A Low-Distortion Modulator Driver With Over 6.5-V_pp Differential Output Swing and Bandwidth Above 60 GHz in a 130-nm SiGe BiCMOS Technology

Abstract

Optimizing a modulator driver for linear and high-speed operation, while simultaneously achieving a high output voltage swing is very challenging. This paper investigates the design of a highlylinear, high-bandwidth yet power-efficient Mach-Zehnder modulator driver based on the breakdown voltage doubler concept, which overcomes the transistors' physical limitations and enables output voltage swings twice as high as conventional differential pair amplifiers can provide. The low-power design was enabled by the use of an open-collector topology for the output stage as well as by employing resistors instead of current mirrors in order to provide the bias currents for the emitter-follower (EF) stages. We show that by means of this EF implementation approach, the power consumption can be reduced by 19% without sacrificing the circuit's bandwidth and linearity. The driver achieves peak-to-peak differential output voltage swings above 6.5 V pp,d and consumes 670 mW of DC power, being one of the most power-efficient drivers in the literature. The 3-dB bandwidth is 61.2 GHz and the total harmonic distortion is 1%, measured at 1 GHz and for the output swing of 6.5 V pp,d . To the best of the authors' knowledge, these are the highest linearity and output voltage swing reported in the literature for modulator drivers with bandwidths above 40 GHz.INDEX TERMS breakdown voltage doubler, linear broadband amplifier, fiber-optic communication, high voltage swing, low THD, Mach-Zehnder modulator (MZM), modulator driver, SiGe BiCMOS

A Linear Modulator Driver With Over 70-GHz Bandwidth 21.8-dB Gain and 3.4-Vppd Output Swing for Beyond 120-GBd Optical Links

“…The performance comparison of the proposed driver with the state-of-the-art designs is presented in Table I lumped drivers proposed in [2], [11], and [13] achieved a low-frequency gain of over 15 dB, but their output stages were designed with an open-collector topology, which intrinsically has 6 dB more gain than the one with the internal 50output matching resistors. Based on TWA topology, [10], [12] presented drivers with bandwidths of over 67 GHz and low-frequency gains of more than 16 dB.…”

“…The output swing of modulator drivers in CMOS is usually below 3 Vppd [2], [8], [9] as it is limited by the device breakdown voltage, which is technology-dependent. In recent years, drivers designed in technologies such as SiGe BiCMOS [10], [11], [12], [13], [14], [15], and InP DHBT [16], [17], [18] have been investigated because they have a collectorto-emitter breakdown voltage BV CEO over 1.5 V and f T / f MAX above 300 GHz. Those drivers achieve excellent performances: a low-frequency gain of more than 20 dB [2], [10], [13], a 3-dB bandwidth of over 67 GHz [10], [12], [13], [14], [18], and an output swing of beyond 4 Vppd [10], [11], [15] with a data rate of more than 90 Gb/s.…”

“…However, a tradeoff exists between a driver's output swing and bandwidth. To avoid too much bandwidth degradation, some techniques have been exploited, for example, a peaking of over 5 dB [11], [13], using an open-collector output stage [2], [11], [13] and removing electrostatic discharging (ESD) diodes in the high-speed path. However, those techniques have obvious drawbacks, such as excess peaking leading to a large amount of group delay variation, open collector topology implying high sensitivity to assembly parasitics, and no ESD cells, meaning poor ESD protection.…”

“…A driver must have good linearity to support a high-order amplitude modulation format. The traditional drivers achieved good linearity but worsened rapidly if the input power exceeded a certain point [10], [11], [12], [13], [14], [15], [16], [17], [18]. The input power was limited to a small range in those drivers to keep good linearity and deliver the targeted output power.…”