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

A voltage controlled linear current source (VCCS lin) is often needed in an analog synthesizer. The VCCS controls the current I_abc through the OTA. R_abc limits the max. current through the OTA for protection of damage. R7 determins the saturation. If R7 is to small, current limit is reached before the controll voltage reaches 5V. In this case the modulation is limited and it may lead to distortion. D1 is for protection against revers bias for T1. The two input OP are used to keep input resistance high and to have the ability to add some CV together.

Specification

Simulation

Schaltplan

The voltage controlled linear 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 follows the CV very close.

Calibration

No calibration necessary. Current limit and saturation are determined through the combination of R7 and R_abc.

Special parts

None

Implementation

Detail from NGF_VCA_Sims

Implementation

The implementation follows the simulation. Only differnce is the voltage divider R13,R14,R15. It adds a small negativ voltage to the input to guarentee that the OTA realy shuts down completely.

2014 August

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