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??


--

Fat, sugar, salt, beer: the four essentials for a healthy diet.

Steve Evans ) writes:
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??


It's there to bias the transistor.

In this case, the base of the transistor (I'm assuming bipolar so I
say base) is at DC ground. But if you just shorted the base to ground,
the signal could not get into the transistor. So you put that RF choke there,
so the base sees ground at DC, but the signal sees a relatively high
impedance.

MIchael VE2BVW

The inductor keeps the transistor base at DC ground potential (probably the
same potential as the not-shown emitter). This makes the transistor only
conduct on positive half-cycles of the drive signal, which is a very
non-linear condition that generates lot's of harmonic content. It's also
common to put a little resistance in series with the inductor, which
slightly reverse-biases the transistor because the RF waveform can then
swing more toward the negative than the positive. A little reverse bias
causes the transistor to conduct over a smaller portion of the input cycle,
which enhances higher-order harmonic generation.

Joe
W3JDR

Steve Evans wrote in message
...
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??


--

Fat, sugar, salt, beer: the four essentials for a healthy diet.

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

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

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.

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

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??

A capacitor has a very low impedance to high-frequency (i.e., 145MHz)
signals and a very high impedance to low-frequency (i.e., DC) signals.

An inductor is the other way around - very low impedance to low
frequency (DC) signals and very high impedance to high frequency (2m).

The inductor allows DC bias currents to flow while not shunting the
desired 2m RF to ground.

The capacitor passes the 2m drive signal from the previous stage without
attenuation, while keeping the DC from the previous stage out of this one.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com