Methods, systems, and techniques for controlling voltage applied across a piezoelectric stack of a downhole acoustic transmitter. At least one of the temperature of the stack and the compressive stress applied to the stack is monitored. At least one of the temperature of the stack and the compressive stress applied to the stack is compared to a temperature threshold and a stress threshold, respectively. When the stack signal is an alternating voltage signal and when at least one of the temperature of the stack and the compressive stress applied to the stack respectively exceeds the temperature threshold and the stress threshold, the stack signal is modified such that a negative polarity portion of the stack signal has a maximum magnitude less than a magnitude of a negative polarity limit.