In power electronics, gate-voltage measurement is used to optimize the design of the gate driver. Therefore a proper measurement technique is vital to ensure the proper operation of the electronic device. Measuring a small signal in a high switching voltage environment is a complicated task especially for high-side switches in a half-bridge configuration with fast semiconductor devices where voltage probes are subject to high common-mode voltages with fast transients. Hence, this article compares experimentally the conventional differential probes with optically isolated probes for measuring a small signal (26V) with a 1200V common-mode voltage and high switching rates created by SiC MOSFET (30kV/µs). The conventional differential probe shows differences of measured voltage amplitude up to 10V compared to optically isolated probe. The experimental results prove that parasitic elements of conventional differential probes change the gate-voltage shape and increase the common-mode current in the experimental setup up to 6dB.