Hi, The whole concept of resistance or impedance assumes a linear circuit behavior (or at least it assumes that you want a linearized version of the real behavior). Metals and a few other elements do exhibit a remarkably linear relationship between voltage and current, so Ohm's law holds very well for them and resistance is a solid concept for these materials. Similarly, when you are dealing with ideal inductors and capacitors, extending resistance to the complex plane (giving impedance) allows you to analyze these linear dynamic systems easily. However, for semiconductors, there is often significant nonlinear behavior within the range of current and voltage which you are concerned about - so asking "what is the impedance of this transistor" may or may not be a very meaningful question. It can be helpful when you are operating the transistor within a small-signal region around a DC operating point, but if the transistor is going in and out of saturation or in and out of cutoff, it isn't such a good approximation and can be very misleading. FETs (both JFETs and MOSFETs) have a region where the drain-source current is fairly linearly related to the drain-source voltage (thus often called the ohmic region). There is no nicely analogous region of operation for BJTs. When a BJT is fully "ON" it is in the saturation region, and it roughly has a constant Vce independent of collector current. When the BJT is partially-on, it has a collector current which depends strongly on the base-emitter voltage and only slightly on the collector-emitter voltage. Thus, it acts like a current sink/source. If you compute the impedance of the collector-emitter pair, you'll get a very high value, even though significant current is flowing and the voltage is moderate (for example, 5V for Vce and 10mA current, but the impedance might be 100K ohms - remember that impedance is dV/dI not V/I - resistance can be either dV/dI or V/I depending on whether it is incremental or absolute resistance) There is also a rough relationship in this region between the base current and the collector current (Beta or Hfe or current gain) - but that depends greatly on temperature, manufacturing variation, and somewhat on Vce. Sean On Sat, Jun 18, 2011 at 11:29 PM, V G wrote: > On Fri, Jun 17, 2011 at 8:44 PM, V G wrote: > >> Hey all, >> >> I'm trying to charge (let's say) a single Li ion battery (float charge) = to >> 4.0V. >> >> The maximum charge current will be controlled with a BJT (tell me if >> there's a better way) and so I've been doing some simulations to see if = I >> can actually get 4.0V across the battery this way. >> >> Screenshot of simulation: http://solarwind.byethost7.com/pic4.png >> >> I'm pretending that R2 is the battery, gradually increasing its impedanc= e >> as it's state of charge increases. The voltage across the battery never >> seems to hit 4.0V. I'm doing another simulation with R2 from 1-100k and = that >> seems to get closer and closer to 4.0V as the current goes to 0 and the >> BJT's impedance rises rapidly. >> >> I can understand that at the beginning, the BJT needs to decrease impeda= nce >> as R2 increases in order to maintain the overall impedance so that the s= et >> amount of current goes through. I don't understand though, why just befo= re >> 40 ohms on the x axis, the BJT impedance increases again. Shouldn't the = BJT >> try and lower its impedance as much as possible so that the most current= can >> go through (even though the maximum amount allowed by the base current c= an >> no longer go through)? >> >> What is this minimum impedance and why is it there? >> > > Nobody? > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .