Current sources in small signal model analysis

[Silenus]

I've been working on some electronics questions regarding BPJ transistors and circuit gains and cant seem to find an answer to a question that affects my analysis. When you have an indpendent current source that is connected between a transistor and voltage rail in a given circuit topology, what do you do in terms of a small signal model? I understand these currents are there to bias the circuit to a particular operating point. Would that mean they are off (open circuit) in small signal model since we are dealing with small signal (not DC conditions)?

[mathias]

Exactly!

You might take into account their impedance, for example when you use a BJT current source to bias a differential pair. In that case, the finite impedance will cause common mode gain.

[LvW]

I've been working on some electronics questions regarding BPJ transistors and circuit gains and cant seem to find an answer to a question that affects my analysis. When you have an indpendent current source that is connected between a transistor and voltage rail in a given circuit topology, what do you do in terms of a small signal model? I understand these currents are there to bias the circuit to a particular operating point. Would that mean they are off (open circuit) in small signal model since we are dealing with small signal (not DC conditions)?

Yes - for a small-signal analysis based on a corresponding (small-signal) model all dc current sources are considered as an open circuit and all dc voltage sources are replaced by a short to ground.  However, dont forget that the VALUES of these sources play an important role for calculating the small-signal parameters used within the model.

[jpb]

Just to add to what every one else has already said, when looking at the small signal impedance of sources you just need to think about what changes (nothing) when you apply small changes of voltage or current.

A fixed current will not pass any more (or less) current what ever small signal change in voltage is applied so it looks like an open circuit. A fixed voltage doesn't change in voltage what ever change of current passes through it so it looks like a short circuit.

Most small signal current sources in transistor models are modeled as voltage or current controlled sources, e.g. the  transconductances gm in a FET model where a voltage change in one place (on the gate) leads to a current change in a different place (the drain current).