for my purposes, I figure. I

I also recently bought a little power-supply kit from Ebay, and have been using that to supply the drive current. It’s adorable:

Now, being driven by the Si5351, the amplifier puts out about 1W on a 5V supply, and about 5W on a 12V supply. Not a particularly clean signal, mind – the 5W is as measured in the power meter of an MFJ-949 – but it is an actual 5W!

Hear you on the air!