A fully differential operational transconductance amplifier with high gain and high power efficiency was designed based on a novel mirror current source circuit with low voltage drop and high output resistance. The OTA was designed in a 018 μm CMOS process at 18 V supply voltage. The higher DC voltage gain was obtained on the premise of keeping 18VPP differential output voltage swing. The NMOS tube differential pairs was used as input sleeve structure. The results showed that the OTA had an open-loop DC gain of 119 dB, a gain bandwidth product of 526 MHz, and a phase margin up to 77° at 23 mA bias current and 2 pF load capacitance. The OTA’s open-loop DC gain could be increased to 153 dB when using an additional gain boost technology.