Electronic Design

Current Amplifier Boosts OTA Output

<p>A bidirectional precision current amplifier can be used to increase the output current of an operational transconductance amplifier, or OTA (<a href="/Files/29/11459/Figure_01.gif">see the figure</a>). The circuit is obtained from a simpler design<sup>1</sup> by adding a complementary stage and two diodes.</p> <p> For positive input currents, diode D<sub>1</sub> is forward biased and the upper half of the circuit acts as a current amplifier for I<sub>in</sub>. D<sub>2</sub>, which is reverse biased and conducts no current, and the lower half of the circuit provide amplification for negative input currents.</p> <p> The amplifier's current gain is 11 (set by the 0.5% tolerance resistors). Thus, the maximum output current has a value of 22mA, if the maximum value is chosen for the OTA's I<sub>ABC</sub>. Most OTAs require a bias current of less than 2 mA. The output offset current is less than 150 µA, with the output voltage range being -7 V to +7 V.</p> <p> The circuit can be used with virtually no crossover distortion at frequencies up to several tens of kilohertz. Operation at higher frequencies is limited by the output capacitance of the OTA.</p> <p> Reference:</p> <p> Alexandru Ciubotaru, "D-to-A converter current booster," <i>Electronics World</i> + <i>Wireless World,</i> Sept. 1991; p. 55. </p>

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