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O: Fachverband Oberflächenphysik
O 16: Scanning probe methods I
O 16.4: Vortrag
Montag, 14. März 2011, 15:45–16:00, WIL C307
Low noise transimpedance amplifyer for STM applications — •Wulf Wulfhekel and Roland Jehle — Physikalisches Institut, Karksruher Institut für Technologie, Wolfgang Gaede Str. 1, 76131 Karlsruhe
Transimpedance amplifiers, or short IV-converters, are used in scanning tunneling microscopes to amplify the small tunneling currents before processing in the feed back loop and thus largely determine the performance of the instrument. While modern operational amplifiers (OPAs) offer extremely low current input noises (≈1 fA/√Hz), the feed back resistor in IV-converters limits their performance due to thermal or Johnson-Nyquist noise when operated at room temperature. At an amplification of 109V/A the latter corresponds to 4.2 fA/√Hz setting the theoretical noise limit of IV-converters. We present a careful analysis of the different noise sources, i.e. noise of the feed back resistor as well as input current noise, shot noise of the input bias current and input voltage noise of the OPA and present a new approach to IV-converters that beats the noise limit given by the feedback resistor using a sophisticated feed back mechanism. Further, internal capacitances of the circuit are compensated to minimize gain peaking. Finally, we compare the performance (noise and band widths) of the home build IV-converter with commercial products.