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I am using this circuit to supply current through R1 which is a 50Ohm coil. Opamp is Lm124 supplied as shown in the schematic. Vin is a voltage that can be between 2.5V and 3.5V. Everything works fine except when Vin is between 2.8V and 3.1V. In this case the output of the opamp gets really noisy with constant noise exceeding 500mV which makes measuring the output voltage with an adc impossible. Did anyone encounter something similar or sees where the problem is? I already changed the opamp and checked all connections and everything seems to be fine. A 100pF capacitor over 100k reduces this but it is far from enough. Does anyone know other circuits that could achieve this voltage dependent bidirectional current through R1? enter image description here

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Where are your decoupling capacitors on the supply to the op-amp? Show the schematic for the 3 V Vref as that could be the problem. \$\endgroup\$ – Transistor Jan 12 at 22:44
  • \$\begingroup\$ There is one 100nF and a 10uF on the supply. I also added 100uF to the Vref voltage but no help. \$\endgroup\$ – refa Jan 12 at 22:53
  • \$\begingroup\$ 100 uF on Vref won't help much - 100 uF caps don't do much for high frequencies. Try a 1 uF and a 0.1 uF(ceramic) in parallel. And put them right next to the + input of the op amp. Also make sure that you are using a ground plane. \$\endgroup\$ – WhatRoughBeast Jan 13 at 0:28
  • \$\begingroup\$ 6V is kind of low for a supply voltage to this 36V capable op-amp. what voltage is on vref? \$\endgroup\$ – Jasen Jan 13 at 1:04
  • \$\begingroup\$ the load is an inductor! Install an additional 100 ohm in parallel, or 22 ohm in series. \$\endgroup\$ – analogsystemsrf Jan 13 at 1:59
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2.8V/50Ω = 56mA where the rising internal current limiter has entered shutdown and gain drops towards 0. THen the inductor reacts with BEMF and the voltage drops in the opposite direction, resulting in a drop in current so the limiter turns on again and gain resumes. THis continued oscillation is due to over current protection.

A solution to increase current such as an emitter follower for one direction or complementary emitter for either polarity is needed. The loop gain will overcome the Vbe drop to eliminate this error.

schematic

simulate this circuit – Schematic created using CircuitLab

Beware of conducted positive feedback from shared Vref=3V . You must attenuate (decouple) any noise ripple voltage.

Proof.

remove 1pF or keep and add 100pF in SIM.

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  • \$\begingroup\$ The other side of the inductor is fixed to 3V so the drop can't exceed +-1V in this configuration. With gain of 1 where 0.5V/50=10mA is the max current the problem still exists. \$\endgroup\$ – refa Jan 12 at 22:57
  • \$\begingroup\$ Then you must have a layout problem with stray positive feedback. Add LPF from Vref to Vin+ for conducted noise and xx pF negative feedback. \$\endgroup\$ – Sunnyskyguy EE75 Jan 12 at 23:20
  • \$\begingroup\$ Have you used a scope to examine that +3v? \$\endgroup\$ – analogsystemsrf Jan 13 at 5:36

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