Can I use one base resistor for several transistors?

In the absence of emitter degeneration resistors, yes, you are missing something obvious.

Your Vbe will vary from one transistor to the next; with one base resistor you will find that one transistor tends to 'steal' the current from the bases of the other three transistors.

With four diodes the one stealing the current would get hot, reducing its voltage drop and making it steal even more current. With transistors that are supposed to saturate the effect would be more complex and less predictable (a half-on transistor will get hotter than a fully on transistor, but one that never really turns on won't get hot at all).

Just use four base resistors. It'll make your life easier in the end.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" came out in April.
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You can't. Unless the transistors' base-emitter voltages are matched (which they won't be), one transistor may hog all the drive current, leaving the other transistors wanting.

For a very light load and relatively heavy drive (e.g. i.drive >

i.load) you might get away with it, but there's scarcely any reason to try: resistors are cheap. But, you don't have that--you're using

4000-series CMOS.

HTH, James Arthur

Thanks, James, that does help.

BTW, speaking of the 4013 I am using, any idea how much current I can get out of it? None of the datasheets list current output, only that maximum dissipation for the chip is 700mW (which would be 140mA at 5V).

Thanks,

Jon

The 4013's drive capability varies with the supply voltage you use. My databook shows the high-state output current i(OH) to be at least

0.5mA for a 5V supply, increasing to 3mA for a 15 volt supply, at 25oC.

Cheers, James Arthur

If it's a true CD4013 then not much. 74HC4013's should be good for

5-10mA, 74ACT4013 more than that. Usually they'll give a high voltage at some current level -- that's usually what I design around, although you can usually get more current if you load the chip more.

My 1984 National Semiconductor data book lists the available current as

0.88mA typical at room temperature, 4.6V at the pin with 5V supply (0.36 minimum at high temperature). Think 'wimpy'.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Oh, I should mention those figures are for logic-level outputs. You can get a lot more current out of 4000B-series parts if you're willing to let the outputs sag, e.g. 2-3mA at 2 volts for a "high" output using a 5v supply. Not very much current any which way you cut it. Be sure not to exceed the power dissipation limits if you do this using higher supply voltages.

--James

get

maximum

Aye, thanks, James, I hadn't figured on the sag. I threw a resistor across the output to draw 50mA and it dropped down to 1.4V, so I guess I"ll have to use one transistor to switch the other ones.

Thanks again,

Jon

Or use FETs which don't draw any current while in the on or off state. Then you don't need base resistors either.

--
Regards, Joerg

http://www.analogconsultants.com

As Joerg noted, just use FETs and you don't have the problem unless there is some particular reason you need to use bipolars. The VN2222LL was designed as a replacement for the 2N2222 in a switch application.

Cheers

PeteS

This may have already been mentioned, but I am coming into this thread a little late. Anyway, putting four base-emitter junctions in parallel runs the risk that the base current will not divide equally. The individual variation in the base-emitter voltage means that matching is critical. Separate base resistors solves this problem.

Robert Scott Ypsilanti, Michigan

are

Thanks Pete, and also Joerg.

I already have the NPN jobbies, and haven't worked with FETs before, but I'll pick some up and play around with them. Am I reading you guys correctly that there is no Ibe when using a FET? Also, would I want to use the same base resistor for one (I'm using 2.2k for my switch mode)?

Thanks,

Jon

On FETs there is no "be" for base and emitter, it would be gate and source. And no, there will be no DC current into the gate. What you do have with large FETs is a substantial gate-source capacitance which datasheets usually call Cgs. The driver needs to charge that up and down and this can slow things down.

Resistors: FETs don't need any but be careful when the traces aren't short. A FET can oscillate and one of the remedies is a series resistor of a few ten ohms right in front of the gate. With proper layout you may not need it.

--
Regards, Joerg

http://www.analogconsultants.com

use

Interesting considerations; thanks for those, Joerg. I see they are pretty inexpensive also, like the regular jobs; I'll pick up a couple tomorrow and play around.

Thanks again,

Jon

You are welcome. Picking some up? That sounds like paradise. Out here there are no electronics stores anymore, it's all mail order now :-(

--
Regards, Joerg

http://www.analogconsultants.com

pretty

and

Aye, I'm kind of lucky there is a store (Norvac Electronics) that sells most of the NTE stuff, among other items.

Unfortunately they didn't have the NTE491 in stock (the NTE version of the VN2222LL), but they did have some other JFETs and MOSFETs there. I'll look over the datasheets to figure out which would be the best choice.

Jon

Every time I saw NTE parts they were way more expensive than regular parts via Digikey. So if you need a lot of components mail order might not be so bad after all.

--
Regards, Joerg

http://www.analogconsultants.com

I'll usually do an online order when I'm doing a big project, since the selection is so much larger, but when I just want a couple of parts it is convenient to just go get it right now. There's also the shipping costs to consider. As for prices on NTE vs. Generic, they might be a little higher, but I've never found it to be too much of a difference, at least on basic semiconductor items and common ICs.

One area that online is a whole bunch cheaper is with things like caps and resistors, though.

I used to order mainly from digi-key, but I pretty much just shop with Mouser now; they charge less for shipping and don't tack on $5.00 for when I only want under $25.00 worth of stuff.

Jon

One time, some years ago, I had a project come up, and concidentally I had just recently signed in at Newark's website. Well, just as this project was materializing, some Newark sales rep gave me a call (I had signed on as an "engineering director" or something ;-) ), and offered to open an account for me, right at the time that I needed to buy a bunch of parts for this project I had just been offered.

I don't know where I got the credit rating, but they sent me the stuff I needed - I just told the sales rep what I wanted, and, like, practically the next day, the stuff showed up by UPS or Fedex or USPS or whatever. The bill showed up in the snail mail about a month later. I'm happy with Newark. :-)

I also bought some stuff from Mouser, but they sent their bill along with the box of parts, and I sent them a check; I'm also happy with Mouser. :-)

I haven't tried Digi-key yet, but since I've known their name for so long, I wouldn't have any qualms about ordering from them if they can beat Mouser and Newark for pricing and delivery. :-)

Good Luck! Rich

Yes, for 1-2 parts it's better to buy local.

Mouser also had better prices on my last order. The reason why I still use Digikey for urgent stuff or large projects is that they have a search engine that is way better. Mouser needs to improve that, big time.

--
Regards, Joerg

http://www.analogconsultants.com