[Amps] Amplifier Input Circuit Consultation

Manfred Mornhinweg mmornhin at gmx.net
Fri Oct 24 13:33:13 EDT 2008

Hi Wayne,

> The 4-1000 has a generally accepted input impedence of 100 ohms.

It depends on the actual operating conditions. But for typical 
conditions, yes.

>  So I am
> planning on using a 2:1 step up unbalanced to unbalanced balun

I can't help making a somewhat pedantic correction here: A transformer 
that is unbalanced both at the input and the output is an unun, not a balun.

This transformer can be wound as a simple autotransformer with the tap 
at 71% of the total turns. And if you need transmission line effect to 
broaden the response, a trifiliar transformer wired as 2:3 stepup will 
produce an impedance ratio of 2.25, which is probably close enough.

 > followed by
> a parallel tuned circuit, Q of 2-3, for each band.  Taking this approach I
> will need only one relay contact per band to switch each tuned circuit (as
> opposed to two needed to switch a pi network).

I don't see a problem. But of course you must evaluate whether saving 
those additional contacts justifies the inclusion of a broadband 
transformer. The additional contacts might be less expensive, and having 
PI circuits there instead of simple tuned circuits gives the possibility 
of tuning the input for best impedance matching AND frequency respone. 
With your approach instead you can tune ONLY the frequency response, 
while the impedance ratio is fixed.

So I think that while the approach with the unun and tuned circuits 
should work, the approach with PI networks will probably produce better 
input SWR.


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