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#1
10-22-2004, 07:21 PM
 dun_noe New Member Join Date: Oct 2004 Posts: 2

Hi all,

I have to do laboratory report for electronics, the laboratory is about the simple BJT inverter circuit. The circuit consists of one RB = 6.8 k (base resistor), one RC = 1 k (collector resistor), and one transistor 2N2222. A 10 kHz TTL signal (from the TTL output of the Function Generator) is supplied to VIN, and the output waveform is observed on the oscilloscope. Then this experiment is repeated for RC = 10 K.

After I observed the waveforms, the switching time for circuit with RC = 1 K is less than the switching time for the circuit with RC = 10 K.

But I get stuck with this question, why the RC value affects the switching time of the output voltage, I've been searching in my textbook and also over the internet, but I still can not understand why.....

Thank you very much for your help
#2
10-23-2004, 01:57 AM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

Hi,

That's a wierdie, alright. I would have expected the 2222 to switch faster with the 10K collector resistor. With 6.8K in the base, the transistor is nowhere near saturated. Just for giggles, drop it to 4.7K and see what happens. That won't really saturate the base either, but 4.7K seems to be the value that makes things work right.

Pure superstition? Probably, but it can't hurt.

There's another variable, too. A 2N2222 in a TO-18 can works differently from a 2N2222A in TO-92 epoxy. The TO-18 works better, for some reason.

Seriously, the transistor has to take less time going into full conduction with a 10K collector resistor than with a 1K. A smaller base resistor should speed things up, too.
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#3
10-23-2004, 08:44 PM
 dun_noe New Member Join Date: Oct 2004 Posts: 2

But I still confuse, I get the same results if I try to simulate my circuit using PSIPE.

Thanks....
#4
10-24-2004, 08:21 AM
 bebop New Member Join Date: Oct 2004 Posts: 1

Quote:
 Originally posted by dun_noe@Oct 23 2004, 03:44 PM Hi, thank you very much for your reply. But I still confuse, I get the same results if I try to simulate my circuit using PSIPE. Thanks.... Quoted post
I'm not that familiar with transistors, but I think your answer has to do with how fast charge carriers are moving between base and collector.

With a bigger collector resistor, will the transistor spend more time in the active region during a transition from on/off or off/on due to the speed charge carriers are limited to moving?

Just an idea. Can anyone elaborate or explain in more detail?
#5
10-27-2004, 08:33 AM
 cyberhehe Member Join Date: Oct 2004 Posts: 60

Quote:
 Originally posted by bebop@Oct 24 2004, 04:21 PM I'm not that familiar with transistors, but I think your answer has to do with how fast charge carriers are moving between base and collector. With a bigger collector resistor, will the transistor spend more time in the active region during a transition from on/off or off/on due to the speed charge carriers are limited to moving? Just an idea. Can anyone elaborate or explain in more detail? Quoted post

Hi Dun_noe,

Did you consider the capacitive effects of your scope when you do the measurements? Assuming there is indeed a capacitance around, you should consider this cap specially when doing the turn off (high to low transistion) in the output side. If we assume the capacitance will be constant, then for a 10K RC, will have definitely slower fall time, than the 1K resistor.

Now in your PSPICE, simulation, you should try to place this cap equivalent from your collector output path to Ground and do the simulation again, this way I think you may have similar results.

Cyber
#6
10-31-2004, 01:10 AM
 avionicsgirl New Member Join Date: Oct 2004 Posts: 7

what configuration is this (ie where is DC ground) CE, CB, or CC
the DC bias of the circuit is coming in on the Collector because your AC signal (by the way was it square, triangle or sine - sounds like square because you say 'switching') is coming in on the Base and GND is on the Emitter.... right?

your results are most likely correct. if i could see a rough schematic of what you are doing, i could do the math.
#7
10-31-2004, 11:57 PM
 cyberhehe Member Join Date: Oct 2004 Posts: 60

Quote:
 Originally posted by avionicsgirl@Oct 31 2004, 09:10 AM what configuration is this (ie where is DC ground) CE, CB, or CC the DC bias of the circuit is coming in on the Collector because your AC signal (by the way was it square, triangle or sine - sounds like square because you say 'switching') is coming in on the Base and GND is on the Emitter.... right? your results are most likely correct. if i could see a rough schematic of what you are doing, i could do the math. Quoted post

I think most discrete (transistor) switches operate in a common Emitter configuration, whether you implement it in NPN or PNP configuration.
#8
11-24-2006, 04:59 PM
 amriths04 New Member Join Date: Nov 2006 Posts: 8

the collector resistor rc and the stray capacitances inherently present in the transistor together give a time constant of (Rc*Cstray). thus if Rc is high, it takes a longer time, else a shorter time.

regards,
amrith.s

 Tags bjt, inverter, simple

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