Designing a Cascode Current Source

A comprehensive guide for enhanced performance and stability

scenic electronic circuit board components

Highlights

High Output Impedance: Cascode configurations dramatically increase output resistance, ensuring stable current despite voltage variations.
Improved Stability: By isolating the lower current source transistor from voltage swings, the design provides consistent performance and minimal sensitivity to supply fluctuations.
Versatile Implementation: Suitable for both MOSFET and BJT technologies, this technique can be tailored for diverse analog circuit applications.

Introduction to Cascode Current Sources

Cascode current sources are an advanced circuit design technique widely used in analog and integrated circuit applications to enhance performance. Traditionally, current mirrors and simple current sources provide basic functionality; however, they suffer from relatively low output impedance and sensitivity to output voltage fluctuations. This is where the cascode configuration comes into play.

A cascode current source comprises two or more transistors arranged in a stack that leverages one transistor’s current-setting behavior and another transistor’s ability to stabilize the operating voltage. The primary benefits include a tremendous boost in the output impedance and improved isolation of the current-setting device from subsequent load variations.

Fundamental Concepts and Design Considerations

Understanding the Cascode Principle

In a basic current mirror configuration, the output impedance is limited. By adding a cascode stage, the design effectively “stacks” a transistor on top of the primary current-setting device. This configuration minimizes the effect of channel length modulation (or Early effect in BJTs), as the voltage at the drain of the lower transistor is stabilized. Consequently, small variations in the output voltage result in very minor changes in the output current.

The cascode current source is especially useful when high performance is needed, particularly in circuits such as RF amplifiers or precision analog circuits where even slight current variations may lead to issues. Stability is enhanced because the transistor lower in the stack operates in a saturated region with a nearly constant drain-source voltage.

Key Design Parameters

When designing a cascode current source, some of the most crucial parameters to consider include:

Transistor Selection

The choice between MOSFETs and BJTs depends on your application’s specific requirements. MOSFETs are often preferred in modern designs due to their high input impedance and ease of biasing for integrated circuits. However, BJTs may be used where higher gain or specific matching is needed.

Bias Voltages and Operating Regions

For the cascode configuration to perform optimally, all transistors must be properly biased. The current-setting transistor needs to be in the saturation region, ensuring its operation as a constant current source. Meanwhile, the cascode transistor must have its gate-source voltage (V_GS) maintained above the threshold level to keep it in saturation. This precise biasing is critical to ensuring that the desired output impedance is achieved and maintained.

Component Sizing and Matching

The matching of transistor characteristics (e.g., threshold voltage, transconductance, and channel length) is vital for optimal performance. Any variation between the devices can lead to mismatches, which could degrade the stability of the current source. Additionally, correct sizing plays a vital role in ensuring that the device operates within safe limits, particularly when stacking transistors.

Designing a MOSFET Cascode Current Source

Circuit Architecture

A MOSFET cascode current source typically involves at least four MOSFETs arranged in two stacks. The basic structure often includes:

Main Current Mirror: Formed by a pair of transistors where one transistor sets the reference current.
Cascode Stage: An additional pair of transistors is placed atop the current mirror. Their primary function is to maintain a constant voltage at the drain of the main current-setting transistor.

In an example design, the current-setting transistor (Q1) is connected with its source to ground (assuming an NMOS implementation), while the cascode transistor (Q2) is stacked above Q1. The gate of Q2 is often tied to a fixed voltage determined by a voltage divider or a bandgap reference to ensure operation in the saturation region.

Example Circuit Diagram

The following schematic outlines a basic MOSFET cascode current source configuration:


         VDD
          │
          │
         ┌┴┐
         │Q3│ <- Cascode transistor for mirror (if using a dual-mirror design)
         └┬┘
          │
         ┌┴┐
         │Q1│ <- Current-setting transistor (current mirror)
         └┬┘
          │
         ┌┴┐
         │R │ <- Reference resistor
         └┬┘
          │
         GND

For implementations that require higher precision, an additional cascode transistor (Q4) can be employed to further isolate the current-setting transistor from load variations, thereby providing an even higher output impedance.

Establishing Biasing Conditions

Proper biasing is the cornerstone of a successful cascode current source design. Here are the steps for establishing the necessary bias conditions in a MOSFET-based design:

Determine the Reference Current (I_REF): The desired output current is set via a resistor connected to a known voltage. A typical relationship might be:
\( I_{REF} = \frac{V_{REF}}{R} \)
Ensure Saturation Operation:
The transistor used to set the reference current (Q1) should be designed to operate in the saturation region. This can be verified by ensuring that the drain-source voltage (V_DS) is above the saturation threshold.
Setting the Cascode Voltage:
The gate of the cascode transistor (Q2) must be maintained at a constant voltage. Often a voltage divider or a dedicated voltage reference circuit is used to determine this bias voltage. The fixed voltage helps in achieving high output resistance by stabilizing the voltage at the interface between the two transistor stages.

Simulating the Circuit

Before fabricating the circuit or implementing it in an integrated circuit design, simulation verifies the expected performance. SPICE (Simulation Program with Integrated Circuit Emphasis) is a widely used tool for such purposes. The simulation typically involves establishing the model for the NMOS devices, applying the necessary DC bias conditions, and performing a DC sweep analysis to verify the current stability against variations in VDD.


* MOSFET Cascode Current Source Example
* Define the MOSFET model
.MODEL NMOS NMOS(VTO=1.0 KP=50E-6)

* Define the circuit components
VDD VDD 0 DC 5V
VSS VSS 0 DC 0V
M1 VSET X VSS VSS NMOS W=10u L=1u      * Current-setting transistor
M2 X Y VSS VSS NMOS W=10u L=1u         * Lower section of cascode
M3 VDD Y VDD VDD NMOS W=10u L=1u        * Cascode transistor
RREF VSET VSS 1k                       * Sets the reference current
VREF VSET VSS DC 1V                     * Reference voltage

* Simulation setup
.DC VDD 0 5 0.1
.PRINT DC I(M3)
.END

Running a simulation ensures that both the desired reference current and the high output impedance are achieved before physical implementation.

Designing a BJT Cascode Current Source

BJT-Based Configuration

In instances where BJTs are preferred, the cascode configuration follows similar principles. Instead of using gate-source biasing, BJTs rely on base-emitter junctions and base biasing networks.

In a typical design using npn BJTs, the basic setup includes:

Current Mirror Stage: Two npn transistors are arranged to form a current mirror, where one transistor sets the reference current.
Cascode Stage: Another npn transistor is placed atop the current-setting transistor to stabilize the collector voltage and hence enhance output impedance.

Biasing for BJTs

In BJT circuits, ensuring that all devices operate in the correct active region is essential. This involves:

Establishing Base-Emitter Voltage:
The base-emitter voltage (V_BE) of the current-setting transistor should be maintained around 0.7V for silicon transistors to ensure that it remains in active mode.
Stabilizing the Collector Voltage:
The cascode transistor is biased such that its collector voltage does not fall out of the active region, thereby minimizing the Early effect and maintaining high output resistance.

Comparison Table: MOSFET vs. BJT Cascode Designs

Parameter	MOSFET Design	BJT Design
Device Type	NMOS / PMOS	npn / pnp
Input Impedance	High	Moderate
Biasing Requirement	Gate voltage (~threshold)	Base-emitter junction (~0.7V)
Output Impedance	Very High (improved by cascode)	High (with proper stabilization)
Sensitivity to Voltage Variations	Low	Moderate to Low
Complexity	Fairly Simple in Integrated Circuits	Requires careful biasing

Advanced Techniques and Practical Considerations

Feedback and Stability Enhancement

Incorporating feedback mechanisms can further enhance the performance of cascode current sources. Negative feedback, for instance, can compensate for variations due to temperature fluctuations or process variations. One advanced method includes the use of a Wilson current source configuration, which builds upon the basic cascode design to achieve superior current matching and higher output impedance.

Designers must also consider the impact of parasitic capacitances and resistances that arise in both discrete and integrated implementations. In high-frequency applications, the layout and careful routing become critical to minimize additional undesired feedback or oscillations.

Temperature and Process Variations

Semiconductor devices exhibit changes in behavior with temperature and process variations. In cascode current sources, transistor parameters such as threshold voltage (for MOSFETs) or the base-emitter voltage (for BJTs) might shift. To compensate for these effects:

Use matched pairs of transistors to ensure consistency across the circuit.
Integrate temperature compensation networks, especially when high precision is required.
Consider using bandgap references to provide stable bias voltages over temperature variations.

Practical Application Scenarios

Cascode current sources find applications in a variety of electronic systems:

Operational Amplifiers: They help create precise bias currents needed for high-gain amplifiers.
RF Circuits: The high output impedance and stability are vital in radio frequency circuits, where signal integrity is paramount.
Sensor Interfaces: In precision sensors and instrumentation circuits, cascode current sources provide stable biasing, leading to improved linearity and performance.
Process Control Systems: These configurations ensure that the bias currents remain stable even in the presence of supply voltage fluctuations or temperature changes.

References

Cascode Amplifier using MOSFET Explained - All About Electronics
Cascode Current Sources - Ques10
Cascode - Wikipedia
Cascode Current Source PDF - Aldinc
JFET Cascode Boosts Current Source Performance - EDN
Current Sources and Sinks - UTK-EECS
Cascode Current Sources Discussion - Reddit ChipDesign
Texas Instruments - Analog Design Resources
Infineon Technologies - Analog Design
Mouser Electronics - Design Components