I am trying to make a self oscillating fluxgate transformer using an LR oscillator with an op-amp. I replaced the op amp with a square wave generator and the h bridge worked fine but when using op amp it doesn't work.

Circuit schematic:

enter image description here

Transient response:


Vno14 is output of U2 and Vn007 is at the non-inverting terminal of U2.

I am using the H-bridge configuration as the op-amp can't output a high current. I initially designed the transistor bases for measuring a current of 0.5 A. I found this type of current clamp meter has good accuracy, but there's only one research paper I found on the internet and I can't access it.

Functional parts are:

LR type oscillator,H-bridge with op-amp, non-inverting op-amp with unity gain for current sense resistor R8.

Why isnt the output of U2 oscillating? What am I doing wrong?

I am expecting a sawtooth waveform with peaks of +- 0.5A in l1 showing charging and discharging of inductor. When I replaced the opamp u2 with just a square wave generator and ran the h bridge worked fine but on connecting the opamp it didn't oscillate.

  • 1
    \$\begingroup\$ Vno14 cannot be the output of U2 because the diode at the output would clamp this to 0.7 V. The maximum allowed reverse base emitter voltage is violated at all NPN. \$\endgroup\$
    – Jens
    Feb 14 at 17:10
  • \$\begingroup\$ No as i said earlier I designed this circuit just for testing at 0.5 A. i kept getting spikes of voltages so i kept it so that the voltage wouldnt be greater than .7 and mess with the u2. \$\endgroup\$
    – Saso
    Feb 14 at 19:33
  • \$\begingroup\$ And what do u mean when u say reverse base emitter voltage is violate at all NPN. \$\endgroup\$
    – Saso
    Feb 14 at 19:33

1 Answer 1


I tried your circuit in the CircuitLab simulator and it basically works.

Because the typical NPN transistor allows only a reverse Vbe of around -5 V I changed the supply configuration. The H-bridge should run with 10 V, so I created a split supply of +/- 5 V to feed it and use the same supply for the OpAmps.

With this modification the output voltages of OA1 (DRV_1) and OA3 (DRV_2) cannot go below -5 V and so Vbe stays within useful limits.

I removed the diode at the output of OA2 because this would affect correct current measurements through R8.

There was an excessive inductive kickback voltage and to catch this I added D1-D4.

There are short periods during takeover where Q2 and Q4 are both conducting, but this is not dramatic.

This is not a good solution but shows, that the general idea of your circuit is OK. Using PNP transistors at the high side could improve the symmetry and takeover.


simulate this circuit – Schematic created using CircuitLab


This approach uses PNP transistors at the low side (Q2 and Q4). This avoids the issue with large negative Vbe for higher supply voltages and produces less critical takeover because a shoot through is not possible.

The split supply must use equal voltages because the trigger level of OA1 depends on this symmetry. If V1 and V2 would be different, the threshold voltages at IN+ would be different as well.

I added a little imbalance with R16, R17 and R18 to help the oscillation startup. A real world circuit would not need this because real OpAmps always have some offset voltages.


simulate this circuit

  • \$\begingroup\$ Thanks for the answer. I tested your circuit on Lt spice and it worked but there appears to be a flat line intially when the output of u2 is high. Why is that? Will it cause problem? And on increasing the voltage to 10v why doesn't this circuit work? \$\endgroup\$
    – Saso
    Feb 15 at 5:42

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