--- In BITX20@..., "Ruud Jongeling" <pe2bs@t...> wrote:
Hello fellow BITX'ers,
Because I want do do some experiments with a ladderfilter on 9 MHz, I
studied the amplifier stage Asshar discribed and used in his BITX20.
I have some questions about it.
- I understand that the input and output impedance depends on the
resistors Rf and Re: Rf*Re=Rout*Rin. In the exemple with Rf=1000,
Re=10 and Rin=50, Rout becomes 200. But what happens if the input
is 100? There will be a mismatch but where will it be? Is the output
impedance fixed because of Rload (220 in the schema) and will be the
mismatch at the input of the amplifierstage?
The putput impedence is affected by both the load resistor and
feedback resistor value. Mismatch under 1:2(2:1) is resonable
for thos application.
- I studied the ladderfilter theory and different C's means different
input and output impedance of the filter. How to match the amplifier
to these impedances? Witch resistors do I have to change for the
exemple of 100? How to calculate them?
- The last question is about the two amplifier stages around the
ladderfilter. Left of the filter the emittor resistance is 220. Right
of the filter the emittor resistance is 470. The other resistors
around Q2 and Q3 are the same.Why is this? Does this has anything to
do with matching the impedance?
The resistors you refer to set bias not impedence. The resistors in
series with capacitors (typical value 10 ohm) are the Re value that
set impedence. The input impedence (first order) is Re * (Beta/F)
[beta/F is the first order AC beta{current gain of transistor} at RF
frequency] and that will be in parallel the base bias resistor and
depressed further by the Feedback resistor. Changing Re and changing
Rfb both alter gain and input as well as output impedence. As you go
up in input impedence the range of values tends to be limited and
circuit stability decreases.
Actually I prefer to set the amplifier impedence and use either an L
network (C-L) or a RF transformer of the ferrite loaded type. For a
small sacrifice in gain a series or parallel resistor at the input and
output of the filter may also be used.
Allison
KB1GMX
