"Steve Evans" wrote in message
...
On Sun, 10 Oct 2004 14:01:44 +0100, Paul Burridge
wrote:

Well then it appears Reg's hunch was right. The transistor in question
has an input capacitance of just under 3pF, so the 0.4uH inductor
forms a parallel tuned circuit with it at 145Mhz. This prevents the
input signal from partially shunted to ground via the input
capacitance were the inductor not there. The idea is to allow as much
of the input signal as possible to develop across the BE diode to
maximise input impedance and gain. The input capacitance is in
parallel with this diode and bypasses RF signals around it - which you
*don't* want. Will a 1uH work instead? Do the maths and find out. But
if you don't know the inductor's Q that probably won't help much...

Sorry, but none of this makes sense to me. There's no diode involved
so I don't know where you get that from. And what's "input
capacitance" and "Q"?
Do try to speak in plain English!

Steve


Hi Steve (swell name, by the way),
From this last comment, it appears that you have a lot to learn. Paul's
was a pretty good explanation of a first step at understanding what might be
going on in the circuit shown some time ago (an inductor in shunt with the
base-emitter of the transistor).

The Base-Emitter in a transistor is a semiconductor junction just like a
diode and in the (higher power) RF amplifiers behaves pretty much like a
diode. With RF applied to the base, there will be conduction on the
positive peaks only and this will constitute a DC current flow which must
have a DC path. The inductor provides such a path since the capacitor can
not. If this makes no sense to you then you, indeed are in over your head
in an attempt to understand because it is pretty basic and simple. You will
need to understand diodes and transistors first.

you say you are "really struggling here with with some of the terminology."
Perhaps you can tell us which words are giving you heartburn?



I will respond to Paul's content, however, with this. The BE
capacitance of this device, in this aparent application, I am pretty sure is
not the dominant effect. The Rrverse biased capacitance is the wrong thing
to focus on. While it is interesting that that it and the inductor are near
resonance, this probaly is not what is happening because this would make the
inpedance looking into the base very high and difficult to get power to the
base, contrary to Reg's hunch. The orignal ASCII cricuit simply had a
coupling cap and a base-emitter shunt cap. It looks like class B or C. C
more likely. Therefore the transistor is in conduction part of the time and
not for another part of the time. Therefore we have a nonlinear, large
signal condition. The base impedance under this condition (pulsed
conduction) will be quite low and dominate and therefore, it will need some
impedance matching to get enough into the base (from the preceeding
collector). SO, I say that the inductor is :
1- Providing the obvious DC path and.
2- Impedance matching along with the "coupling" capacitor (did it have a
value??)...BUT!

The one monkey wrench I will throw in, is that the Miller effect will also
have a very significant effect on the input impedance of the stage. The Ccb
is a path providing significant feedback and probably dominating the input
impedance.

If you don't recall, the Miller Effect describes the capacitance looking
into the base which looks like Ccb times the voltage gain (call it A). This
is due to the fact that Ccb connects between the input / base and output /
collector. Because the collector voltage is ~~ 180 degrees out of phase,
with the base voltage, an input voltage change of, say one millivolt, on the
input side of Ccb results in a change in voltage on the output side of Ccb
of one milivolt times the voltage gain, A. This results in a total change
across Ccb of A+1 milivolts and therefore a current change A+1 times a value
that the 1 milivolt input change expected to see. This makes the capacitor
look A+1 times as big as it actually is.

Finally, and possibly the most difficult to quantify (ok two monkey
wrenches--nobody expects the Spanish inquisition), in RF circuits there is
*very frequently* one other confounding factor and this is the circuit board
layout and/or the actual physical construction. All the previous talk about
how inductors and capacitors behave differently at high frequencies (I
believe by Roy Lewallen) is nicely put, but the actual connection methods
also can have a very significant effect on what value components are used.
The "wiring" can add other capacitances and inductances which, very often,
do not show up on the schematic. This can have profound effect on the
components used, completely masking any hope of understanding of the circuit
from the schematic diagram. As the power level in the circuit goes up, the
impedances go down and short wires or PC board runs can become significant
impedances, either to help or hurt the desired matching circuit.
--
Steve N, K,9;d, c. i My email has no u's.