Saturday, September 5, 2020

putting a filter on an amp

The FB amp as built before gives a gain of 100 which is established by R3 and R4. Rc and Rl are equal so we see 1/2 on the output. If we make Rl 10 times Rc we will get a gain of 100 if the transistor is capable (Hfe or Beta are at play here also) This could be the RF amp on a BCB receiver and serve quite well but assume we have a short wave station or flame thrower of a local at the high end of the band what can we do? Consider the filter we used in the last post.

I placed the filter across the FB resistor and ran a frequency response. As you can see it will virtually kill any short wave signal. The filter I used earlier was set to 1600Khz. You can adjust the values and pass a higher or lower frequency.
This series was in response to a question about the LM386 output filter. I think a review of the datasheet may shed more light on that. I have hear more than once the "cheap" LM386 amps are trash and prone to oscillations. I have also heard the "expert" on a radio forum say the filter is not needed. This makes me wonder if they left the did not use the filter and created the stability issues? The number one issue we have as home builders is keeping the output from the input especially when higher frequencies are concerned.
Consider a short wave build operating at 4 Mhz with an AF amp that will amplify 4 Mhz. Now let the AF amp oscillate. You just "built a regenerative receiver".
Use a ground plane.
Keep lead short and run them close to the ground plane.
Separate (shield) input and output.
Consider using filters to control the band pass for your amps.
Most of all enjoy the hobby!

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