Question about common-collector circuits.
in [Basics]
|
Prev: anti spam technique?
Next: photons and reflection
From: krw on 27 Nov 2009 17:01 On Fri, 27 Nov 2009 13:39:56 -0800, Rich Grise <richgrise(a)example.net> wrote: >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. No, a collector resistor does two things; reduces the transistor's power dissipation and limits the emitter current. The collector resistor is quite common, particularly if the emitter is a primary output.
From: John Larkin on 27 Nov 2009 20:08
On Fri, 27 Nov 2009 13:37:49 -0800, Rich Grise <richgrise(a)example.net> wrote: >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. That would increase the output impedance. A collector resistor acts more like a hard current limit, with little effect up to the point of limiting. To protect it from sharp transients on the BASE, then use a base >resistor. (or slow it down with a parallel cap.) A cap from base to ground is an excellent way to get an emitter follower to oscillate at 100 MHz or so. John |