Question about common-collector circuits.
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From: Tim Wescott on 27 Nov 2009 13:34 On Fri, 27 Nov 2009 23:44:57 +0530, pawihte wrote: > Tim Wescott wrote: >> On Fri, 27 Nov 2009 14:42:22 +0530, pawihte wrote: >> >>> I remember reading a long time ago (I think it was a magazine article) >>> a cautionary note about operating a common-collector BJT >>> with the collector tied directly to the power supply rail. It >>> recommended inserting a resistor between the collector and Vcc. What >>> I don't remember is the reason given. The only thing I can think of >>> is where there's a possibility of the base being driven above Vcc, >>> forward biasing the base-collector junction. In that case, a resistor >>> in series with the base would also serve the purpose of >>> limiting the current. Is there another factor I haven't thought of? >>> TIA. >> >> A base resistor would have a much less reliable current limiting >> action, as the HFE of the device varies over temperature and from >> device to device. >> >> If you're only ever driving one thing and it's under your control and >> it won't pull too much current with the transistor on hard, then you >> don't need a current-limit resistor on the collector side. >> > I'm pretty sure the article was talking about low-power c-c circuits in > general where it can be assumed that there's enough resistance on the > emitter side to keep current and dissipation within safe limits, no > matter what the HFE is. The exception would be if the base is driven > higher than the collector without an appreciable resistance on either > base or collector. Remember that just because someone's an idiot doesn't mean they can't get published in a trade journal. Look at me -- I'm an idiot, and I have articles published in Embedded Systems Design. -- www.wescottdesign.com
From: Rich Grise on 27 Nov 2009 16:35 On Fri, 27 Nov 2009 14:42:22 +0530, pawihte wrote: > I remember reading a long time ago (I think it was a magazine article) a > cautionary note about operating a common-collector BJT with the collector > tied directly to the power supply rail. It recommended inserting a > resistor between the collector and Vcc. What I don't remember is the > reason given. The only thing I can think of is where there's a possibility > of the base being driven above Vcc, forward biasing the base-collector > junction. In that case, a resistor in series with the base would also > serve the purpose of limiting the current. Is there another factor I > haven't thought of? TIA. I can't think of any reason that makes sense, unless you've got a really, really crappy power supply, in which case the PS design needs to be fixed. Frankly, I think your writer was blowing smoke out his a$$. Don't forget the supply pypass caps, however. :-) Hope This Helps! Rich
From: Rich Grise on 27 Nov 2009 16:37 On Fri, 27 Nov 2009 10:04:27 -0800, John Larkin wrote: > On Fri, 27 Nov 2009 14:42:22 +0530, "pawihte" <pawihte(a)invalid.com> wrote: > >>I remember reading a long time ago (I think it was a magazine article) a >>cautionary note about operating a common-collector BJT with the collector >>tied directly to the power supply rail. It recommended inserting a >>resistor between the collector and Vcc. What I don't remember is the >>reason given. The only thing I can think of is where there's a >>possibility of the base being driven above Vcc, forward biasing the >>base-collector junction. In that case, a resistor in series with the base >>would also serve the purpose of limiting the current. Is there another >>factor I haven't thought of? TIA. >> >> > Possibly they were concerned about the possibility of a shorted external > load. > > But there's an immense amount of nonsense in magazine articles and web > sites. > To protect it from a shorted output, the proper place is in the emitter lead. To protect it from sharp transients on the BASE, then use a base resistor. (or slow it down with a parallel cap.) Cheers! Rich
From: Rich Grise on 27 Nov 2009 16:39 On Fri, 27 Nov 2009 23:44:57 +0530, pawihte wrote: > Tim Wescott wrote: >> On Fri, 27 Nov 2009 14:42:22 +0530, pawihte wrote: >> >>> I remember reading a long time ago (I think it was a magazine article) >>> a cautionary note about operating a common-collector BJT >>> with the collector tied directly to the power supply rail. It >>> recommended inserting a resistor between the collector and Vcc. What >>> I don't remember is the reason given. The only thing I can think of >>> is where there's a possibility of the base being driven above Vcc, >>> forward biasing the base-collector junction. In that case, a resistor >>> in series with the base would also serve the purpose of >>> limiting the current. Is there another factor I haven't thought of? >>> TIA. >> >> A base resistor would have a much less reliable current limiting >> action, as the HFE of the device varies over temperature and from >> device to device. >> >> If you're only ever driving one thing and it's under your control and >> it won't pull too much current with the transistor on hard, then you >> don't need a current-limit resistor on the collector side. >> > I'm pretty sure the article was talking about low-power c-c circuits in > general where it can be assumed that there's enough resistance on the > emitter side to keep current and dissipation within safe limits, no matter > what the HFE is. The exception would be if the base is driven higher than > the collector without an appreciable resistance on either base or > collector. > If that happens, there's something wrong with your design. And to protect it from overvoltage on the base, use a diode reverse-biased from the base to the Vcc rail, probably preceded by some resistance. Cheers! Rich
From: Rich Grise on 27 Nov 2009 16:42
On Fri, 27 Nov 2009 23:56:40 +0530, pawihte wrote: > John Larkin wrote: >> On Fri, 27 Nov 2009 14:42:22 +0530, "pawihte" <pawihte(a)invalid.com> >> >>> I remember reading a long time ago (I think it was a magazine article) >>> a cautionary note about operating a common-collector BJT >>> with the collector tied directly to the power supply rail. It >>> recommended inserting a resistor between the collector and Vcc. >>> What I don't remember is the reason given. The only thing I can >>> think of is where there's a possibility of the base being driven >>> above Vcc, forward biasing the base-collector junction. In that >>> case, a resistor in series with the base would also serve the purpose >>> of limiting the current. Is there another factor I haven't thought of? >>> >> I can't see any reason to do that in general, without some specific >> hazard to avoid. >> >> Emitter followers do tend to oscillate, but a series base resistor is >> a better fix than a collector resistor. >> > That's what I thought, to both statements. As I'm pretty sure the article > was about low-power common-collectors in general, in most cases there > would be enough resistance in series with both base and emitter to keep > currents and dissipation within safe limits. > > Anyway, my question does not concern any practical design I have in mind. > The thing keeps popping up in my mind now and then, and it bugs me that I > can't remember exactly why they made that point. Because, as has been noted, the writer of the article was an idiot. There could have been some valid reason, but if there were, you'd have remembered it because it'd have been so unusual. ;-) Cheers! Rich |