Feedback amplifiers are marvelous, reliable circuits. They have gained popularity in the amateur literature thanks to many articles featuring them in the popular magazines like the QST, they have been extensively used in the works of Wes Hayward, W7ZOI.
This is a discursive video that I recorded for my club to show how one can easily design feedback amplifiers, build them and measure their performance. I have used the Antuino to do the measurements. The Antuino firmware that is used in this video is available on https://github.com/afarhan/antuino2.2
In the video, I have glossed over two important points: 0.6v base-emitter potential difference and towards the end I have rushed through the IMD measurment, these will be covered in a upcoming videos.
As an adjunct to the method in the video, based on the calculations set forth in Fig 2.69 of Experimental Methods in RF Design (Hayward, Campbell and Larkin, ARRL), I have created a small feedback amplifier calculator that is available below. To understand what’s going on, read W7ZOI’s introductory article and then try his Paper on Transistor Models. It is worth your time to understand what amplifiers do.
There are a few caveats though:
- This calculator does not handle complex impedances. It works as a rough estimation for HF to lower VHF range
- The emitter current Ie should be limited to 100 mA or less
- The transistor beta should be at least 5 times higher than the frequency of operation
You can play with values, however if you want to get started, here are some tips:
- The Ratio of Rf/Zin determines the voltage gain
- Keep the ratio of Rf/Zin same as Zout/Re
- For the small transistors like 2N3904, 2N2222, etc., keep Ie to less than 20 mA
- This model is very basic. For better modelling learn to use Spice and model it there
|N:1 N=||(Output Transformer)|
(The results appear below the circuit. The circuit diagram is to show what we mean by each resistor in the calculator).