Javalab
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Lars Drud Nielsen ldn@oersted.dtu.dk |
In the (normal) case of a large open-loop gain of the opamp (typically >100 dB) the feedback mechanism will force the opamp input voltage difference to be virtually zero. In this limit the Thevenin resistance, RTh, of the circuit will be determined solely by the bridge resistance values.
When the bridge is balanced (upper resistance ratio equal to lower resistance ratio) RTh will approach infinity (only limited by the open-loop gain of the opamp). This special case is known as a Howland controlled current source. The load independent current, Isc, of the Howland source will be proportional to the difference between the two controlling voltages.
With the bridge being balanced, verify the load independency of the current through the load, as well as the virtual absence of influence of a possible common-mode input voltage (as long as it does not drive the the opamp into saturation).
With the bridge being not balanced, observe the occurrence of either positive RTh values (load current decreasing with increasing load resistance) or negative RTh values (load current increasing with increasing load resistance).
Note 1: As pointed out above, the Howland module is equivalent to an ideal current source, controlled by the voltage difference between two arbitrary circuit nodes, P1 and P2. The device injects Isc into a third circuit node, P3, extracting this current from the ground node (via the opamp power supplies). To realize an ideal current source of value Isc between two arbitrary circuit nodes, P4 and P3, two Howland modules may be introduced, one to inject Isc at node P3, another to inject the opposite current -Isc at node P4 (i.e. a quite similar device with the roles of the controlling nodes, P1 and P2, interchanged!).
Note 2: If the controlling nodes, P1 and P2, are not by themselves ideal voltage sources (as presumed in the applet diagram) two additional opamps might be needed as buffer amplifiers (voltage followers). The entire and fully generalized controlled current source may stil be implemented with four opamps on a single chip and eight identical high precision resistors.