Two-Watt Driver Amplifier

 
 

Since my final amplifier needs at least 1 W input, I looked around for some options on a driver amplifier and decided on a fairly common broadband approach using bipolar transistors and binocular core transformers at the input and output. The circuit is loosely based on Motorola application note AN779, the famous 20 W amplifier that you can buy as a set of parts from Communications Concepts, Inc. (CCI). I stole the first stage, used some generic CB transistors, and customized the transformers a bit to get “good enough” performance. This was my first attempt at a broadband amplifier of this type...


The first task was to try and optimize the winding of the transformers. For a constant turns ratio of 2:1, which is better: 2, 4, or 6 turns on the primary? Using 2843000202 binocular cores and 28 ga wire, I made three test articles and connected them via SMA connectors to my spectrum analyzer and tracking generator. Here are the frequency responses:

A Two-Watt Wideband Driver Amplifier





MEASUREMENTS




Gain:

    16 dB max, down 2 dB at 3

    and 30 MHz


Max Power at 7 MHz:

   +34.7 dBm, 3.0 W


Input return loss:

   >11 dB, 1 to 30 MHz

(SWR < 1.2:1)


Distortion at 2 W output:

  HD2  -48 dBc

  HD3 -25 dBc









 

Full power at 7 MHz is about +34.7 dBm (2.95 W) when higher-order harmonics begin to jump up. At 2 W, HD2 = -48 dBc, HD3 = -25 dBc. Here is a typical distortion spectrum.

I never did get a current draw measurement. It runs off of 12 V and the PC board was fabricated by the Sharpie method. A full ground plane is used on the component side. There’s an overkill heatsink, about 2 x 3 inches and 0.5 inches thick and is the same size as the board. It runs comfy-cool and is totally stable and blowup-proof.

Here is the final frequency response, which is reasonably flat though not spectacular. Considering how un-flat the power amp turned out to be, this is more than sufficient.

Performance

With too few turns, there is insufficient inductance and low frequency response suffers. With too many turns, a premature high-frequency rolloff occurs. The optimum appears to be the 4:2 turns ratio, so I went with that for the input transformer. It was flat within 0.3 dB from 3-30 MHz. The output transformer was a 4:3 ratio, plus a single turn for the feedback network.


Since the specified transistors were not on hand, I used some fairly generic low-cost devices (2SC2075), which meant that the input matching and feedback parameters from AN779 were invalid. So the tweaking began, attempting to flatten the response while maintaining a good input and output match. Having the tracking generator and analyzer, and a return loss bridge, were incredibly valuable.