Gang,
1.? First of all you are going to need something like the
following RF probe to measure the output of the f_T
tester we have been discussing in previous posts.
There are a lot of RF probes that use the one diode,
a 4.7M resistor and ....? For this project I wanted more
sensitivity.
RF probe? <>
The reason for this is that I find that with the Rigol scope there
is some jitter on the max values of the output from the
CAL and OUTPUT.? I am going to use the probe and
an analog VTVM, B+K Model 177 VTVM, for a more
steady reading.??? This meter I found at a swapmeet
for $20 USD and I refurbished to like new.? Big meter.
2.? You will need the f_T fixture found at
f_T fixture <>
Copyright by Charles Wenzel.? No call sign that I see.
This will now allow you to check all your transistor stock,
old and new (see below), and document your builds.
Call it f_T#1 by CW for your measurements using the original
unmodified version.
Now make your measurements according to the instructions
in the schematic.? V_in=1.0V_pk or 0.707V_RMS.? Whatever
voltage you get a CAL, and it may not be 100mV, make that
the voltage out, V_OUT, to increase f_input to match.? Then multiply
the f_in by 20 to get f_T.? Easy as pie.? They just need to match.
The value is not critical.
3.? [Advanced]? Now, Steve, AA7U, and I have mods to the fixture.
Simple.? Change the 150ohm resistor from V_cc to the 1K and 82ohm
node to a variable resistor.? I used a 2K and Steve uses a 500 ohm
variable.? The 500 will work better, but I didn't have one.
Use this variable to adjust the bias to get 10mA through the 100ohm
emitter resistor for the transistor you are testing.? Since Steve and
I are doing measurements for the V_ce in the spec sheet, we have
a variable lab supply for the 5V source and we vary it to get the
V_ce in the datasheet and adjust the variable R to get 10mA.
You measure V_c, the collector voltage to ground, subtract 1.0V
to get V_ce.? This is going to be like adjusting a regen receiver.
Also, use a SPDT switch and just switch V_CAL and V_OUT to your
voltage measurement scope or meter so you won't be moving
leads back and forth.? Idea from Steve, AA7U.? Speeds things up.
4.? Cheap transistors. <>
I have the 1,000 2N3904 NPN transistors received yesterday.? Using the
f_T#1 configuration, so that you can compare your 2N3904s to these, here is
what I get.? The cost was $14.64 USD from the site.? He has about 50,000,000
left, so rush and get yours before they run out.? There are some other
transistors there and 1,000x2N7000 JFETs for $23.89+$7.94S&H.
Now I did the following measurements as a first pass test to determine how
critical the V_ce voltage is to the resulting f_T.? It does matter. Also, I'm
using the Rigol scope, so mileage may vary until I can get round to it
using the diode probe and eliminate the small jitter.
V_cc replaces the 5V supply in the schematic.
V_c is voltage from the collector of the DUT to ground.
V_ce is 1.0V subtracted from V_c, since there is a 1.0V
drop across the 100 ohm emitter resistor due to 10.0mA
current through it.
V_cc?? ? ? I_C??? ? ?? V_c???? V_ce?????? f_T
5.25V? 10.0mA? 1.58V ? 0.58V? ? ----? couldn't get a match V_OUT too low
9.00V? 10.0mA?? 5.42V? 4.42V?? 104MHz
12.0V? 10.0mA?? 8.44V? 7.44V?? 142MHz
23.6V? 10.0mA? 21.00V 20.0V?? 176MHz
h_FE=331 as measured on the Chinese Critter Checker
h_FE between 100 min and 300 max on Philips Semiconductor datasheet at I_C=10.0mA.
Don't know what the emitter current is in the tester, not sure it's a valid comparison.
f_T is shown to be 300MHz minimum in the datasheet.
Because the h_FE=331 value, I'd expect the f_T value to be lower, but by how
much is a guess.? At f_T=176MHz, the transistors should be good enough for
up to 0.1*f_T or so, but that is to be determined.? For smaller quantities
and better quality, you may want to shop around.? I'm just pointing
out how cheap these things were and I have a large number of projects
to experiment using them in.? It would be a waste to not use them up.
They came in 12 days from the order date.
These are the transistors I plan on using for the Manhattan projects.
Attached is photo of bag open containing the 2N4401 critters.
FYI
--
chuck adams, aa7fo
QRP WAS 17m started Apr 2, 2025
WAS-17 QRP --> TX,FL,SC,GA,OK,AR,KS,AL,AZ,MN
PA,CA,CO,WI,NE,ID,IL,NC,NY,AL
MA,OH
DXCC W,VE,JA,PY,ZL
