[HomeBrew] 72 MHz amp

[John Marshall]

Hi Richard,

Your diagram looks fine to me as long as I view it in a fixed width 
font.

I don't see any obvious major problems with this circuit.  The .01uF 
bypass cap in the emitter circuit might be a suspect.  If that cap has 
a large parasitic inductance (or long leads) it won't provide an 
effective bypass at 72 MHz and the 22 ohm resistor will add to the 
degenerative feedback and reduce the gain.  I would try a small .001uF 
ceramic cap with very short leads here.  I would use similar caps for 
input and output coupling, but I doubt those are the problem.

Your collector choke looks like it's sized just right, assuming a 50 
ohm load.  I've occasionally misread choke markings.  If you had a 
larger value choke with a lot of self capacitance, it might kill the 
gain.  I've used 2 - 4 turns through a small ferrite bead as a 
collector choke with good results.

GL es 73,
John, KU4AF
Pittsboro, NC


On Nov 28, 2004, at 10:56 AM, R. Blackman wrote:

> ** Please do NOT cross-post messages in the TO: or CC: field of the 
> e-mail message. **
>
> Hi to the group.
>
> I am trying to build a low power amp for 72 MHz IF in a homebrew
> transceiver.
> The following circuit  works great  at 25 MHz but is dead at 72 MHz.
> Any advice would be appreciated.
>
>
>                   |--------------|-----2uH---------12vdc+
>                 1000 ohm         |
>                   |              |
>                   |              |--------0.01uf --output
>                   |              |
>                   |  2N3866  b  /  c
> input---0.01uf----|------------[
>                   |             \--2.2 ohm----|-- 22 ohm---gnd
>                   |          e                |
>                   |                           |
>                  470 ohm                      |--0.01 uF---gnd
>                   |
>                   |
>                  gnd
>
> Thanks for any comments.
> Richard / VA3NDO
>
>
>  
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