B.L. Hart

A novel bipolar-drive circuit for medical applications

A novel drive circuit, useful for medical electronics, is capable of supplying a sample of human tissue, across which there should be zero direct voltage (dc), with a well-defined test current from a source having an output impedance exceeding 16 MOmega at 100 kHz.

A novel current-feedback op-amp exploiting bootstrapping techniques

The operation of the conventional current feedback operational amplifier (CFOA) is reviewed and its performance parameters used as benchmarks in the development of a new input stage architecture that provides a common-mode rejection ratio (CMRR) improvement of some 45 dB and offset voltage less than 10 mV.

A wide bandwidth voltage-follower with low distortion and high slew rate

A symmetrical BJT voltage-follower is presented which combines low harmonic distortion and high slew rate. Unlike conventional class-AB voltage-followers the proposed design provides large signal swing, some plusmn4.5 V on a plusmn5 V power supply, as well as large positive and negative output currents for driving capacitive loads, resulting in high slew-rate capability. The circuit exhibits gain flatness of 0.1 dB at 728 MHz with an inductive behaviour coming in at higher frequencies. The operational range is -20degC to +100degC with a power consumption of 55 mW at room temperature.

A hierarchy of input stages for current feedback operational amplifiers

This paper considers the trade-offs involved in the design of six new input stages intended to improve the performance of a current feedback operational amplifier (CFOA), over that possible using an established input circuit configuration, with respect to three major characteristics, viz, common mode rejection ratio (CMRR), offset voltage and skew rate.

Cascode amplifier: A cautionary tale

The cascode amplifier has the potential of providing high gain and high bandwidth simultaneously. However, the design is not as intuitive as one might at first think. In this paper we present a detail analysis of the single cascode amplifiers. The relationship between gain and bandwidth is important. When used to achieve maximum bandwidth the voltage gain of the common-source stage is close to unity. However when the cascode is designed to obtain a high voltage gain, then the gain-bandwidth trade-off, typical in the common source amplifier, re-appears. This analysis is used to provide the basis for practical cascode amplifier design.

Designing a very high output resistance current source for medical applications

A new technique is proposed for the design of a current source, in bipolar technology, with a very high output resistance and low output capacitance. Such a technique is useful in electrical impedance tomography applications. The DC relationships of the basic topology are established. Then, based on the analysis of a simplified equivalent circuit, an expression is formulated for the output resistance and tested against simulation results for both ideal and practical biasing circuits. The effect of circuit capacitances, and in particular, the role of the collector-base capacitance of the output transistor, in determining the output impedance as a function of frequency is considered. Finally, a new circuit is shown to have an incremental output resistance exceeding 200 GΩ.

High performance current-feedback op-amps

This paper presents the designs of the two new CFOAs (current feedback op-amps), one employing a bootstrapping technique, the other a cascoding technique, that provide both high CMRR (common-mode rejection ratio) and slew rate. Moreover, the new CFOA designs exhibit a low DC offset voltage, high bandwidth, and improved gain accuracy, enabling them to be used in applications requiring variable closed-loop gains with constant bandwidth, such as in automatic-gain-control, video, graphics and multimedia applications.

Rail-supply insensitive CMOS current generator

The theoretical background and a selection of simulated results are presented for a 10 µA PTAT current generator that is a modified CMOS version of an existing bipolar configuration. Using standard 0.35 µm CMOS technology, the circuit exhibits a current variation of less than 0.01% when the supply rail voltage is changed from 0.5 to 10 V.

A very high output resistance current source

The vertical stacking of two identical sub-circuits—improved versions of a bipolar transistor configuration proposed by Baxandall and Swallow—driven by dual output current mirrors, facilitates the design of a current generator producing a direct current of 1 mA with an incremental output resistance exceeding 200 GΩ.

A High Frequency Low Distortion Voltage-Follower

This paper presents a novel voltage-follower, implemented in bipolar technology, designed for operation up to 250 MHz. It features high input and low output impedances, low total harmonic distortion (THD) and low offset-voltage