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Conversely, a current source provides a constant current, as long as the impedance of the load is sufficiently lower than the current source's parallel impedance (which is preferably very high and ideally infinite). In the case of transistor current sources, impedances of a few megohms (at low frequencies) are typical. Because power is current ...
Current mode logic (CML), or source-coupled logic (SCL), is a digital design style used both for logic gates and for board-level digital signaling of digital data.. The basic principle of CML is that current from a constant current generator is steered between two alternate paths depending on whether a logic zero or logic one is being represented.
It can also be used to model a more realistic current source (since ideal current sources do not exist). The circuit topology covered here is one that appears in many monolithic ICs. It is a Widlar mirror without an emitter degeneration resistor in the follower (output) transistor. This topology can only be done in an IC, as the matching has to ...
The "long-tail" current source (R E) sets the total current flowing through the two legs of the pair. The input voltage controls the current flowing through the transistors by sharing it between the two legs, steering it all to one side when not near the switching point.
This is the only first order source of mismatch in the three-transistor Wilson current mirror [8] Second, at high currents the current gain, β, of transistors decreases and the relation of collector current to base-emitter voltage deviates from = (). The severity of these effects depends on the collector voltage.
Open outputs configurations thus differ from push–pull outputs, which use a pair of transistors to output a specific voltage or current. These open outputs configurations are often used for digital applications when the transistor acts as a switch, to allow for logic-level conversion, wired-logic connections, and line sharing.
Depletion-load processes replace this transistor with a depletion-mode NMOS at a constant gate bias, with the gate tied directly to the source. This alternative type of transistor acts as a current source until the output approaches 1, then acts as a resistor. The result is a faster 0 to 1 transition.
Figure 1: A version of the Widlar current source using bipolar transistors. Figure 1 is an example Widlar current source using bipolar transistors, where the emitter resistance R 2 is connected to the output transistor Q 2, and has the effect of reducing the current in Q 2 relative to Q 1.