**EMRFD Message Archive 1470**

MessageDateFromSubject1470 2008-02-25 20:18:07 kanewderfish Zout of a 2N4401 tuned RF amp Hi gang,

I've been doing some experimenting with measuring some

impedance/bandwidth parameters of parallel tuned tank circuits. I

have been using the methods similar to those outlined in "Solid

State Design for the Radio Amateur" and "Experimental Methods in RF

design. Specifically, connecting a 1pF cap from a 50 Ohm generator

to the high side of the tank (other side of the tank to ground), and

another equal value cap from the same point to a 50 Ohm resistor

connected to ground. A 10X scope probe and 100MHz scope is then used

to measure the voltage across the 50 Ohm. Work is being done around

10MHz.

I appear to be getting reasonable results. What I really wanted to

investigate however, was the effect that a common emitter 2N4401 BJT

amp would have on a given tank circuit. So, I did the following:

I built up dead bug style a voltage divider bias with emitter

stabilization amplifier circuit with the following values:

Resistor from decoupled Vcc (9V) to base--150K

Resistor from base to ground-- 16K

Resistor in emitter lead--100 Ohms (with a 0.01uF bypass cap)

Collector connects to one side of parallel resonant tank, other side

of tank has 100 Ohm to Vcc and a 0.01uF cap to ground (decoupling

network).

Emitter current is ~1mA

There is no feedback network from collector to base.(Other than

internal effects and stray).

I tried connecting the fixture described above (1pf coupling caps,

and 50 Ohm load) to the collector/tank junction to compare the

bandwidth of a given tank circuit alone or connected into this

powered amp circuit. This was problematic as I saw multiple resonant

points that did not seem to correlate with the values obtained with

the tank alone. I presume this is due to the transistor circuit with

it's associated internal feedback paths upsetting the readings

somehow, but I'm not certain.

I then connected a 50 Ohm signal generator to the base through a

2.7pF cap, and coupled the collector to a 50 Ohm resistor through

another 2.7pF and powered up the circuit. This seemed to produce

much more stable results. Many of my results were as

expected.....the tank circuits are heavily loaded by what I presume

to be the output impedance of the transistor.

Here is a sample result:

A coil wound on a T50-2 to produce 2.717uH resonated with 99pF when

tested alone in the fixture described above, produced these results:

Fr......9620kHz

3dB bandwidth....56kHz

Q.....171.8

X....167.5 Ohms

As Z = Q x X Z = 28.76k Ohms

When I placed the same tank in the amplifier circuit as described

above, I got these numbers:

Fr.....9694kHz (A bit higher freq.......??)

3dB bandwidth.....1640kHz

Q.....5.91

X.....167.5 Ohms

As Z = Q x X Z = 994 Ohms

Since the raw tank Z was ~29k Ohms, and the value for Z was ~1k Ohms

for the transistor circuit, I assume the impedance at the collector

must be ~1k Ohms to cause that much of a drop.

Any thoughts? Does ~1000 Ohms seem plausible as the total Z seen at

the collector of this stage? Is my technique reasonable? "Why did

the 2 caps to the collector fixture" not work well?

BTW, I have more data with other tank values that seem to confirm,

(at least >my< understanding) of things like L/C ratios, loaded Q,

and bandwidth. If there is any interest I'll put it together and

post it.

Tnx

Bob WB0POQ