Linear Current Sources
Op Amp Current Sources
Because in the feedback path of inverting and non-inverting amplifier the current is proportional to the incoming voltage these two circuits may be used as linear voltage controlled current sources.
The current through is given by . The main drawback of this design is that the load connected instead of must not be grounded (or generally speaking: connected to a fixed potential) because this would cause a short circuit of the op amp's output.
TODO: op amp current source for grounded loads, derivations, input+output impedance
Transistor Current Sources
Simple NPN current sink
First, to make the transistor conduct, we choose a control voltage . Thus . The emitter current is
If the transistor is not saturated
which gives the current that the NPN transistor sinks.
Simple PNP current source
The derivation for a simple PNP transistor current source is similar to that for the NPN case. This time we have the current flowing into the emitter. If (breakpoint voltage of a typical BJT) the current sourced at the collector is given by
This configuration is useful for instance to drive the control current input of an OTA.
- simple FET current source
- precision current sources with transistor in feedback path of op amp
A current mirror is a special variety of a current controlled current source. The incoming current controls an outgoing current of opposite sign, hence the term current mirror. The simplest realisation is a voltage controlled current source in which the incoming control current is converted to a control voltage using a resistor according to Ohm's law. To create current mirrors with different input impedances one may instead connect a voltage controlled current source in series with a current to voltage converter (cf. OpAmpCircuits).
TODO: derivations, circuits, is this interesting because of OTAs or expendable?