filter Strange behavior from variable-gain amplifier

Posted on Feb 5, 2014

It takes a voltage between 0. 4 and 1. 2 V on pin 5 to control its gain. In the first two versions of the circuit, I used logic ICs and/or counters and an op amp with a resistor network for a DAC to provide the voltage, and all worked well. This last week, I decided to try my hand at doing the same with an Arduino doing the logic work, and instead of an op amp DAC, I`m filtering a PWM

filter Strange behavior from variable-gain amplifier
Click here to download the full size of the above Circuit.

signal and feeding it to a darlington-pair voltage follower. It took a while to get it right, but now it works. The odd part is, the voltage range that produces the correct gain variation is completely different: 1. 22 V to 2. 25 V. I`m happy that it works, but I`m very curious as to why that is.

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