On Sun, 03 Oct 2004 15:08:22 GMT, Steve Evans
wrote:

Hi everyone,

Below you will find my attempt to show in text-form, a circuit
fragment from a 145Mhz amplifier:


--------------capacitor-------------------------------transistor base
|
|
I
|
coil
|
|
|
|
------------------------------------------------------------GND

The cap's value is 1nF; the inductor's is 0.4uH.
The cap (I assume) is to couple one amplifier stage into the next
(50ohm source/load) with minimal attenuation of the desired VHF
signal. But like what's the purpose of this inductor to ground??

---

At 145 MHz, the reactance of the cap is:

1
Xc = -------
2pifC


1
= ------------------------------ ~ 1.1 ohms
6.28 * 1.45E8 Hz * 1.0E-9 F

So it's likely not effecting a match to 50 ohms.


The reactance of the inductor is:


Xl = 2pifL



= 6.28 * 1.45E8 Hz * 4.0E-7 H ~ 364 ohms

so they're not resonant at 145MHz.



Since the resonant frequency of the LC is:


1
f = --------------
2pi(sqrt LC)

it's tuned to

1
f = ----------------------------- ~ 7.96MHz
6.28 * sqrt (4E-7H * 1E-9F)

which is nowhere near 145MHz.


If that's all there is to the circuit, my guess is that it's a
highpass filter with the coil doing double duty as a DC return for the
base as well as a fairly high reactance load for the driver. Also,
(WAG) since the transistor's input resistance and capacitance will
appear effectively in parallel with the coil, it may wind up looking
like something closer to 50 ohms than 364 ohms to the driver.

--
John Fields

The input impedance of the transistor is capacitive. So the inductor very
likely resonates with it at the working frequency.
----
Reg, G4FGQ

On Sun, 3 Oct 2004 19:20:22 +0000 (UTC), "Reg Edwards"
wrote:

The input impedance of the transistor is capacitive. So the inductor very
likely resonates with it at the working frequency.

You might be on to something here, Reg. Maybe the inductor's there to
'neutralise' the transistor's input capacitance.
The parallel tuned circuit formed by the inductor and the transistor
input capacitance would have a maximum impedance at 145Mhz if the
transistor's (capacitive) input impedance were about 3pF., which
doesn't sound far out for an RF small-signal tranny. Without that
inductor, sure there'd be no bias on the base, but additionally, the
input capacitance of the transistor will shunt away much of the VHF
input signal to ground.
Does that make sense?
--

"What is now proved was once only imagin'd." - William Blake, 1793.