Electronic feedback circuits

Fast scan measurements, i.e. for investigations of the dynamics of surface diffusion or reconstruction are done preferably in constant height instead of constant current mode because no electronic feedback circuit, limiting response time and scan speed, is involved in this mode. Obviously this works only with very smooth electrode surfaces. An electronic setup (bipotentiostat) that allows fast transient methods combined with scanning probe microscopies has been reported [21]. 

For in situ investigations of electrode surfaces, that is, for the study of electrodes in an electrochemical environment and under potential control, the metal tip inevitably also becomes immersed into the electrolyte, commonly an aqueous solution. As a consequence, electrochemical processes will occur at the tip/solution interface as well, giving rise to an electric current at the tip that is superimposed on the tunnel current and hence will cause the feedback circuit and therefore the imaging process to malfunction. The STM tip nolens volens becomes a fourth electrode in our system that needs to be potential controlled like our sample by a bipotentiostat. A schematic diagram of such an electric circuit, employed to combine electrochemical studies with electron tunneling between tip and sample, is provided in Figure 5.4. To reduce the electrochemical current at the tip/solution... 

Figure 6. CFT feedback circuit and instrumentation for AC measurements. Reproduced with permission from reference 17. Copyright 1982 Institute of Electrical and Electronics Engineers.

Four-electrode system — Figure. Electronic circuit of a four-electrode potentiostat (X, potential input Y, current output RE1 and RE2, reference electrodes CE1 and CE2, counter electrodes PF, positive feedback circuit for IR drop compensation)... 

Positive feedback circuit — Electronic circuit incorporated in a -> potentiostat, which is used for the - IR drop compensation. Through this circuit, a part of the voltage at the current output of a potentiostat is positively fed back to the potential input, so that the - IR drop can be automatically compensated for. However, note that the positive feedback makes the system unstable and occasionally leads to oscillation. See also - four-electrode system. 

This controllable shift of a laser frequency V] against a reference frequency yR can be also realized by electronic elements in the stabilization feedback circuit. This omits the Pockels cell of the previous method. A tunable laser is frequency-offset locked to a stable reference laser in such a way that the difference frequency / = vl — can be controlled electronically. This technique has been described by Hall [5.98b] and is used in many laboratories. 

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