On Sun, Jun 19, 2011 at 1:06 AM, Sean Breheny wrote: > 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. > I thought I was operating in the DC small signal region. Aren't I? > 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) Could you please explain this? I don't understand this > 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. > > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .