Electrical – LM358 differential voltage and amplifier circuit

currentoperational-amplifier

I designed a circuit that's rather common:

schematic

simulate this circuit – Schematic created using CircuitLab

And I have a problem. I have got few LM358s, some from ST, some from other companies. (I tested LMV358 also – it puts ~5 V as V_OUT_FINAL).
I made a designed circuit and already put every one of those LM358s in it and none works the way I want them to do. I always get a constant error of 1-3 V as V_OUT_FINAL. I also get wrong result as V_OUT_1 (because it's around 0,5-1 V).
I don't know what's wrong. The values in my real project are wrong in terms to the designed and simulated. I simulated it on many tools already and all give me results like those from attached circuit.

V_IN_1 = 12 V, V_IN_2 ~ 12 V (little less than 12V).
V_OUT_1 = few mV.
V_OUT_FINAL ~ 3 V.

Best Answer

  1. First disconnect second stage input from first stage output.

  2. Biasing (DC) of Pin 2 and Pin 3. Pin3 is held at approximately 1.4 volt and Pin 2 at about 12 volts. Your inputs at those Pins have to be of level which would not require any amplifier circuit. A resistor from Pin2 to ground is required to bias the pin properly. Else connect it to input through a series resistance of 10K. Use coupling capacitors to avoid disruption of dc condition of pins.

  3. Feed back. Pin 2 is fed back through 100K (R4). The present configuration gives stage gain of (R4/R2 = 0.25). R2 value (input impedance) should be (for example) 10K for gain of 10.

  4. Preferably use Variable Resistor (one for a pair) for setting DC conditions of Pins. Later on, you can use fixed value resistance.

  5. After fixing stage 1 check stage 2 dc conditions, connect stage 1 output to stage 2 through a coupling capacitor and check.