In this paper, a differential SiGe power amplifier (PA) is designed using a bipolar differential pair in a 0.35-μm SiGe BiCMOS technology with through-silicon-via (TSV). Measured using continuous wave (CW) and Long Term Evolution (LTE) modulated waveforms, significant gain expansion (3-5 dB) is observed. The PA reaches power-added-efficiency (PAE) of 61.7% / 51.2% / 40.0% at Pout = 25.6 / 25.4 / 25.7 dBm at supply voltages of Vcc = 2.8V / 3.3V / 4.2V, respectively, with 800 MHz CW input. With the help of the envelope-tracking (ET) technique, the measured PAE improves by 7.3% / 10.4% / 15.4% compared to the fixed supply PA at power back-off regions at Pout = 19.9 / 22.1 / 22.4 dBm, achieving PAE of 38.4% / 43.4% / 38.6% at 800 MHz for LTE 16QAM 5 MHz and passing the LTE spectrum emission mask (SEM) without predistortion. This SiGe ET-PA shows promise for operation as the medium power (MP) PA for efficiency enhancement in the back-off regions.