---
title: "Voltage-Controlled Current Source | Intro to EE"
description: "Voltage-controlled current source is a dependent source that outputs current proportional to an input voltage, a core model in Intro to Electrical Engineering."
canonical: "https://fiveable.me/introduction-electrical-systems-engineering-devices/key-terms/voltage-controlled-current-source"
type: "key-term"
subject: "Intro to Electrical Engineering"
unit: "Unit 5"
---

# Voltage-Controlled Current Source | Intro to EE

## Definition

A voltage-controlled current source (VCCS) is a dependent source that produces an output current proportional to a controlling voltage, usually written I = kV. In Intro to Electrical Engineering, you use it to model circuit behavior and small-signal devices.

## What It Is

A voltage-controlled current source is a dependent source in Intro to Electrical Engineering that makes one current depend on a voltage somewhere else in the circuit. If the control voltage changes, the source changes its output current by the same proportional rule, often written as I = kV, where k is the transconductance or gain factor.

That phrase, dependent source, matters. Unlike an independent current source, a VCCS does not just push a fixed current all by itself. Its output is tied to a circuit variable, so you have to track both the controlling voltage and the direction of the current arrow in the diagram. If the control voltage is positive, the current may flow in the arrow direction; if it is negative, the current can reverse.

In circuit diagrams, a VCCS is usually drawn as a diamond-shaped source with an arrow inside, plus a label showing what voltage controls it. The controlling voltage might be measured across two nodes somewhere else in the circuit, not necessarily across the source itself. That is why you often see expressions like i = gmvx, where the output current depends on a voltage labeled vx.

A VCCS shows up a lot in analog electronics and small-signal transistor models. When you linearize a transistor around an operating point, the device can behave like a voltage-controlled current source, which is a cleaner way to analyze how a small change in input voltage changes the output current.

This is also a great place to use superposition. Since the circuit is linear as long as the dependent source stays in the model, you can turn off independent sources one at a time and solve the circuit response. The dependent source stays active, because it is controlled by the circuit variables, not by an independent battery or generator.

A common mistake is treating a VCCS like an independent current source and shutting it off during superposition. You do not do that. You only deactivate independent sources, while the VCCS remains part of the circuit equations because its current depends on the controlling voltage.

## Why It Matters

Voltage-controlled current sources show up whenever Intro to Electrical Engineering moves from simple resistor circuits into models that behave like real devices. They are one of the main ways you represent amplifiers, transistor small-signal behavior, and feedback systems without drawing every physical detail inside the component.

Once you can recognize a VCCS, you can translate a circuit diagram into equations faster. That matters in nodal analysis, small-signal models, and any problem where a voltage at one node causes a current somewhere else. Instead of guessing what the circuit does, you write the control relationship directly and solve for the unknown node voltages and branch currents.

It also connects directly to superposition and circuit decomposition. If a circuit includes a dependent source, you still can analyze the contribution from each independent source separately, but you must keep the dependent source in every version of the circuit. That distinction is easy to miss, and it changes the answer.

In lab or homework settings, VCCSs help you read models of op-amps, transistors, and other active circuits as simplified block diagrams. The better you understand the source, the easier it is to see whether a circuit is acting like an amplifier, a buffer, or a feedback-controlled system.

## Connections

### Dependent source

A voltage-controlled current source is one type of dependent source. The key idea is that its output is not fixed on its own, it depends on another voltage or current elsewhere in the circuit. That dependency is what makes it useful in modeling active devices like transistors and op-amp stages.

### Superposition theorem

VCCSs stay active when you apply superposition, because they are dependent on a circuit variable. You only turn off independent sources one at a time. This is a common exam trap, since students sometimes try to deactivate every source and end up removing the behavior they were supposed to analyze.

### [Current-controlled current source](/introduction-electrical-systems-engineering-devices/key-terms/current-controlled-current-source)

Both are dependent current sources, but the control variable is different. A VCCS responds to voltage, while a current-controlled current source responds to current. Recognizing which variable controls the output is the first step before you write the correct circuit equations.

### [Voltage-Controlled Voltage Source](/introduction-electrical-systems-engineering-devices/key-terms/voltage-controlled-voltage-source)

This is the voltage-output version of the same idea. A voltage-controlled voltage source produces an output voltage based on a controlling voltage, while a VCCS produces output current based on voltage. Comparing the two helps you remember whether the source is giving you current or voltage at the output.

## On the AP Exam

Problem sets and quizzes usually ask you to identify the controlling voltage, write the source equation, or include the VCCS correctly in nodal analysis. If a circuit uses superposition, you keep the dependent source active and only turn off the independent sources one at a time. A typical question might give you a diamond-shaped source and ask you to find the branch current or solve for a node voltage after plugging the VCCS into the equations.

You may also see it in a transistor small-signal model, where you interpret the device as a voltage-controlled current source and predict how a small input change affects output current. The main skill is translating the diagram into a math relationship without mixing up the controlling voltage and the output current direction.

## Voltage-controlled current source vs Current source

A current source sets a current directly, while a voltage-controlled current source sets current based on another voltage in the circuit. In other words, a current source is independent if its value is fixed, but a VCCS is dependent because its output changes when the control voltage changes.

## Key Takeaways

- A voltage-controlled current source is a dependent source whose output current is proportional to a controlling voltage.
- You usually write it as I = kV or i = gmvx, which tells you the current follows the input voltage by a fixed gain.
- In circuit analysis, a VCCS stays active during superposition because it depends on circuit variables, not on an independent source.
- You will often see VCCSs in transistor small-signal models, amplifier circuits, and feedback systems.
- The biggest mistake is turning off a dependent source or confusing the control voltage with the current the source delivers.

## FAQs

### What is a voltage-controlled current source in Intro to Electrical Engineering?

It is a dependent source that produces an output current based on a controlling voltage elsewhere in the circuit. The relationship is usually linear, so you can write the source current as I = kV. In circuit problems, that means the source is part of the equations, not just a fixed current value.

### How do you draw a voltage-controlled current source?

It is usually shown as a diamond-shaped source with an arrow inside to show current direction. The controlling voltage is labeled next to the symbol, often with node names or a variable like vx. That label matters because it tells you which voltage determines the output current.

### Do you turn off a VCCS when using superposition theorem?

No. You turn off only independent sources, such as independent voltage or current sources. The dependent source stays on because its value comes from a circuit variable, and removing it would change the behavior you are trying to solve.

### Is a VCCS the same as a current source?

Not exactly. A current source delivers current, but a VCCS delivers current that depends on a voltage. A fixed current source is independent, while a VCCS is dependent and has to be analyzed through the controlling relationship.

## Related Study Guides

- [5.3 Superposition theorem](/introduction-electrical-systems-engineering-devices/unit-5/superposition-theorem/study-guide/e3xStvJ59EWLOiUF)

## About This Document

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