EMRFD Message Archive 824
Message Date From Subject 824 2007-06-03 16:59:17 Thomas Butchers Beta / Transconductance (was new with questions) All,
Wes w7zoi wrote:
Now regarding the beta question: It is not as bad as you might
guess. If you go to the amplifier discussion in Ch 2 of emrfd, you
will see that we can do gain calculations without using beta at
all. This is because we can often regard the bipolar transistor as
a voltage driven device with a transconductance of Ie/26 where Ie is
the DC emitter current in mA. This is not always true. However,
the better circuit designs are often those that are not beta
dependent. If you are ever around engineers who are designing
integrated circuits, you will note that beta rarely come up in the
I'm a little new to all of this so hopefully this is not too basic a
question. I'm not sure I understand the term transconductance. In my
research I see that transconductance is the ratio of output current to input
voltage; but what are the units? Conductance is in Mho's or now I guess they
are called Siemens but is there a unit for transconductance....
In working through the example given on page 2.3, using figure 2.10 and
assuming an input signal of 1 microvolt I get the following:
Ie = 1mA
Transconductance = 0.03846
Signal current = 38.46 nV
But I'm not sure of any of these numbers; based on that the voltage gain is
192.3 which is a little hard for me to believe. Am I missing something here?
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828 2007-06-04 10:54:13 Wes Hayward Re: Beta / Transconductance (was new with questions) Hi Tom,
You are correct with your units. The unit of conductance is the
siemen, which is reciprocal ohm. Indeed, we used to call
them "mho". Generally these days we use lower case when talking
about a unit, but upper case for a more direct reference to the
classic fella or gal who did the work. So we might measure a
resistance of 1000 ohms when we apply Ohm's Law. The "trans" in
transconductance just refers to "transfer," for we refer to the
current in a lead that is different than the terminals where we apply
Your calculated gain of 192 is quite correct for the transistor
biased with 1 mA of emitter current. Kick it up to a few mA and the
gain is even higher. On the other hand, adding 100 Ohms of emitter
degeneration to the transistor biased at 1 mA would put the net
emitter R at 100+26=126. The gain drops to 5K/126=40. If you run
the emitter current up to 10 mA with the 100 Ohm degeneration, net Re
is 102.6 and the gain is 5K/102.6=48, but it is now determined almost
completely by the resistors and not by any transistor properties.
Go ahead and throw some circuits together and measure some of these
things. It can do wonders to establish the things you read in your
831 2007-06-04 21:19:39 KF4YYD Re: Beta / Transconductance (was new with questions) Hey Wes,
Thanks for the input! Wow, looks like I may be learning something after
I don't know, guess I'll have to put off trying to buy a multimode/band
rig yet again (at this rate I'll never have one) and see if i can't
scare up some inexpensive test equipment. It would be nice to have a
scope to actually see what is going on instead of doing all this stuff
on paper and hoping.
832 2007-06-05 05:20:12 Mike Scott Re: Beta / Transconductance (was new with questions) Tom
50 MHz scopes are usually available at swap meets for under $100.
Sometimes you can find a 100 MHz scope for that price.
833 2007-06-05 21:59:01 jr_dakota Re: Beta / Transconductance (was new with questions) Buy a used scope, you can build most of the rest yourself ... many of
the circuits you experiment with can be used as, (or as a block in)
your own test equipment .... You experiment with a LC oscillator and
when you're done you have a RF signal generator
One of the most useful things I've found (I don't know how I did
without it for so long) is the digital milliwatt meter I built ...
Being able to measure gain, insertion loss, oscillator drive, etc
directly in db's makes RF design (and troubleshooting) a lot simpler
... Kanga US offers a kit that's even better than the one I built ...
EMRFD has an analog meter versi