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??

The inductor provides a bias path to ground, to hold the average
transistor base voltage at zero volts, while passing the base
current. It also forms a resonant circuit with the capacitor (and
base capacitance) that has a peak response at some frequency,
hopefully in the middle of the band being amplified. This resonance
lowers the impedance at the input side of the capacitor and raises it
at the base node, stepping the input voltage up and the input current
down.

--
John Popelish

With a 1nf coupling cap, there's no impedance matching happening because the
capacitive reactance is so low that the impedances on both sides of the cap
are essentially connected together.

Joe
W3JDR

John Popelish wrote in message
...
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??

The inductor provides a bias path to ground, to hold the average
transistor base voltage at zero volts, while passing the base
current. It also forms a resonant circuit with the capacitor (and
base capacitance) that has a peak response at some frequency,
hopefully in the middle of the band being amplified. This resonance
lowers the impedance at the input side of the capacitor and raises it
at the base node, stepping the input voltage up and the input current
down.

--
John Popelish

Joe Rocci wrote:

John Popelish wrote in message
...
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??

The inductor provides a bias path to ground, to hold the average
transistor base voltage at zero volts, while passing the base
current. It also forms a resonant circuit with the capacitor (and
base capacitance) that has a peak response at some frequency,
hopefully in the middle of the band being amplified. This resonance
lowers the impedance at the input side of the capacitor and raises it
at the base node, stepping the input voltage up and the input current
down.

With a 1nf coupling cap, there's no impedance matching happening because the
capacitive reactance is so low that the impedances on both sides of the cap
are essentially connected together.

Right. I didn't pay any attention to the given values. They produce
a resonance around 8 megahertz.

--
John Popelish