On 19/06/2011 16:57, V G wrote: > On Sun, Jun 19, 2011 at 12:40 AM, Oli Glaserwrot= e: > >> Because of the bipolar saturation voltage. A FET would be better as a >> pass element, which is why they are used in LDOs. >> It seems you are still seeing the bipolar as a variable resistor. It >> doesn't work like that - better to see it as a voltage controlled >> current source. >> > No no, I'm not. I fully understand that it's a voltage controlled current > source. I just want to observe how the dynamic impedance of the BJT chang= es. > I see - my mistake, sorry. For the dynamic impedance see below. >> Try your simulation again, but set the base current to 1uA and plot >> this: d(V(n002))/d(Ic(q1)) >> This will give you the dynamic impedance of the collector. >> > Is it the derivative d(V)/d(Ic)? If so, what is the idea behind that? To calculate the impedance of the collector d(Vc)/d(Ic) However (I didn't notice the step simulation) you would have to do it=20 with a transient simulation and ramp the collector voltage. Hard to do=20 it with a step .op simulation. It can probably be done but I haven't=20 time to look at the manual right now. I think Sean explained it well. Dynamic as opposed to static, so on a=20 computer where it is a finite difference based derivative=20 (Vx-(Vx-1))/(Ix-(Ix-1)) rather than V1/I1, I2/I2 and so on. So if two points of a changing voltage and current are: V1 =3D 2, V2 =3D 1= ,=20 I1 =3D 1.0001A, I2 =3D 1A (V1-V2)/(I1-I2) =3D 1/0.0001 =3D 10Kohm as opposed to: V1/I1 =3D 2ohm and V2/I2 =3D ~1ohm I have probably not explained it very well. Give it a go - run the=20 simulation and plot the formula I gave you: set the base current to 1uA,=20 resistor to 1 ohm, ramp the collector voltage from 0 to 10V in a=20 transient simulation, and see what you get. Should give you an idea of the collector impedance. If you have problems=20 let me know and I'll send you an LTSpice file over. The Art of Electronics goes into great detail on transistor models, and=20 explains it all very clearly. There are other books which go into more=20 mathematical detail, but the AOE explanation is sufficient to understand=20 enough to use them in most applications. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .