Small Signal Analysis of DiodesSmall Signal Analysis of DiodesD. W. ParentSo far as long as we stay below the output rails or f3dbour circuit circuits work under any inputor f3dbour circuit circuits work under any input voltage.• Resistors, V=IRes sto s,– Sure R has a toler ance and parasitic capacitance and inductance, but that is still easily modeled.• Capacitors, Z(s)=1/Cs– Parasitic R and L• Inductors, Z(s)=Ls– Parasitic R and LOPAMPS•OPAMPS– Equations can be modified to work under all conditions (fdVOS rail to rail)conditions (f3dB, VOS, rail to rail)The full diode equation works under all conditions as well, but this does not mean it willconditions as well, but this does not mean it will act like a ln or exp function under all conditions.If t t thi di d hIf we want to use this diode we have to add or subtract a dc voltage to move our signal into the proper range.gOnly works anlnorOnly works an lnor exp function here.Many times in EE we are interested in making sure our circuit behaves in a linear gfashion!•EE110 and EE112 are all based on theEE110 and EE112 are all based on the assumption that the system being designed/studies is linear (time invariant asdesigned/studies is linear (time invariant as well).•OPAMPS are made up of:•OPAMPS are made up of:– Diodes, MOSFETS, BJTSN li !!!–Not linear!!!We use small signal analys is to design/analyze circuits th t li ltIt h t tthat use non linear elements. It has two parts.•SuperpositionSuperposition– We can break up the problem into a DC response, and an AC response, and then add them together for the full response.• Linearization of the nonlinear device –Find a DC “Q” point– Assume signal does not deviate from the point “t h”“too much”– Calculate conductances and resistances around this pointthis point.As easy example is a resistive divider because all h l li bi ihthe elements are linear to begin with.All the parts are linear so the equation is:Do the example according to small signal lianalysis Find the DC Q point.The resistance of the R2 is already Linear.Add AC and DC together and you get the full response!.OP can be used to rapidly find the DC operating point h th ili lti th iitwhen there is a non‐linear element in the circuit.VDCWorks when Vin~10mVFull ResponseHow to use ssa diode.ascHow to use ssa_diode.ascFullresponseFull
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