Current Source Symbol
Definition: The Current Source symbol represents an ideal two-terminal circuit element that forces a constant or controlled current through a circuit regardless of the voltage across its terminals, defined in IEC 60617-02 and ANSI Y32.2 / IEEE 315 as a circle enclosing an arrow that indicates the direction of conventional positive current flow.
Also known as: ideal current source, constant current source, independent current source, current generator.
What the Current Source symbol means
The Current Source symbol denotes a two-terminal element that maintains a specified current (in amperes) independent of the load resistance or the voltage that develops across it. An ideal current source has infinite internal impedance, meaning it can supply any voltage required to sustain the defined current. In practice, real current sources approximate this behaviour over a limited compliance voltage range.
In circuit analysis the current source symbol is dual to the voltage source symbol: where a voltage source fixes the potential difference across its terminals, the current source fixes the current through the branch. Current sources appear in small-signal transistor models (the controlled current source in a MOSFET or BJT equivalent circuit), bias networks, and precision reference circuits.
How to identify the Current Source symbol
The Current Source symbol is a circle with an arrow drawn inside it. The arrow points in the direction of conventional positive current flow (from the positive terminal, marked '+', toward the load and returning to the negative terminal, marked '−'). In some notations the circle may be enclosed by two external terminals drawn as short lines. The symbol is visually similar to the voltage source (circle) but uses an internal arrow instead of '+'/'-' or a sine wave, distinguishing it clearly.
Function in a circuit
An ideal current source forces a defined current I (measured in amperes, A) through any connected external circuit. Because its internal impedance is theoretically infinite, it can sustain the required current even if the load voltage swings from zero to the compliance limit. In electronic design, dependent (controlled) current sources model the transconductance of transistors: the drain current of a MOSFET equals g_m × V_gs, represented as a current source controlled by the gate-source voltage.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-02 (Symbol 02-02-07): a circle with an internal arrow indicating current direction. The '+' terminal is where current exits the source. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315: identical circle-with-arrow symbol. The arrow points from the '−' terminal to the '+' terminal inside the circle, representing the direction of conventional current inside the source. |
| Key difference | IEC and ANSI use essentially identical glyphs for the independent current source. Both show a circle with an internal arrow; the '+'/'-' terminal labels follow the same convention. Dependent (controlled) current sources are shown with a diamond shape in some ANSI/IEEE texts rather than a circle. |
Terminals / pins
| Pin | Name |
|---|---|
| pos | + |
| neg | - |
Typical values
Current magnitude in amperes (A), milliamperes (mA), or microamperes (µA). Real current source compliance voltage is a key specification (e.g. 0–12 V compliance for a 100 mA source). Dependent current sources are parameterised by transconductance g_m (A/V) or current gain β.
Where the Current Source symbol is used
- Small-signal equivalent circuit models of BJTs and MOSFETs (g_m × V_gs or β × I_b controlled current sources).
- LED driver circuits where constant current maintains brightness independent of supply voltage variation.
- Precision sensor biasing where a fixed current through a thermistor or RTD sets a measurable voltage.
- Current-mode DAC outputs where each bit cell contributes a weighted current to a summing node.
- Differential pair tail current sources in op-amp input stages to set quiescent current.
- Electrochemical impedance spectroscopy (EIS) test equipment that injects a known AC current into a cell.
Example
In the small-signal model of a common-source MOSFET amplifier, the current source symbol appears between the drain and source terminals with a value of g_m × V_gs. This controlled current source captures the amplifying action: a small AC voltage V_gs on the gate causes a proportionally larger AC current in the drain branch, which develops an output voltage across the drain resistor R_D.
Key facts
- The Current Source symbol is a circle with an internal arrow; the arrow points in the direction of conventional current flow from the positive terminal through the external circuit.
- An ideal current source has infinite internal impedance, forcing a constant current regardless of the terminal voltage across it.
- The schematic designator for a current source is I or CS; the SI unit of current is the ampere (A), symbol A.
- A dependent (controlled) current source is distinguished from an independent source by using a diamond shape (ANSI/IEEE convention) instead of a circle.
- IEC 60617-02 (Symbol 02-02-07) and ANSI Y32.2 / IEEE 315 both define the current source as a circle with an arrow; the glyphs are essentially identical across the two standards.
- Practical current sources have a finite compliance voltage range—the maximum terminal voltage over which the constant-current behaviour is maintained.
- The current source is the Norton equivalent element, dual to the Thevenin voltage source; every Norton equivalent circuit consists of an ideal current source in parallel with an internal resistance.
Frequently asked questions
What does the current source symbol look like?
The current source symbol is a circle containing an arrow. The arrow points in the direction of conventional positive current flow. Two terminals extend from the circle; the terminal from which the arrow points away is the positive terminal. This distinguishes it from the voltage source (circle with + and − signs or a sine wave inside).
What does the current source symbol mean in a circuit diagram?
The current source symbol means that a fixed current (specified in amperes) flows through the branch regardless of the voltage across the element. In circuit analysis it is the dual of a voltage source, enforcing current rather than voltage.
What is the difference between IEC and ANSI current source symbols?
Both IEC 60617-02 and ANSI Y32.2 / IEEE 315 represent the current source as a circle with an internal arrow indicating current direction. The two standards are essentially identical for the independent current source. The main difference arises with dependent sources, where IEEE 315 uses a diamond shape.
What is the designator letter for a current source on a schematic?
The designator letter for a current source is I (for independent current source) or IS. In SPICE simulation netlists the element identifier starts with 'I'. In some documentation 'CS' (current source) is used as the reference designator.
What is the difference between a current source and a voltage source?
A voltage source (circle with + and − or a sine wave) fixes the terminal voltage regardless of current; a current source (circle with arrow) fixes the current regardless of terminal voltage. They are mathematical duals: a voltage source has zero internal impedance, while an ideal current source has infinite internal impedance.
What is a dependent current source?
A dependent (controlled) current source is a current source whose value is proportional to a voltage or current elsewhere in the circuit. It models the amplifying action of transistors. In ANSI/IEEE schematic notation, dependent current sources are drawn as a diamond (rhombus) with an arrow inside, rather than the circle used for independent sources.
What standard defines the current source symbol?
The independent current source symbol is defined by IEC 60617-02 (Symbol 02-02-07) and ANSI Y32.2 / IEEE 315. Both standards use a circle with an internal arrow. SPICE simulation follows the IEEE convention, using the letter 'I' to identify current source elements in a netlist.
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