For determining the dynamic on-resistance of a power transistor, the voltage and current waveforms have to be measured during the switching operation. The novel heterostructure wide-bandgap (eg. AlGaN/GaN) transistors inherently suffer from the current collapse phenomenon, causing the dynamic on-resistance to be different from the static. Also, with traditional switching components, power electronic designers want to determine the on-resistance with the circuit in operation, where the transistor is heated up above ambient temperature. Measuring voltage waveforms using an oscilloscope distorts the characteristics of an amplifier inside the oscilloscope when the range of the measurement channel is not set wide enough to measure both on-state and off-state voltage, resulting in failure to accurately measure the voltage waveforms. A novel voltage clamp circuit improving the accuracy of the transistor’s on-state voltage measurement is presented. The measurement accuracy is improved by clamping the off-state voltage across the transistor to a lower voltage that is still greater than the on-state voltage. Unlike traditional clamping circuit, the presented voltage clamp circuit does not introduce delay caused by RC time constants keeping the voltage waveform clear, even during state transitions of the evaluated semiconductor device. The performance of the presented circuit is illustrated by measurements on a 2 MHz inverted buck converter. The dynamic range of the measurement circuit can be further increased by using a power converter that switches at higher frequencies and integrating the measurement circuit on a chip.