Operational Transconductance Amplifier - Subsequent Improvements

Subsequent Improvements

Earlier versions of the OTA had neither the I_bias terminal shown in the diagram nor the diodes shown adjacent to it. They were all added in later versions. As depicted in the diagram, the anodes of the diodes are attached together and the cathode of one is attached to the non inverting input (Vin+) and the cathode of the other to the inverting input (Vin−). The diodes are biased at the anodes by a current (I_bias) that is injected into the I_bias terminal. These additions make two substantial improvements to the OTA. First, when used with input resistors, the diodes distort the differential input voltage to offset a significant amount of input stage non linearity at higher differential input voltages. According to National Semiconductor, the addition of these diodes increases the linearity of the input stage by a factor of 4. That is, using the diodes, the signal distortion level at 80 mV of differential input is the same as that of the simple differential amplifier at a differential input of 20 mV. Second, the action of the biased diodes offsets much of the temperature sensitivity of the OTA's transconductance.

A second improvement is the integration of an optional-use output buffer amplifier to the chip on which the OTA resides. This is actually a convenience to a circuit designer rather than an improvement to the OTA itself; dispensing with the need to employ a separate buffer. It also allows the OTA to be used as a traditional op-amp, if desired, by converting its output current to a voltage.

An example of a chip combining both of these features is the National Semiconductor LM13600 and its successor, the LM13700, the data sheet for which can be found here: