Amplifier Feedback Loop Research Articles

Balance in the feedback loop components of the mammalian cochlear amplifier

The objective of this study is to identify the components of the cochlear amplifier feedback loop, viz., the system transfer function and the feedback gain. For maximum positive feedback amplification, loop gain, which is the multiplication product of these transfer functions, would approach unity. Near-unity loop gain requires that the system transfer function and the feedback gain be almost reciprocally matched. Whether such a matched condition exists remains to be shown and would depend on the mechanism of the cochlear amplifier. For the outer hair cell (OHC) electromotility hypothesis, a local lumped model shows that around the characteristic frequency, the feedback gain, which is proportional to the OHC transmembrane potential, is almost reciprocally matched with the system transfer function. This finding emphasizes that the low-pass filtering of the OHC transmembrane potential is not a challenge; instead, it is necessary for positive feedback amplification in the cochlea. Furthermore, such a balance of the feedback loop components exists over the tonotopic axis despite tonotopic parameter variations. These observations made from the local model are shown to remain valid despite global coupling using the global coupled mechanical–electrical–acoustic model of the cochlea.

ID-TIMS U–Pb geochronology at the 0.1‰ level using 1013 Ω resistors and simultaneous U and 18O/16O isotope ratio determination for accurate UO2 interference correction

We document recent advances in analytical protocols that employ 1013 Ω resistors in the Faraday cup amplifier feedback loop for high-precision U–Pb geochronology by isotope dilution thermal ionisation mass spectrometry (ID-TIMS).

High-precision zircon U/Pb geochronology by ID-TIMS using new 1013 ohm resistors

Accessory mineral U–Pb geochronology by isotope dilution thermal ionization mass spectrometry (ID-TIMS) requires precise and accurate determinations of parent–daughter isotope ratios.

Monitoring the pressure impact of a runner's footstep on the inner sole of the shoe

The research goals are to learn about the biomechanics of human footsteps and apply the knowledge to the understanding of the subsequent wave energy that is transferred into the ground or track surface. Phase (1) research is focused on learning about the time development of the footstep pressure that a runner’s foot is transferring to the innersole of an athletic shoe, along with accelerometer information. A Tekscan FlexiForce Model A401 force sensor (25.4 mm sensing diameter and 0.208 mm thick) coated with a polyester substrate was chosen due to its versatile force range if biased and placed in an operational amplifier feedback loop. The response time is <5 microseconds. Two force sensors will be placed on the upper side of a removable innersole to measure force near the ball and heel of the foot. Phase (2) is to use a Digi Intl XBee 802.15.4 Development Kit to communicate (using a wireless link) the transducer voltage responses. The transmitting module is strapped to the runner’s ankle. A receiving module receives data streams and connects to a USB computer interface. Phase (3) converts the data streams to measurements of pressure vs. time. Preliminary experiments and a data analysis will be presented.

Operational Amplifier Based Test Structure for Quantifying Transistor Threshold Voltage Variation

A new test structure has been developed, which is comprised of MOSFET arrays and an on-chip operational amplifier feedback loop for measuring threshold voltage variation. The test structure also includes an on-chip clock generator and address decoders to scan through the arrays. It can be used in an inline test environment to provide rapid assessment of Vt variation for technology development and chip manufacturing. Hardware results in a 65-nm technology are presented. The significance of the bias dependence of Vt variation is discussed for SRAM product designs.

A 155 Mbps laser diode driver with automatic power and extinction ratio control

An integrated laser diode driver (LDD) driving an edge-emitting laser diode was designed and fabricated by 0.35 μm BiCMOS technology. This paper proposes a scheme which combines the automatic power control loop and temperature compensation for modulation current in order to maintain constant extinction ratio and average optical power. To implement temperature compensation for modulation current, a novel circuit which generates a PTAT current by using the injecting base current of a bipolar transistor in saturation region, and alternates the amplifier feedback loop (closed or not) to control the state of the current path is presented. Simulation results showed that programmed by choice of external resistors, the IC can provide modulation current from 5 mA to 85 mA with temperature compensation adjustments and independent bias current from 4 mA to 100 mA. Optical test results showed that clear eye-diagrams can be obtained at 155 Mbps, with the output optical power being nearly constant, and the variation of extinction ratio being lower than 0.7 dB.

A wide-band AC-coupled current source for electrical impedance tomography

A current source suitable for application in electrical impedance tomography (EIT) is described. The first stage of the commercially available current-feedback amplifier AD844 constitutes a current-conveyor implementation and allows the construction of wide-bandwidth current sources, thus avoiding the mismatching and temperature-induced problems that arise in discrete realizations. The lack in gain accuracy of this circuit is overcome by the inclusion of its input buffer in an operational amplifier feedback loop. Saturation problems that appear when placing a DC-blocking capacitor between the source and the electrode are solved by a DC feedback that maintains DC voltage at the output near to 0 V without reducing the output impedance of the source. Two AC-coupled current sources, in both inverting and non-inverting configurations, are described and their possible applications to EIT are listed.

A digital integrator and scan generator coupled with dynamic scanning for scanning tunneling microscopy

A novel digital integrator circuit has been engineered for constant-current mode STM feedback control. In contrast to analog integrators that rely on charging or discharging of a capacitor in an amplifier feedback loop, the integrator uses a 16-bit digital counter that is rapidly incremented or decremented depending on whether the tunneling current is higher or lower than a reference value. The counter drives a digital-to-analog converter (DAC) whose output is amplified and used to control the STM Z-axis scanner. A computer’s parallel input port records the DAC output word at each sampling point, directly giving the digital value of the tip position. The integrator is used in conjunction with a two-axis 16-bit scan generator. The Z-axis feedback reference circuit is used to dynamically control the STM’s lateral scanning rate, and scan rates of 300 μm s−1 are demonstrated. The digital integrator and scan generator facilitate fast switching between different modes of STM operation, allowing glitch- and drift-free locking of tip position, interchange of axes, scanning or switching the sign of the tip bias, and flexible tailoring of the feedback response.