# BJT simple inverter

Discussion in 'Homework Help' started by dun_noe, Oct 22, 2004.

1. ### dun_noe Thread Starter New Member

Oct 22, 2004
2
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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. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
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.

3. ### dun_noe Thread Starter New Member

Oct 22, 2004
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But I still confuse, I get the same results if I try to simulate my circuit using PSIPE.

Thanks....

4. ### bebop New Member

Oct 24, 2004
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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. ### cyberhehe Well-Known Member

Oct 6, 2004
61
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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. ### avionicsgirl New Member

Oct 30, 2004
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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. ### cyberhehe Well-Known Member

Oct 6, 2004
61
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I think most discrete (transistor) switches operate in a common Emitter configuration, whether you implement it in NPN or PNP configuration.

8. ### amriths04 New Member

Nov 24, 2006
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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