Voltage controlled logarithmic current source for operational transconductance amplifiers (OTA) with high input impedance

A voltage controlled logarithmic current source (VCCS log) is often needed in an analog synthesizer. The VCCS controlls the current I_abc through the OTA. R_abc limits the max. current through the OTA for protection of damage. R8/R9 determins the saturation. If R9 is to big, current limit is reached before the controll voltage reaches 5V. In this case the modulation is limited and it may lead to distortion. If R9 is to small I_abc is to low. R8 determins the slope of the logarithmic curve. The two input OP are used to keep input resistance high and to have the ability to add some CV together. together.

Specification

Simulation

Schaltplan

The voltage controlled logarithmic current source is build around the operational amplifier on the right. Straight forward as you can see in many books. The two left OpAmps are needed to keep the input resistance high and to build an voltage adder for adding different contoll voltages. D1 and V3 are only used for simulation.

Oscillogram of simulation

Oszilogram

As you can see I_abc (red line) follows an logarithmic curve very well.

Calibration

Offset is trimmed with Tr1. Current limit and saturation are determined through the combination of R8/R9 and R_abc.

Special parts

None. In some application it might be usefull to match the transistors.

Implementation

Detail from NGF_VCA_Sims

Implementation

The implementation follows the simulation. Adjust TR3 to shut down the OTA cpmpletely with no CV attached.

2014 August

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