Question about common-collector circuits.
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From: pawihte on 27 Nov 2009 12:58 Jon Kirwan 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. > > With a high supply rail, the collector resistor might allow a > smaller/survivable Vce for the emitter follower? I'm just > imagining > that the emitter voltage is Vb less a diode drop and that Vc is > the > supply rail (without the resistor), so it's possible to have > quite a > fair sized Vce and thus quite a lot of dissipation in the BJT > itself > if there is no collector resistor. Does any of that connect > with your > recollection? > It was about emitter followers in general and not about any specific design, and I'm fairly sure the example they gave had a 100-ohm resistor in series with the collector. I don't remember what the emitter resistor was, but I have a strong impression that it was a low-power circuit where the emitter resistor would keep dissipation well within safe limits.
From: John Larkin on 27 Nov 2009 13:04 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. John
From: pawihte on 27 Nov 2009 13:14 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. > Having a current-limit resistor on the collector side is > certainly a > trick to keep in your bag-o-tricks, but I wouldn't think you'd > need to > use it universally.
From: krw on 27 Nov 2009 13:20 On Fri, 27 Nov 2009 23:44:57 +0530, "pawihte" <pawihte(a)invalid.com> 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. Or, as mentioned earlier, you short the emitter. >> Having a current-limit resistor on the collector side is >> certainly a >> trick to keep in your bag-o-tricks, but I wouldn't think you'd >> need to >> use it universally. >
From: pawihte on 27 Nov 2009 13:26
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. >> > > 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. |