Designing a Capacitance Multiplier as a Power Supply Filter

Key Takeaways

• Learn how to design a capacitance multiplier circuit using real-world transistor models in OrCAD X.
• Discover current best practices and simulation-driven design techniques for analog power filtering.
• Use PSpice simulation to verify ripple filtering and component tolerance sensitivity.

All these components are important parts of your power conditioning strategy

Capacitance multiplier circuits are a great circuit block for removing residual ripple voltage and other noise sources. In terms of design and layout, you can easily design a capacitance multiplier from discrete components or an operational amplifier IC. This circuit causes a capacitor to behave like a much larger capacitor, which provides much better smoothing in power supply circuits.

Where Are Capacitance Multiplier Circuits Used?

Building a Capacitor Multiplier Circuit with a Transistor

The first way to build a capacitance multiplier circuit is to use a transistor, such as a MOSFET. A transistor with higher DC gain provides a larger capacitance multiplication factor in a capacitance multiplier circuit. 

This circuit is inherently nonlinear and takes advantage of saturation in a transistor.

A simple capacitance multiplier circuit with a transistor is shown in the figure below. Note that 

• The two resistors function as a voltage divider that regulates the voltage applied to the base of the transistor and the voltage drop across the transistor. 
• Simple capacitance multiplier circuits sometimes omit the R2 to provide a higher output voltage, but this reduces the level of noise suppression provided by this circuit.
• By using the R2 in parallel with the capacitor, the transistor is more easily driven into saturation as the collector-base voltage is lower. Thus, the output from the transistor will saturate at a lower level. 
• However, this increases the collector-emitter voltage drop, which increases power dissipation as heat.
• Note that a BJT transistor is depicted. However, MOSFETs can be used as well.

Simple capacitor multiplier with a transistor

Driving the transistor to saturation is important here as this suppresses the ripple on the input voltage from changing the output voltage. If you are driving the capacitance multiplier at a lower input voltage, then you need to apply a lower collector-base voltage (i.e., R2 > R1) to ensure the transistor enters saturation. If the input voltage is sufficiently high, you can set the output voltage by adjusting the values of R1 and R2.

If using a BJT in this circuit, the capacitance C is amplified to (1 + beta)*C, where beta is the gain provided by the transistor. In other words, the capacitor C behaves as if its capacitance is (1 + beta)*C. While the gain of a single transistor is limited, you can provide much higher capacitance multiplication factors by using a Darlington pair.

Capacitor Multiplier with an Operational Amplifier

A capacitor multiplier can also be built with an operational amplifier instead of a transistor. In this case, the operational amplifier must be operating in the linear regime, i.e., the inputs must be unsaturated. This limits the range of input voltage values you can use in your capacitance multiplier circuit.

Capacitance amplification factors of ~100 or larger are possible with this circuit, where the amplification factor is equal to the gain in the amplifier circuit, as long as the amplifier does not enter saturation. If the gain is less than 1, then this circuit could be viewed as a capacitance divider. This circuit is shown below.

Capacitance multiplier with an operational amplifier.

This circuit is intended to amplify the capacitance in a series RC circuit, where R2 and C are in series, effectively creating a much more powerful low-pass filter. If you are looking to design a capacitance multiplier with an operational amplifier, you can generate a number of Bode plots with different values of C to examine the filtration provided by this circuit.

Why Use a Capacitance Multiplier in a PCB?

Working with a capacitance multiplier provides the same level of filtration and smoothing as a much larger capacitor using smaller discrete components. 

• Extremely strong low-pass filtering with an RC circuit requires one or more large capacitors to provide a strong roll-off, which consumes a large amount of board space.
• Alternatively, you could build a higher-order low-pass filter from stages using discrete components, but you still have the same problem, and you will need to set the 3 dB frequency to a very low value to properly filter ripple. This provides better ripple suppression, but it still uses a significant amount of board space.

Best Practices for Designing Capacitance Multiplier Circuits

When designing a capacitance multiplier circuit, a few best practices ensure optimal performance and stability. 

• First, select a transistor with stable current gain (beta) across temperature and operating current ranges—this consistency directly affects the effective capacitance and ripple suppression. 
• Use a low-noise BJT or MOSFET if your application is audio or analog-sensitive.
• Choose high-quality capacitors (like film or low-ESR electrolytics) for the input filter cap to ensure long-term reliability and minimal signal degradation. 
• Ensure proper transistor biasing, typically using a voltage divider with a bypassed base node, to keep it in its linear operating region. 
• In high-gain configurations, consider adding a base resistor or emitter resistor to enhance stability and limit inrush current. 
• Always simulate the design under varying conditions (supply voltage, load, temperature) using tools like PSpice to validate performance before committing to layout.

Leveraging OrCAD X for Capacitance Multiplier Circuit Design

Designing a capacitance multiplier begins with choosing the right transistor. For instance, a BJT with medium β or a low-noise MOSFET. OrCAD-X PSpice gives you instant access to a rich built-in library of over 33,000 analog and mixed-signal models, including BJTs, MOSFETs, diodes, and more. You can use the Part Search feature in Capture to quickly locate a suitable transistor model—say, matching criteria for β, noise, or package size—and drop it directly into your schematic with associated SPICE parameters. 

If you need a part not in the default library, you can import a .lib file from a manufacturer and use OrCAD’s Model Editor to automatically link the SPICE model with the schematic symbol 

OrCAD X PSpice Design

Once your transistor, resistor, and capacitor are in place, PSpice gives you the tools to fully validate performance:

• Run Transient Analysis to observe startup behavior—how the output node charges and how ripple is reduced over time, mimicking a much larger capacitor.
• Use AC Sweep to plot the effective cutoff frequency and ensure the multiplier acts like the desired large capacitance in low-frequency filtering.
• Employ DC Bias and DC Sweep to verify the correct transistor biasing and see how supply variations affect performance.
• For robust design, especially when targeting sensitive analog applications, use Advanced Analysis features:
• Monte Carlo can test how β variations and resistor/capacitor tolerances impact ripple suppression.
• Sensitivity/Worst-case Analysis helps pinpoint the most critical parameters (like transistor β or resistor mismatch) that affect filtering effectiveness.

With these capabilities, you can model real-world behavior and confidently optimize your capacitance multiplier to meet ripple and stability specifications before board layout or prototyping. OrCAD X provides everything you need. With tools for schematic capture, SPICE analysis, and worst-case design validation, OrCAD X streamlines every step from concept to production. Ready to try it yourself? Start your free trial of OrCAD X here and bring cleaner, more stable analog designs to life.