Blackmer’s VCA Cell Implemented Using Subthreshold MOSFETs

In 1973, David Blackmer introduced a voltage-controlled amplifier consisting of a pair of negative-positive-negative transistors performing logging and antilogging functions coupled with a complementary positive-negative-positive pair. This “Blackmer cell” has been widely used in studio equipment such as compressors and mixers. The authors consider replacing the bipolar junction transistors with metal-oxide-semiconductor field-effect transistors in the subthreshold region to exploit the resulting exponential voltage/current relationship, particularly focusing on its implementation on a field-programmable analog array. Resistors used for input voltage-to-current conversion and output current-to-voltage current conversion are replaced with operational transconductance amplifiers. Simulations show that the circuit may be expected to work well as a voltage-controlled attenuator but may react catastrophically when attempting gains greater than unity due to the nonlinearity of the output operational transconductance amplifier. The effect of various mismatches in transistor parameters is studied.