Universal Gain Cells for Broadband Amplifiers
Background: Designing distributed amplifiers by using heterojunction bipolar transistors (HBTs) is a great challenge compared with using field effect transistors. The reasons causing this challenge are a) the input capacitance, C?, of HBT is typically an order of magnitude larger than input capacitance, Cgs, of FET for the same operating current leading to lower bandwidth, b) the HBT lossy input characteristic is the shunt resistance, r?, compared with FET having infinite value of input shunt resistance, which causes large attenuation at the input artificial transmission line of distributed amplifier resulting in less number of DA sections, and c) the HBT has high output conductance compared with FET resulting in larger attenuation in the output artificial transmission line of the distributed amplifier. These prevent HBT distributed amplifier to achieve high gain-bandwidth product. The attenuation compensation for HBT distributed amplifiers has proposed by others in order to solve the mentioned problems. Its circuit topology is the distributed amplifier with each stage as a common collector cascaded by cascode amplifier. The reasons enabling HBT distributed amplifiers to achieve high gain-bandwidth product are as followings; the common collector in cascade with cascode amplifier has higher input shunt impedance and lower input capacitance compared with common emitter amplifier, in addition, it generates the negative resistance compensating loss in the artificial input transmission line. The cascode amplifier has high output impedance leading to low loss in the artificial output transmission line. As a result, this topology can dramatically increase the gain-bandwidth of HBT distributed amplifiers. Since this technique can effectively increase gain-bandwidth of HBT distributed amplifiers, many publications have reported the designs of HBT distributed by implementing attenuation compensation technique achieving high gain-bandwidth products amplifiers. Technology: University researchers have developed gain cells for broad-band amplifiers which can be applied to fiber-optic communications as modulator driver, limiting, automatic gain control and transimpedance amplifiers. The amplifiers are more robust to process variations while high-gain-bandwidth can be maintained. Also, the gain peaking frequency and group delay can be actively controlled in contrast to passive ones currently employed in industry. Thus the amplifier bandwidth and group delay can be optimized for each application; i.e., high GBP with low group delay for fiber optic communications and maximum GBP with bandwidth selection for broadband wireless communication. Application: This invention can be used in fiber optic communication systems.
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