On the TI-99/4A, the cartridge port does not have the facility to offer a traditional RAM expansion. Therefore, a sidecar solution is the best option.
Get the most universal RAM expansion for your TI-99/4A. This 32K RAM expansion uses native 5V static RAM and CPLD for size, simplicity and low power consumption. Typical power consumption < 100mA. If you want a case, there is a design available.
Available on backorder