0
\$\begingroup\$

I am new on designing power management for circuits.

schematic

simulate this circuit – Schematic created using CircuitLab

I want to design a circuit to stabilize the 12v power input. I have a buffer amplifier at the signal input. I use OPA743, which is a rail-to-rail op-amp. I add a bypass capacitor (C3) at the power input and connect the input to both the power supply (pin5) and positive signal input (pin3) for the op-amp. I connect the negative signal input (pin4) to the output (pin1) and negative power supply (pin2) to GND.

I want to have a unity gain from this buffer amplifier. I want the circuit to buffer the 12v power input. I follow figure 4 of page 10 on the datasheet:

Op Amp Figure

But I get an output voltage of around 4v, instead of 12v. Does anyone know what the problem is? Thank you!

\$\endgroup\$
  • \$\begingroup\$ Could you explain a little about what exactly you think this circuit should do and how? \$\endgroup\$ – brhans Feb 13 '15 at 19:51
  • \$\begingroup\$ Sure. I want to have a unity gain from this buffer amplifier. I want the circuit to buffer the 12v power input. I follow figure 4 of page 10 on the datasheet: ti.com/lit/ds/symlink/opa2743.pdf \$\endgroup\$ – Tracy Fu Feb 13 '15 at 19:58
  • \$\begingroup\$ I uploaded a circuitlab schematic. The one you had was really hard to understand - in the future please try to use circuitlab to help us help you! \$\endgroup\$ – FullmetalEngineer Feb 13 '15 at 20:06
  • \$\begingroup\$ Is Vin the same 12V as on the output? Is that 12V on the output what you want or is it the 12V power supply? (No offense, but it's clear you are new to design.) \$\endgroup\$ – George Herold Feb 13 '15 at 20:11
  • 1
    \$\begingroup\$ What is the voltage of Vin? The opamp cannot "stabilize" it, since the op amp will faithfully try to follow it. \$\endgroup\$ – WhatRoughBeast Feb 13 '15 at 21:22
3
\$\begingroup\$

The op-amp can't do anything useful in this configuration. You need to give it more supply voltage than the desired output voltage, even though it's "rail to rail".

If you ask it to output 8V or even 11V with a 12V supply, it may be able to do that, provided the load is not too much for the op-amp to supply. In this case, that might be 10mA or so.

I don't know exactly why you're only getting 4V at the output, but I suspect it's because the load is something like a couple hundred ohms or equivalent, so the output current is limiting at 20-30mA.

Even if you put a lighter load on there, my first comment applies- this circuit can not do anything useful without some changes (more supply voltage compared to desired output voltage).

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.