A current dependent current source (CDCS)—often called a current-controlled current source (CCCS)—is an active circuit element that delivers an output current strictly proportional to a controlling current measured elsewhere in the network. When you introduce a CDCS into a design, it fundamentally changes the circuit's behavior by decoupling the output current from the load resistance, forcing the load to accept whatever current the control loop dictates until the power supply's voltage compliance limits are hit.
Think of a traffic roundabout where the exit flow is strictly dictated by the entry flow from a specific feeder road, regardless of how clear or congested the downstream streets are. The downstream traffic (load resistance) cannot dictate the flow; the feeder road (control current) holds absolute authority.
The Core Mechanics and SPICE Implementation
In theoretical circuit analysis, a CDCS is represented by a diamond-shaped symbol with an arrow indicating current direction, alongside a gain multiplier (often denoted as $\beta$, $h_{FE}$, or $\alpha$). In the physical world, ideal dependent sources don't exist as standalone two-terminal components you can buy from DigiKey. Instead, they are the fundamental operating principle of semiconductor devices and integrated circuit blocks.
When simulating these circuits in LTspice or PSpice, you don't draw a BJT; you use the F-type independent/dependent source primitive. The syntax requires you to specify the output nodes, the controlling voltage source (which must be a 0V dummy source to measure current), and the dimensionless gain factor.
CCCS vs. VCCS: The Semiconductor Divide
Engineers and hobbyists frequently confuse the CDCS with the Voltage-Dependent Current Source (VCCS). The distinction dictates which transistor family you choose for your design.
| Feature | CDCS (Current-Controlled) | VCCS (Voltage-Controlled) |
|---|---|---|
| Physical Proxy | Bipolar Junction Transistor (BJT) | Metal-Oxide-Semiconductor FET (MOSFET) |
| Control Variable | Base Current ($I_B$) | Gate-Source Voltage ($V_{GS}$) |
| Gain Parameter | $h_{FE}$ or $\beta$ (dimensionless) | $g_m$ (Transconductance, in Siemens) |
| Input Impedance | Low to Moderate (k$\Omega$ range) | Extremely High (G$\Omega$ range) |
| Primary Use Case | Current mirrors, linear amplifiers | Switching regulators, digital logic |
Worked Numeric Example: The 2N3904 BJT
Let's look at the most common physical manifestation of a CDCS: the NPN Bipolar Junction Transistor. We will use the ubiquitous 2N3904 NPN transistor in a common-emitter configuration.
- The Setup: You apply a base current ($I_B$) of $50 \mu A$ (0.00005 A) into the base pin.
- The Gain: According to the Onsemi datasheet, the DC current gain ($h_{FE}$) for a 2N3904 at a collector current of 10 mA is typically 200.
- The Calculation: The CDCS equation dictates $I_C = h_{FE} \times I_B$. Therefore, $I_C = 200 \times 50 \mu A = 10 mA$.
What this changes in the real circuit: If your collector load resistor ($R_C$) is $1 k\Omega$, the voltage drop across it is 10V. If you swap that resistor for a $500 \Omega$ resistor, a standard voltage source would double the current. But because the BJT acts as a CDCS, the collector current remains locked at 10 mA. The voltage drop simply falls to 5V. The transistor automatically adjusts its internal collector-emitter resistance to maintain the 10 mA flow, right up until the transistor hits saturation (where $V_{CE}$ approaches 0.2V and the dependent source model breaks down).
Where You Meet This in Practice
You won't just see CDCS theory in textbooks; it is the backbone of several critical analog and power electronics topologies.
- Current Mirrors: The foundational building block of analog IC design (like op-amps and voltage regulators). A reference current is fed into a diode-connected transistor, and the dependent source action copies that exact current to a parallel output transistor.
- Current-Mode PWM Controllers: In switch-mode power supplies using chips like the UC3842, the peak inductor current (the control current) directly dictates the termination of the PWM duty cycle. The inductor current acts as the controlling variable for the internal dependent source driving the gate logic.
- Hall Effect Current Sensors: Devices like the Allegro ACS712 output a voltage, but internally, the magnetic field generated by the primary current controls a dependent current source in the Hall element, which is then converted to a voltage across an internal resistor.
Real-World Scenario Walkthrough: Building a Discrete Current Mirror
Theory assumes perfectly matched components. The bench reality is much messier. Here is a walkthrough of a real-world failure and fix involving a CDCS-based current mirror.
The Setup
You need to bias a discrete differential audio amplifier pair. You decide to build a simple current mirror using two 2N3904 transistors (Q1 and Q2) to provide a rock-solid 1 mA tail current. You tie their bases together, tie Q1's base to its collector (diode-connected), and set the reference current ($I_{REF}$) using a $10 k\Omega$ resistor from a 12V rail. Q2's collector is the output ($I_{OUT}$) feeding the differential pair's emitters.
The Numbers
Assuming a $V_{BE}$ drop of 0.65V, the voltage across the $10 k\Omega$ reference resistor is $12V - 0.65V = 11.35V$. This yields an $I_{REF}$ of 1.13 mA. Because the bases are tied together and the transistors are theoretically identical, $I_{OUT}$ should mirror this at 1.13 mA.
The Outcome
On the bench at an ambient 22°C, your multimeter reads $I_{OUT}$ at 1.11 mA. Close enough for a quick prototype. You power up the audio amp, and it plays music cleanly.
What Went Wrong
After 15 minutes of operation, Q2 (which is dissipating more power due to the voltage drop across its collector-emitter junction to the differential pair) heats up to 55°C. Q1, acting only as a diode, stays near 30°C.
Because the base-emitter voltage ($V_{BE}$) of a BJT has a negative temperature coefficient (dropping roughly 2 mV/°C), the hotter Q2 requires less base voltage to conduct. Since both bases are locked to the exact same node, Q2 begins to hog the current. Your $I_{OUT}$ drifts from 1.11 mA up to 1.6 mA. This skew shifts the DC operating point of your differential pair, introducing severe even-order harmonic distortion into the audio output.
The Fix: Emitter Degeneration
To fix this, you must introduce local negative feedback to break the ideal CDCS assumption and stabilize the thermal drift. You add 47 $\Omega$ emitter degeneration resistors to both Q1 and Q2. Now, as Q2 heats up and tries to draw more current, the voltage drop across its $47 \Omega$ resistor increases, effectively reducing its $V_{BE}$ and choking off the thermal runaway. The current mirror is no longer a pure, unadulterated CDCS, but it is stable.
Common Confusions and Bench Mistakes
When troubleshooting or designing with dependent sources, avoid these common pitfalls:
Confusion 1: Treating a Dependent Source like an Independent Source
An independent current source (like a precision LM334 current regulator diode) will push its rated current regardless of what the rest of the circuit does, provided it has enough voltage headroom. A dependent source will only push current if the controlling branch is active. If your base current drops to zero due to a broken bias resistor, your collector current immediately drops to zero (ignoring leakage). You cannot troubleshoot the output branch without first verifying the control branch.
Confusion 2: Ignoring the Early Effect
In textbook models, a CDCS has infinite output impedance—meaning current never changes regardless of $V_{CE}$. In reality, the Early effect causes the effective base width to shrink as collector-base reverse bias increases. This gives the physical CDCS a finite output impedance. If your load voltage swings wildly, your 'constant' current will slope slightly. For high-precision analog designs, you must use a Wilson current mirror or cascode topology to mitigate this.
Confusion 3: Forgetting Compliance Voltage
A CDCS cannot create energy. If your control current demands 20 mA, but your load resistance and supply voltage only allow 10 mA before the transistor saturates ($V_{CE}$ hits ~0.2V), the dependent source model collapses. The transistor enters hard saturation, the gain ($\beta$) plummets, and the output current becomes entirely dependent on the load resistance and supply voltage, not the control current.
Understanding the current dependent current source bridges the gap between abstract schematic symbols and the physical silicon on your workbench. Whether you are simulating a small-signal amplifier in SPICE or biasing a discrete power stage, respecting the thermal and voltage compliance limits of the physical devices acting as your dependent sources is the hallmark of robust circuit design. For deeper reading on analog mirror topologies, the Analog Devices current mirror guide provides excellent advanced topologies.
