VCCS (Dependent Current Source) Symbol
Definition: The VCCS symbol represents a voltage-controlled current source — a dependent source whose output current equals a transconductance gm (in siemens, amps per volt) times a controlling voltage sensed elsewhere in the circuit — drawn per IEEE 315 / ANSI convention as a DIAMOND containing a current-direction arrow, the diamond distinguishing dependent sources from independent-source circles.
Also known as: voltage-controlled current source, dependent current source, controlled current source, G source, transconductance source, gm source, transconductance amplifier.
What the VCCS (Dependent Current Source) symbol means
The VCCS symbol denotes an idealized two-port element: a controlling port (Ctrl+, Ctrl−) senses a voltage vx while drawing no current, and an output port (Out+, Out−) forces a current iout = gm·vx through whatever is connected, regardless of the voltage that develops. The gain gm is a transconductance — 'trans' because the controlling voltage and the output current appear at different ports — measured in siemens (S), historically called mhos (ohm spelled backwards, symbol ℧). An ideal VCCS has infinite input impedance at the control port and infinite output impedance at the output port, the defining properties of a perfect transconductance amplifier.
The VCCS is arguably the most physically important dependent source, because it is the natural small-signal model of every field-effect transistor and of the bipolar transistor's collector current versus base-emitter voltage: a MOSFET or JFET channel delivers a drain current gm·vgs, which is precisely a VCCS. Operational transconductance amplifiers (OTAs) such as the LM13700, and gm-C filter stages, are deliberate circuit-level VCCS implementations. In SPICE it is the G element: G1 out+ out− ctrl+ ctrl− gm.
How to identify the VCCS (Dependent Current Source) symbol
Look for a DIAMOND with an ARROW inside pointing in the direction of positive output current flow. The arrow (rather than ±) marks a current source; the diamond (rather than a circle) marks it as dependent. The controlling voltage is shown either as a labeled terminal pair (Ctrl+, Ctrl−) — commonly drawn to the left of the diamond — or as an annotation such as 'gm·vx' beside the symbol with vx defined across some element elsewhere.
Distinguish it from its three siblings by two questions: what is inside the diamond (arrow = current output, ± = voltage output), and what variable appears in the gain expression (a voltage vx = voltage-controlled, a current ix = current-controlled). IEC-style schematics may draw controlled sources as annotated circles rather than diamonds, so when the shape is ambiguous the dependency expression decides.
Function in a circuit
In analysis, the VCCS contributes the constraint iout = gm·vx. In nodal analysis it enters the current-balance equations directly — one reason transistor small-signal analysis is usually done nodally — with vx expressed in terms of node voltages. The output port behaves as an ideal current source: infinite output impedance, current independent of output voltage.
As a model, the VCCS captures transistor action: replace a MOSFET with a VCCS of value gm·vgs between drain and source (plus an output resistor ro for channel-length modulation) and the whole amplifier can be solved with linear algebra. As a real circuit block, an OTA is a VCCS whose gm is electronically adjustable via a bias current, enabling voltage-controlled filters, amplifiers, and oscillators; gm-C filter design builds integrators from a VCCS driving a capacitor.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 has no dedicated diamond for dependent sources; IEC-style schematics may show a controlled current source as a circle with the current arrow and an annotation of the controlling law (gm·vx). International textbooks nonetheless use the diamond convention almost universally. |
|---|---|
| ANSI/IEEE 315 | IEEE 315 / ANSI practice draws the dependent current source as a diamond enclosing an arrow, with the gain expression written beside it. SPICE encodes it as the G element (G = VCCS in the E/G/H/F dependent-source letter scheme). |
| Key difference | As with all dependent sources, the operative difference is diamond (dependent) versus circle (independent) in IEEE/textbook practice, against IEC's annotated-circle tradition. The internal arrow marking current direction is common to both; the gain units (siemens) and the controlling-variable annotation identify the element unambiguously. |
Terminals / pins
| Pin | Name |
|---|---|
| out_pos | Out+ |
| out_neg | Out- |
| cp | Ctrl+ |
| cn | Ctrl- |
Typical values
Transconductance values span decades: a small-signal JFET offers gm of 1–6 mS, a BJT at 1 mA collector current has gm = IC/VT ≈ 38.5 mS, a power MOSFET can exceed 10–100 S, and an LM13700 OTA provides a gm programmable up to about 9.6 mS via its amplifier bias current. In filter design, gm-C integrators use VCCS blocks of microsiemens to millisiemens against picofarad-to-nanofarad capacitors. SPICE declaration: G1 out+ out− ctrl+ ctrl− 0.02 defines a 20 mS transconductance; ideal elements assume infinite input and output impedance.
Where the VCCS (Dependent Current Source) symbol is used
- Small-signal models of MOSFETs, JFETs, and BJTs, where the channel or collector current is gm times the input voltage
- Operational transconductance amplifier (OTA) circuits — voltage-controlled amplifiers, filters, and oscillators (LM13700, CA3080)
- gm-C continuous-time filters in integrated circuits, built from VCCS blocks driving capacitors
- SPICE behavioral modeling of transconductance stages, loudspeaker current drive, and sensor interfaces
- Long-tailed pair and differential-stage analysis, where each transistor contributes a gm·vgs current
- Laboratory transconductance amplifiers that convert a signal voltage into a proportional test current
Example
Replacing a MOSFET with its small-signal model, the VCCS's Ctrl+ and Ctrl− pins connect across gate and source to sense vgs while its Out+ and Out− pins connect from drain to source injecting iout = gm·vgs; with gm = 5 mS and a 2 kΩ drain resistor the stage gain is −gm·RD = −10 V/V, and in SPICE the equivalent element is written G1 drain source gate source 5m.
Key facts
- The VCCS equation is iout = gm·vx; gm is transconductance in siemens (S), historically mhos (℧) — amps of output per volt of control.
- Symbol: diamond (dependent source) containing an arrow (current output, pointing in the direction of positive current flow).
- SPICE letter G denotes the VCCS in the E/G/H/F dependent-source scheme; syntax G<name> out+ out− ctrl+ ctrl− gm.
- The ideal VCCS has infinite input impedance AND infinite output impedance — the perfect transconductance amplifier.
- Every FET's small-signal drain current is a VCCS (id = gm·vgs), making this the most physically ubiquitous dependent source.
- A BJT's transconductance is gm = IC/VT, about 38.5 mS per milliamp of collector current at room temperature.
- OTAs (e.g. LM13700) are practical VCCS ICs whose gm is set by a bias current, enabling voltage-controlled filters and amplifiers.
- Like all dependent sources, a VCCS is never zeroed during Thevenin/Norton source-killing; it stays active and requires the test-source method for Rth.
Frequently asked questions
What is the gain unit of a VCCS?
Siemens (S), the unit of conductance — output amps per controlling volt. The historic name is the mho (ohm backwards, symbol ℧), still seen on older datasheets as µmhos. The gain is called transconductance (gm) because the controlling voltage and the resulting current are at different ('trans') ports. A gm of 5 mS means every millivolt of control voltage produces 5 µA of output current.
How do I tell a VCCS from a VCVS on a schematic?
Look inside the diamond. An arrow means the output is a CURRENT (VCCS if the gain expression references a voltage, CCCS if it references a current). Plus and minus signs mean the output is a VOLTAGE (VCVS or CCVS). Both VCCS and VCVS are voltage-controlled, so their gain expressions contain a vx — the internal marking is what tells you what the source delivers.
Why is the VCCS used to model transistors?
Because that is physically what a transistor does in small-signal terms: a MOSFET's drain current changes by gm·vgs, and a BJT's collector current changes by gm·vbe. Neither output current depends (to first order) on the output voltage, which is exactly an ideal current source controlled by an input voltage. Adding an output resistance ro in parallel models the second-order dependence (channel-length modulation or Early effect).
What is an OTA and how does it relate to the VCCS?
An operational transconductance amplifier is a real IC implementation of the VCCS: it outputs a current proportional to its differential input voltage, iout = gm(v+ − v−), with gm adjustable via an external bias current. Parts like the LM13700 and CA3080 use this to build voltage-controlled filters, amplifiers, and oscillators — anywhere the gain of a stage must itself be a signal.
What does the G element mean in SPICE?
G is the SPICE letter for a voltage-controlled current source. The card G1 out+ out− ctrl+ ctrl− 0.005 creates a source pushing iout = 0.005 × v(ctrl+, ctrl−) amps from Out+ to Out−. The companion letters are E (VCVS), H (CCVS), and F (CCCS). Modern simulators also allow behavioral expressions, e.g. G1 out 0 VALUE={0.005*V(in)}.
Does a VCCS have high or low output impedance?
Infinite, ideally. A current source must maintain its programmed current no matter what voltage appears across its terminals, which by definition is infinite output impedance. This is opposite to the VCVS (zero output impedance). Real transconductance stages have large-but-finite output impedance, modeled by placing a resistor ro in parallel with the ideal VCCS.
Related symbols
- Current Mirror symbol
- Current Source symbol
- N-Channel MOSFET symbol
- Op-Amp symbol
- Transimpedance Amplifier symbol
- Voltage Source (DC) symbol
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