The J2103A POwer Stage Isolator allows you to measure power supply open-loop and control loop gain including plant gain.

The J2103A Power Stage Isolator is a general device that replaces feedback control circuits to control the power output circuit (Power stage), also, known as the power stage plant.

The power supply plant gain represents the amplification factor of the power supply unit within this feedback control loop.

Integrated Design for Efficiency

With the J2103A, you can measure the frequency response of the power output circuit (Power stage) before designing the feedback control circuit and design the feedback control circuit according to the actual measurement results.

The design process can be completed at one time, with the integration of the power output circuit (Power stage) and the feedback control circuit, the design cycle can be greatly shortened.

Versatile Feedback Control

The J2103A can be used with almost any power circuit that requires feedback control. Its feedback input (Vin) and reference voltage (Vref) use differential inputs without reference ground problems. Therefore, designs with isolated or non-isolated power sources, switching or linear power supplies, and positive or negative voltage feedback can all use the J2103A.

The plug-and-play J2103A simplifies accurate open-loop and control loop gain parameter measurement quickly and easily while using the Picotest P9611A/P9610A power supply with an FRA, such as the OMICRON Lab Bode 100 VNA.

This is a powerful tool for power supply designers to be able to quickly access hidden or difficult-to-bound parameters for their power supply design or to support accurate parameter testing for state-space average VRM model development.

Picotest sells the instruments and access to perform BOTH open and closed-loop tests. The test setups are different with the open loop being simpler. Please see the Application Note for more details.


In the world of power electronics, the focus is on the power supply. The challenge is developing an accurate model to use for simulation or measuring the gain to determine the power supply stability and performance. To create an accurate model for the power supply, the open-loop gain (AOL) and closed-loop plant gain of the power supply need to be understood. Many power supply manufacturers do not include the AOL characteristics in the datasheet, and the closed-loop gain depends on the control loop designed around the power supply.


A proper understanding of AOL at DC and over-frequency is crucial to the understanding of closed-loop gain, bandwidth, and stability analysis of the power supply. The power supply’s closed-loop gain or power supply rejection ratio (PSRR) is an important parameter for power supplies and voltage references. PSRR is a significant performance concern, as even small amounts of high-frequency ripple voltage at the input can significantly degrade the output precision of voltage reference and LDOs and impact downstream circuitry. The output of the power supply is not only a function of the inputs but also of the power supply’s control loop and power distribution network (PDN) loading. Therefore, it is important to understand these parameters when designing or simulating a power supply.

The power supply plant needs to receive a feedback signal to determine the control loop adjustment to maintain the output voltage under dynamic load conditions. However, this control loop into the plant provides an additional gain to the overall system. To understand the gain of the plant, open the control loop to assess the open-loop gain.

Create a State-Space Average VRM model in Keysight PathWave ADS or SPICE. Measuring the open-loop gain is critical to determining the power supply slope compensation and Ri terms. 

Power engineers often struggle to measure the open-loop gain of the power supply. How do you determine the power stage gain on a power supply? This is where the Picotest J2103A power stage isolator, along with a frequency response analyzer (FRA), can help. Measuring the plant’s open-loop gain or control loop gain is relatively simple to do if the power supply includes a COMP pin for external compensation.

Application Note: “Using the J2103A Power Stage Isolator to Measure Power Supply Open-Loop and Control Loop Gain”