Robert M. Fox

Analog AGC Circuitry for a CMOS WLAN Receiver

The IEEE 802.11a standard uses orthogonal frequency division multiplexing (OFDM) to allow high data rates in multipath WLAN environments. The high peak-to-average power ratio (PAPR) of OFDM signals, along with stringent settling-time constraints, make conventional closed-loop automatic gain control (AGC) schemes impractical for WLAN receivers. In a direct conversion receiver, AGC and channel-select filtering are performed by analog baseband circuitry. A baseband signal processor using a new open-loop analog gain-control algorithm for OFDM is described. The new AGC algorithm uses switched coarse gain-setting steps followed by an analog open-loop fine gain-setting step to set the final gain of variable gain amplifiers (VGAs). The AGC was implemented in a 0.18-mum CMOS process using newly designed circuits including linear VGAs, RMS detectors, and current-mode computation circuitry. Simulation and measurement results verify that the new AGC circuit converges with gain error less than 1dB to the desired level within 5.6 mus

Thermal impedance extraction for bipolar transistors

This paper describes a method for extracting the thermal impedance of bipolar transistors. The measurement is a two-step process: first the fractional temperature coefficients are calibrated at dc and then a transient step response is measured to extract the thermal spreading impedance. Measurement configurations and an example measurement cycle are shown. The measurement results can be fitted to multiple-pole models for use in compact circuit modeling in SPICE.

The effects of BJT self-heating on circuit behavior

This study demonstrates the circuit and device conditions under which self-heating can significantly affect bipolar junction transistor (BJT) circuit behavior. Simple quantitative measures are supplied that allow estimation of thermally induced errors in BJT small-signal parameters, based on knowledge of the transistor geometry and its Early voltage. It is shown that errors in output admittance and reverse transadmittance can be significant without much power dissipation, especially when the base and emitter driving impedances are small. Other small-signal parameters are less affected unless the power dissipation becomes significant. Thermal effects in large-signal DC analysis can be significant in precision analog circuits that depend on close transistor matching; such circuits can also exhibit long settling-time tails due to long thermal time constants. ECL (emitter-coupled logic) delay is shown to be insensitive to self-heating. These effects are demonstrated through simulations of a variety of circuits using versions of SPICE modified to include physics-based models for thermal impedance. >

A Micromachined Dual-Backplate Capacitive Microphone for Aeroacoustic Measurements

This paper presents the development of a micro-machined dual-backplate capacitive microphone for aeroacoustic measurements. The device theory, fabrication, and characterization are discussed. The microphone is fabricated using the five-layer planarized-polysilicon SUMMiT V process at Sandia National Laboratories. The microphone consists of a 0.46-mm-diameter 2.25-mum-thick circular diaphragm and two circular backplates. The diaphragm is separated from each backplate by a 2-mum air gap. Experimental characterization of the microphone shows a sensitivity of 390 muV/Pa. The dynamic range of the microphone interfaced with a charge amplifier extends from the noise floor of 41 dB/ radicHz up to 164 dB and the resonant frequency is 178 kHz.

Compact modeling of BJT self-heating in SPICE

Self-heating effects in bipolar junction transistors (BJTs) have been incorporated into PSpice DC and AC analyses. The effects are intrinsic to the operation of the transistor, and are treated within the device model, avoiding the need for thermal subcircuits. A physical thermal impedance model is provided, which allows prediction of the thermal impedance for devices with rectangular emitters from device geometry. A simple approximation is used to predict thermal frequency response. The predictive model can be overridden by measured thermal model parameters. The modifications made to the PSpice code are presented, along with some discussion of implementation alternatives. An example simulation is presented, demonstrating the significance of thermal effects in a typical circuits. Run-time comparisons show that the modified code is about half the speed of unmodified PSPice, mostly because of slower convergence. It is believed that this performance can be improved with suggested implementation changes. >

Bipolar Microwave RMS Power Detectors

A peak/RMS power detector with ges40 dB dynamic range is presented. The simulated frequency response is flat to 60GHz and the measured response is flat to 20 GHz. Analysis shows that the Meyer detector, originally developed as a peak detector, can be used for RMS detection with an error less than 0.5dB over an approximately 20 dB range, comparable to the popular RF/microwave diode detector. The range for RMS detection is extended by cascading several stages of attenuators and detectors, leading to a circuit suitable for applications such as embedded RFIC test. The power detector is only 700times550 mum2 including all AC and DC bond pads

Multiple operating points in a CMOS log-domain filter

A second order bandpass filter was designed by converting a conventional bipolar junction transistor-based log-domain filter circuit to its weak-inversion CMOS equivalent. When biased as designed, the filter performed as expected, but the integrated circuit at times self-biased to an unintended operating point, rendering the circuit useless. This behavior is related to the positive feedback loops used in log domain (and many other) circuits. Such situations cannot, in general, be predicted by inspection of the circuit topology. A method is presented for predicting and analyzing multiple operating points in SPICE circuit simulation. Of course, the methods predictions apply only to the circuit as modeled in the simulator. Detailed analysis illustrates that the number of operating points can depend on the modeling of effects (in this case, well-substrate leakage currents) that are often modeled carelessly, if at all.

MOSFET behavior and circuit considerations for analog applications at 77 K

We present an investigation into the behavior of silicon MOS transistors and analog circuits operated at liquid-nitrogen temperature (LNT). Simple scaling rules are used to predict the LNT performance of CMOS operational amplifier circuits designed for room-temperature operation. Measurements show that unity gain frequency and slew rate can be improved by the same amount as the mobility increase with no loss of stability if bias currents are properly controlled. We also show that room-temperature CMOS amplifier circuits can be redesigned for 77-K operation by reducing channel widths and compensation capacitor area, giving performance equal in most respects to that of unscaled circuits at room temperature. However, 1/f noise is degraded by such redesign. Similar considerations of NMOS amplifiers show that such circuits do not benefit greatly from operation at liquid-nitrogen temperature. To aid in studying the temperature dependence of the sheet resistance of diffused resistors, a computer program was developed based on available models for bulk mobility and carrier freeze-out. Accurate predictions require a temperature dependence for lattice scattering that differs from previously reported values.

Biocompatible, High Precision, Wideband, Improved Howland Current Source With Lead-Lag Compensation

The Howland current pump is a popular bioelectrical circuit, useful for delivering precise electrical currents. In applications requiring high precision delivery of alternating current to biological loads, the output impedance of the Howland is a critical figure of merit that limits the precision of the delivered current when the load changes. We explain the minimum operational amplifier requirements to meet a target precision over a wide bandwidth. We also discuss effective compensation strategies for achieving stability without sacrificing high frequency output impedance. A current source suitable for Electrical Impedance Tomography (EIT) was simulated using a SPICE model, and built to verify stable operation. This current source design had stable output impedance of 3.3 MΩ up to 200 kHz, which provides 80 dB precision for our EIT application. We conclude by noting the difficulty in measuring the output impedance, and advise verifying the plausibility of measurements against theoretical limitations.

Scalable small-signal model for BJT self-heating

The effects of self-heating on BJT (bipolar junction transistor) behavior are demonstrated through measurement and simulation. Most affected are the small-signal parameters Y/sub 22/ and Y/sub 12/. A frequency-domain solution to the heat-flow equation is presented. It applies to any rectangular emitter geometry. This model, although simple enough for CAD, predicts thermal spreading impedance with good accuracy for a wide range of frequencies.<<ETX>>