Hi Partzman,
Yes, we've certainly discussed this before. But you may have possibly misunderstood what I was asking, because of my use of the term 'FET'. I'm thinking of a hypothetical source that limits bias current to a varactor to less than the datasheet's leakage current. For instance, say the varactor bias has a leakage current of 20 nA. This hypothetical gadget would supply way less than this current, say 20 pA in the 1-10 V range. The question then becomes, does this very low current still allow the varactor C to be changed, as bias V is raised?
F.
The polarization current is a question of a not perfect varactor, not a question of operating principle, it is a pure and simple loss of energy, in addition to that at the origin of the change of C and which will be due to the current already mentioned i(t) = dQ/dt = d(C(t).U(t))/dt. Indeed, the electrical charge being conserved, if the one retained in a capacitor changes, it is because it is input or output from the capacitor, so it is a current. There is no dynamic current if and only if C and U are covariant in the opposite direction. This is the case when the plates of a capacitor are moved away from each other: U increases proportionally as C decreases. The energy is proportional to U² so it increases. In this case, it is the mechanical energy of separating the plates that is transformed into electrical energy, and the charge is conserved. In the case where the variation of C is obtained by modifying U, we always have this work of separating the charges of the 2 plates, this follows from the Coulomb law. Here this work is obtained from U, we will consume a power U(t).i(t) = U(t) . d(C(t).U(t))/dt. For this reason, even if the static bias current were zero, or limited, we would still have COP <= 1. The trick of a parametric system is to tap a free energy to change the parameter (heat, ZPE, nuclear, electronic spin...). If we use the same energy as the one we want to produce, in this case electrical energy, and on the same port as where we want to recover it (a capacitor electrode), we go round in circles.
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Topic: parametric pumping of L's and C's (Read 48319 times)
