# Simulation of inverting Op-Amp

I am trying to build a circuit which is controlled from an analog input signal Vin to produce a -24V signal with >50mA output. My analog card can only produce ~10mA, therefore I generate the signal using an Op-Amp. I used the analog simulation tool from Microchip (MINDI) and the simulation result differs from my hand calculation.

The circuit represents the standard layout of a inverting amplifier. I removed most of filter capacitors and other resistors from the design guideline. Building a non-inverting amplifier with the same principle gives the correct results. Is there a bug in the calculation of the simulation or does my circuit has a significant error?

I am trying to build a circuit which is controlled from an analog input signal Vin to produce a -24V signal with >50mA output.

To produce -24 volts from an op-amp means that it has to have a negative power rail because an op-amp cannot conjure up any voltage that is outside the range of the power rails and, your negative rail is in fact ground (0 volts): -

The MCP6V51 is a rail-to-rail output device so, if the negative rail were (say) -25 volts you could expect -24.9 volts at best.

To produce circa 50 mA requires a more specialized op-amp because the MCP6V51 cannot normally be expected to deliver more than a few mA - it has a short circuit output current of -36 mA and, under normal load conditions, the data sheet implies about 5 mA in the table on page 4.

• I used the simulation for verification of my calculation. The opamp in the circuit is one of the MCP devices supported from the MINDI tool.
– v3xX
Jan 27, 2020 at 11:11
• I don't believe what you have said affects my answer. Jan 27, 2020 at 11:12
• My pick would be ti.com/lit/ds/symlink/opa192.pdf which should provide the necessary current
– v3xX
Jan 27, 2020 at 11:14
• That's the short circuit current (into a zero ohm load). If you look at figure 20 it implies that the voltage dropped in the output stage when driving 50 mA is about 3 volts. You need to account for this and give your negative supply enough headroom (maybe make it -28 volts). However, on some loads you may overheat the device. If for instance you wanted to deliver 50 mA at (say) -10 volts on the output, the op-amp would be burning 0.05 x (28 - 10) watts = 0.9 watts and would go into thermal shutdown. Jan 27, 2020 at 11:36
• If your load is always connected to ground (positive rail) then you can buffer the op-amp output with a BJT emitter follower and take negative feedback from the emitter (R1). Then you choose the BJT that has the appropriate thermal characteristics and copper surrounding it. Jan 27, 2020 at 12:16