transistor curves - Page 1
Forum section: electronics / beginners

author: KTP
author_rank: Frequent Contributor
timestamp: May 25, 2010, 05:37:59 pm
content: Hi again guys,We have a lab due this week on mosfets, and one of the sections requires plotting of the output characteristics of a particular fet (drain current vs drain-source voltage for several values of gate-source voltage). The procedure was to set Vgs and then using two multimeters, measure the voltage across a 100 ohm drain resistor and the voltage at the drain over a range of 0 to 10 volts presented at the resistor. We were required to do this for Vgs values of 2V to 5V in 0.5 volt increments. A *lot* of measurements!After performing these hundreds of measurements, I had the idea to use our supplied velleman usb 2 channel scope/fg (utter crap, but it does work for the coursework so far) to sweep the mosfet drain resistor with a ramp or triangle wave of 10Vpp amplitude with a 5V dc offset and use the two channels to plot the IV curve in real time. I had a problem though, because I couldn't use the two oscilloscope probes to get the voltage across the drain resistor (to get drain current) AND get the voltage at the drain itself because of the common ground. If I used the difference in channel 1 and channel 2 to get the drain current, then I would need a third channel to get the drain voltage or find a way to plot ch1 - ch2 vs ch2 on the scope, which didn't seem possible. I knew I would want to run the scope in x-y mode, so I came up with the idea of using a differiential amplifier op-amp configuration to give me the voltage across the drain resistor referenced to ground and also amplified by 10. This gave me a single channel that represented drain current at 1mA per division and allowed me to use the scope in x-y mode to plot this vs the drain voltage (with the triangle wave set at 50hz)...beauty! I now could investigate any Vgs curve for the mosfet instantly on the scope screen instead of having to make measurements by hand and plot.I posted my circuit and description on the class mailing list but nobody seems interested....I guess they are just there to get the grade and a job. Kind of sad...I was hoping to have people to share ideas. Anyway, I attached a picture of the x-y output for several values of Vgs so you can see the pretty curves. They matched my multimeter measurements exactly.

author: armandas
author_rank: Frequent Contributor
timestamp: May 25, 2010, 06:25:18 pm
content: Good stuff. I'm not surprised that students are not interested though. I can imagine many of my course mates going into sales after graduation

author: A-sic Enginerd
author_rank: Regular Contributor
timestamp: May 25, 2010, 06:33:44 pm
content: I've found that when you step that far outside the box and come up with something really cool in the classroom, if you don't hear any interest it's because they aren't able to keep up with you and have no clue what you just did. Hell, it even happens on the job sometimes.

author: A-sic Enginerd
author_rank: Regular Contributor
timestamp: May 25, 2010, 06:34:21 pm
content: Oh yeah - nice job!

author: Polossatik
author_rank: Frequent Contributor
timestamp: May 26, 2010, 08:19:01 pm
content: The first few times you try to explain, then you just give up and use your ingenuity to your own advantage (and maybe share it with the few people who are actually interested) .If they want to do it the hard way, let them. Then you can use the gained time to do something that is actually interesting....To avoid to be daily confronted with the same kind of "blank" reaction , most of the time I limit the amount of info to the level I hope they can handle/ are interested in and then move on.I'm a product expert (software) and get sometimes questions why I document so much of the things I know/discover/test for all to see in our organization, if i'm not afraid that this will make me less "indispensable" (really.. just the idea alone that someone would be "indispensable" for a multibillion $ company... people are silly.).Here is your answer, most people don't want to know... and the few who do can actually learn *you* somethingsonever be afraid of sharing, but don't expect lot of reply's/interest , even even from people who should

author: Simon
author_rank: Unknown
timestamp: May 28, 2010, 08:45:41 pm
content: Quote from: polossatik on May 26, 2010, 08:19:01 pmKind of reminds me of where I work, they knew about back emf diodes on relay's but didn't figure they were needed for motors too duuuuuuhhhhh

author: EEVblog
author_rank: Unknown
timestamp: May 29, 2010, 03:00:33 am
content: Quote from: KTP on May 25, 2010, 05:37:59 pmExcellent work!And yes, sadly that is very common. It's not unusual to be the only one in the class or course who actually takes an active interest and passion in what they are studying.Dave.

author: TheWelly888
author_rank: Frequent Contributor
timestamp: May 29, 2010, 09:19:05 am
content: KTP - Your circuit would be of more interest to this forum, how about a diagram please? Just to see if I can do the same again with my analogue scope.

author: KTP
author_rank: Frequent Contributor
timestamp: May 29, 2010, 07:06:20 pm
content: Thanks for all of the nice responses...I wish our class were as interested in electronics as you guys are!TheWelly: it is pretty silly simple, but attached to this post is the diagram I drew using LTspice (I didn't bother to try and make a simulatable circuit here, just using LTspice only as a drawing program in this instance). You just need a 2 channel scope capapable of displaying in x-y mode (but are there any that don't?) and a function generator capable of producing a ramp or triangle wave.

author: TheWelly888
author_rank: Frequent Contributor
timestamp: May 30, 2010, 10:09:18 am
content: Thanks! I'll have a go with that once I get the bat detector project out of the way!