Increase the drive strength of the circuit
The voltage "sag" is because the source of the signal is using AC coupling (which is to say there is a capacitor in series with the signal).

When the current passes through this capactior it either charges or discharges until there is no more current (which happens when voltage across the load is zero).

To decrease the effect, you can either:


increase the value of the source capacitor so it takes longer to charge or discharge
bypass the source capacitor entirely (this is most ideal).
increase the load resistance so there's less current
increase the frequency so there's less time for the capacitor to charge or discharge