EMRFD Message Archive 9837

Message	Date	From	Subject
9837	2014-03-18 17:58:58	gt25psi2002	Biasing a NPN for Optimal Q-Point
Hi there, 1.    I've read a book saying that higher Ve (voltage across emitter) value is better for distortion free circuit. Does this imply that Ve can go as high the figure that make V across Ic is 0 i.e. Rc=0? [I've seen many good circuits using 2n5109 with Rc=0]. 2.    The above advice complies to another source (referring to a web site) saying that the lower Rbb/Re ratio is better. 3.    While another source referring to a web site says the ratio of Vc/Ve should be about 75%/25%. Please advise me which direction is correct. Regards.
9838	2014-03-19 00:54:42	Russell Shaw	Re: Biasing a NPN for Optimal Q-Point
9839	2014-03-19 07:07:09	Chris Trask	Re: Biasing a NPN for Optimal Q-Point
> >1. I've read a book saying that higher Ve (voltage across emitter) value is better for distortion free circuit. >Does this imply that Ve can go as high the figure that make V across Ic is 0 i.e. Rc=0? >[I've seen many good circuits using 2n5109 with Rc=0]. > It is correct to say that a higher Vce will resort in lower distortion as doing so will place the operating point away from the transition zone, where most of the distortion occurs. Take a look at my two papers on transistor selection, which show curve families for a variety of devices with some discussion about distortion: http://www.home.earthlink.net/~christrask/Paper015.html http://www.home.earthlink.net/~christrask/Paper016.html > >2. The above advice complies to another source (referring to a web site) saying that the lower Rbb/Re ratio >is better. > Haven't seen this comment. Could you provide a URL for it? > >3. While another source referring to a web site says the ratio of Vc/Ve should be about 75%/25%. > Haven't seen that one either (URL?). However, for an amplifier having a resistor from the collector to Vcc the best dynamic range is obtained when Vce is 0.5*Vcc. Far better dynamic range is obtained with an inductor or transformer winding between the collector and Vcc. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
9840	2014-03-19 08:01:51	gt25psi2002	Re: Biasing a NPN for Optimal Q-Point
Dear Russell Shaw & Chris Trask, Thank very much. The book I referred to is "RF Circuit Design" by Chris Bowick, 2nd Edition, page 128:- a).    "Equation 6-1 tends to imply that the higher VE is, the better". b).    "This equation (Eq. 6-2) indicates that once a transistor is specified, the only control that the designer has over the effect of β changes on collector current is through the resistance ratio RB/RE. The smaller this ratio, the less the collector current varies. c).    Whereas reference to "the ratio of Vc/Ve should be about 75%/25%" is a URL which I still cannot relocate where it is. Regards.