How to design the front end of a radio

Discussion in 'Homebrew and Kit Projects' started by KG6NEI, Jun 8, 2008.

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  1. AC0FP

    AC0FP Ham Member QRZ Page

    A check of my designs (which can still be purchased today) show I always use a L-network to match 50 ohms to the input of the 1st LNA.


  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    There's a ton of great stuff on this, but you should start with the ARRL Handbook (almost any edition).

    A few ground rules before you start, however.

    1) GAIN is the easiest thing in the world to achieve. Anyone can build a really lousy preamp with a lot of gain. Don't go there.

    2) Noise is always a function of temperature. The lower the temperature, the lower the noise. There are two ways to keep the temperature down....low current or large mass. R.F. power transistors make great low-noise front ends for receivers!

    3) Never design a stage for maximum gain. Transistors are cheap. Design for stability...which means several low gain stages are better.

    4) Use lossless negative feedback if you can. This is a cool...but advanced...technique.

    5) Front end selectivity is a good thing.

    Have fun!

  3. AC0FP

    AC0FP Ham Member QRZ Page

    Modern transistors are "HotRods"

    Eric has a great point about gain. Modern transistors have a tremendous amount of gain and low noise figures available and there relatively cheap. Keeping that gain under control and maintaining stability can be a real challenge. Many times I have chosen grounded Gate JFET's (J310) for use up to and including two meters for this reason.


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