Question about common-collector circuits.
in [Basics]
|
Prev: anti spam technique?
Next: photons and reflection
From: pawihte on 27 Nov 2009 04:12 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.
From: Jon Kirwan on 27 Nov 2009 05:56 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? Jon
From: krw on 27 Nov 2009 10:29 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. It's normally used to limit the current in the circuit. If the base voltage rises the emitter current rises, causing the collector current to rise and the collector voltage to fall (I*R in the collector resistor). Eventually the CB junction becomes forward biased, the transistor saturates, "Beta" (Ic/Ib, anyway) drops, and the current is limited at that point. You also may need a small resistor (~50R) in the base to keep the transistor form oscillating.
From: John Larkin on 27 Nov 2009 12:01 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. John
From: Tim Wescott on 27 Nov 2009 12:23
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. 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. -- www.wescottdesign.com |