A compact 4-channel 224Gb/s PAM4 optical transmitter in 130-nm SiGe BiCMOS technology is presented employing 3-tap feedforward equalizer (FFE) and continuous time linear equalizer (CLTE) techniques. One pre-emphasis tap and CLTE structures broadened the bandwidth to 26GHz. Moreover, the peak-peak value of group delay for this system is reduced to 6.7ps by adopting one pre-fall tap structure. The output stage employs current mode logic circuits to achieve high modulation currents. This chip occupies an area of 1.8mm×1.95mm with a 250um channel pitch. An operating rate of 56Gb/s/ch is achieved when inputting 28Gbaud PAM-4 signal. The single-ended output voltage amplitude is 240mV, corresponding to 5.6mA modulation currents.