Optical Modulator Driver Circuit for High-Frequency CMRR
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Solution Overview
Problem
Optical modulator drivers using travelling wave amplifiers face challenges in maintaining a high common-mode rejection ratio (CMRR) in high frequency regions, leading to increased jitter and identification errors in optical receivers, particularly when performing pulse amplitude modulation.
Innovation Solution
The optical modulator driver incorporates a differential amplifier structure with a series circuit and resistor configuration that includes a third resistor connected to one end of a capacitor and a fourth resistor connected to the other end, enhancing the CMRR in high frequency regions through improved current source characteristics and voltage gain linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a conventional travelling wave amplifier structure is used, then the device can operate at high frequencies, but the common-mode rejection ratio (CMRR) deteriorates in the high frequency region (10-30 GHz)
Solution Approach 1:
The patent modifies the electrical parameters of the amplifier circuit by introducing a series circuit comprising a capacitor and resistors connected to the differential pair. This changes the frequency-dependent behavior of the circuit, improving CMRR in the 10-30 GHz range while maintaining high-frequency operation capability.
Solution Approach 2:
The series circuit with capacitor and resistors acts as an intermediary element that mediates between the differential pair and the load. This intermediate circuit modifies the signal path to enhance common-mode rejection without compromising the overall amplification function at high frequencies.
2Speed
If the amplifier is designed for high frequency operation, then speed performance is improved, but waveform quality deteriorates due to increased jitter
Solution Approach 1:
By introducing the series circuit with specific capacitor and resistor values, the patent optimizes the frequency response and damping characteristics of the amplifier. This reduces jitter and improves waveform quality at high symbol rates (25-40 Gbaud) without sacrificing speed performance.
3Measurement precision
If linear amplification is implemented for PAM-4 modulation, then modulation accuracy is improved, but the frequency range with adequate CMRR is limited
Solution Approach 1:
The series circuit modifies the frequency-dependent gain and phase characteristics of the amplifier, extending the frequency range over which linear amplification with adequate CMRR can be maintained. This enables PAM-4 modulation to be performed accurately across a broader frequency spectrum.
Data Source
AI summary
In the optical modulator driver, a plurality of differential amplifiers are arranged in one direction, and input terminals and output terminals of each adjacent differential amplifier are connected via independent wirings. Each differential amplifier includes a differential pair and a series circuit. In the series circuit, a resistor, a capacitor, and a resistor are connected in series. The series circuit is provided between emitters of two transistors of the differential pair, one of the differential input terminals of the differential amplifier and one transistor of the differential pair are connected via a resistor, and the other of the differential input terminals of the differential amplifier and the other transistor of the differential pair are connected via a resistor.