824	2007-06-03 16:59:17	Thomas Butchers	Beta / Transconductance (was new with questions)
Message	Date	From	Subject
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 discussions. 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? Tom kf4yyd "Beer is proof that God loves us and wants us to be happy." -Ben Franklin No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.472 / Virus Database: 269.8.7/829 - Release Date: 6/2/2007 5:26 PM
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 the voltage. 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 mind. 73, Wes w7zoi
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 all. 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. Tom kf4yyd
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. Mike AE6WA
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