For real-time monitoring of the on-resistance of a power transistor, the voltage and current should be measured during the switching operation. When the voltage waveform is measured, the amplifier inside the measuring oscilloscope may be distorted if the range of the measurement channel is not set wide enough to measure both on-state and off-state voltage levels, resulting in failure to accurately measure the voltage. Conventional circuits partially solve this problem by clamping the off-state voltage to a lower value. However, they introduce problems such as voltage peaks, measurement offset, and delays caused by RC time constants. In this paper, traditional clamping circuits are explained and discussed to illustrate their disadvantages first. Then, a new voltage clamp circuit is presented. The proposed circuit can solve the problems of RC time constants and voltage peaks during state transitions of the device under test. Finally, the performance of the proposed circuit is illustrated by measurements on a 100-kHz Buck converter.