🚀 go-pugleaf

I would suggest treating the problem as a DC coupled amplifier rather than
a narrowband RF approach. Make a preamplifier as a differential pair (or
AC couple into a single transistor amplifier). Instead of using a matching
network to match the input impedance, synthesize the input impedance with
feedback. If you apply feedback around a fixed gain, high input impedance
amplifier (inverting, of course!), the input impedance is Rf/(gain+1). So
if your preamp has a gain of -9V/v, and a feedback resistor value of 500
ohms, you will have an input impedance of 50 ohms. This will be a
broadband amplifier; low end set by either DC (for a diff pair) or by your
AC coupling. High end will depend both on the response of your amplifier
open loop and on parasitics associated with Rf. You will get fewer
parasitics if you do NOT use spiral trimmed metal film resistors. Use
either chip R's or non-spiral trimmed resistors (such as from Precision
Resistive Products). The spiral trim gives the resistors extra L and C.
You can also use carbon comp resistors, but I've found that they have
about the same capacitance as spiral trim for some reason.

Steve Smith

Nev wrote:

> I am interested in building a high gain / low noise amp to use in
> masthead aplications for TV reception.
>
> The freqs response required is 180 to 200 Mhz
>
> I have built narrow band units using BF981 and other dual gate Mosfets
> and gasfets but cant seem to get the response to be broadband enough
> for this application.
>
> Yes I could buy one ...... but wheres the fun in that !!
> Some of the kits available (ie MAR series, OM350, uPC 1688G etc )
> are too broadband and have NF of over 4 Db
>
> Suggestions / circuits appreciated
>
> Nev   ynotme2@lycosmail.com