Constant-Current Source

[dzafar]

I understand that we use an NMOS to form a constant current source. My question is, can we create a constant current sink using a similar approach with PMOS? Is constant current sink even a thing?

 

[FvM]

The usual terminology is current source for both polarities. If not, we would designate the PMOS current source and the NMOS current sink, isn't it?

 

[dzafar]

Isn't PMOS used as a current SINK? I am new to microelectronics

Thanks in advance

 

[BradtheRad]

A comparator's output is said to sink current, because it has an N-device at the output terminal. When turned on it provides a path to ground.

 

[d123]

Hi,

Current source: It provides current to another device. "Transmitter".
Current sink: It accepts current from another device. "Receiver."

Typical designs, I guess you've "Googled" a few constant current sources/sinks and/or CURRENT MIRRORS and seen both types: PMOS on V+ and NMOS on GND.

Consider a circuit from a conventional top to bottom viewpoint, simplest way to understand perhaps..., PMOS sources current from V+ and NMOS sinks current to GND.

 

[CataM]

When an IC is sinking current, in datasheets it is represented with a negative value.

 

[asdf44]

I would think that 'sink' or 'source' refers to the voltage/current polarity or the circuit rather than the device doing it. And under that definition you can certainly make either an nmos or pmos sink or source.

 

[Eric Best]

I understand that we use an NMOS to form a constant current source. My question is, can we create a constant current sink using a similar approach with PMOS? Is constant current sink even a thing?

Constant Current Source is a term used in the circuit theory (analysis/synthesis), as a construct to model real sources. It is meant as a source of energy/power, it sources energy, so its opposite is load not sink (from this point of view a load "sinks" energy). The constant current sink is not "a thing".

P.S.: The ideal CCS gives, according to its name, a specified constant value of current into any arbitrary load, i.e. it does it under any circumstances (voltage across it is defined by the current and that load; so, for instance, CCS can give that current into a short circuit with zero voltage across the load, or if you disconnected the load, its voltage would go to infinity, trying to "push" the current through its disconnected terminals). Look at it as at a black box, at one component (here a source) of set of components used in the circuit theory. It does not distinguish source/sink orientation in terms of your question because here this does not give a sense. One pole of such a source is sourcing (taking into account the consensual current direction), the other is sinking current of the same value.

A real CCS always works as CCS only within specified range of conditions, outside them it does not (such a state is usually called saturation). In principle, it usually consists of a voltage source (a power supply) and a regulator that controls voltage across the intended load so that current flowing through it, measured as a voltage drop across a sensing resistor in series with the load, is constant. The output/power component of the regulator is often a BJT or MOSFET and we can speak about sourcing or sinking current in relation to their electrodes but not in relation to the whole current source (created by all the above mentioned components).
Btw., you've mentioned an NMOS to form a CCS, the drain of which acts as a sink not a source of current, as FvM also reminded.