# What does a two overlapping circles symbol mean in an electrical schematic/diagram?

In the datasheet for a A3503 Hall Effect Sensor, I saw a circuit diagram labeled 'Functional Block Diagram', and it shows a symbol I have not seen before. It's shown here between 3 - Output and 2 - Ground.

What is this supposed to represent?

The full datasheet is available here

• Great question, I've never seen that symbol before either, and it's for such a common device. I wonder where it originated? – Shamtam Jan 31 '13 at 16:57

It's the symbol of a current source.

There are different symbols used to represent a current source:

Two overlapping circles represents a current source. In this case, it is being used to sink a certain minimum amount of current through the output transistor, in order to keep its dynamic impedance low and improve the overall frequency response.

Current sources (and sinks) are commonly used in IC design, because they're actually easier to implement than high-value resistors. They also give the circuit better performance in many cases, because the effective impedance of a current source is very large, which can be used to create high gain without a requirement for a lot of voltage "overhead".

To answer the question about high-value resistors, consider the materials that are available to the IC designer: silicon (doped to various levels) and metal (aluminum or copper). The resistivity of metal is very low, so that just leaves the silicon. Unfortunately, the specific resistivity of the silicon is difficult to control tightly, so it's difficult to create precision resistors. In any case, it takes a significant amount of silicon area to create a resistor of more than a few kOhms in value.

The effective (dynamic) impedance of a current source is defined by how much the current through it changes with a change in the voltage across it, specifically Reff = ΔV/ΔI. It's relatively easy to build a current source whose current changes only a few parts per million with a 1-V change in voltage. For example, a 1-mA source whose value only changes by 1 µA would imply an effective resistance of 1 MΩ.

The transistors to do this take up much less room than an actual 1-MΩ resistor in silicon would. Besides which, you would have to put 1000 V across that resistor to get 1 mA through it!

• I have 2 doubts: 1) You said current sources are easier to implement than resistors, why ? 2)What is effective impedance of a current source and how to measure it ? Please explain. – Nishu Jan 31 '13 at 15:31
• Current sources are easier to implement than resistors in ICs is the key. Resistors take up a significant amount of space on a wafer, compared to the transistor(s) necessary to make the current source. – Shamtam Jan 31 '13 at 16:56

It could also be, more specifically, a norator. I've never used this symbol before, but I saw it in a paper that mentions a nullor equivalent circuit.

_Dependent Current Source_ _Independent Current Source_ _____Norator_____