I am feeding a 7809 with a 12v input to generate 9v. The problem is my op amp needs a range of -9v to 9v. How do I set this up?

  • \$\begingroup\$ Basically how do I set up /-9v \$\endgroup\$ – Raaj Mar 22 '13 at 4:58
  • \$\begingroup\$ What are you trying to do with the op amp? A schematic would be helpful. You may not actually need -9V. \$\endgroup\$ – Matt Young Mar 22 '13 at 4:59
  • \$\begingroup\$ I am trying to build an inverting amplifer..which means now my signal will be in the negative range..yes \$\endgroup\$ – Raaj Mar 22 '13 at 8:11
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    \$\begingroup\$ You can build an invert amplifier off a single supply. \$\endgroup\$ – Matt Young Mar 22 '13 at 15:39

If you only have battery power, and need to bias a negative rail on some linear parts, a voltage inverter like this one is a quick and easy way to do it.

block schematic

You will need the IC solution, plus three capacitors. The IC switches the input capacitor \$C_p\$ between +V and ground at a rate determined by \$C_{osc}\$. This makes the output voltage appear inverted, and charges \$C_r\$ to the inverted voltage.

If you go this route, you need to be aware of the current you're pulling from the newly created negative rail. These parts can only source about 10 mA best case.

  • \$\begingroup\$ +1, Coincidentally there was a discussion about the 7660 just yesterday in EE Chat with @jippie. \$\endgroup\$ – Anindo Ghosh Mar 22 '13 at 5:33
  • \$\begingroup\$ oh my..is it possible to do this using a non-IC method? i am not allowed to use other IC's only discrete components.. \$\endgroup\$ – Raaj Mar 22 '13 at 8:11
  • \$\begingroup\$ The Op-amp isn't a discrete component but you say you are using one of these - @user1436508. Use an op-amp to make an oscillator and use diodes and caps to produce a negative rail of (say) 10V then a -9V regulator if needed. It can be done but I think you should have said up-front you only use discrete components. \$\endgroup\$ – Andy aka Mar 22 '13 at 8:24
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    \$\begingroup\$ @user1436508 Please keep in mind that 7809 is an IC too! \$\endgroup\$ – AndrejaKo Mar 22 '13 at 11:19

Edited because of missing the battery bit, new suggestion: http://www.dz863.com/circuits-1-899100259363-SCHEMATIC_Boost-Converter-Generates-Three-Analog-Rails/

Old suggestion which forgot about the 12v battery - use two 7809's on top of each other to create a three-rail supply and decouple the output.

  • \$\begingroup\$ In the @user1436508 comment above he says he can't use ICs. Bit like trying to design a bike without wheels; but he can use op-amps?!? \$\endgroup\$ – Andy aka Mar 22 '13 at 9:51
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    \$\begingroup\$ Yeah, either it's a homework question or the restriction is rather arbitrary and he should challenge it. \$\endgroup\$ – John U Mar 22 '13 at 12:25
  • \$\begingroup\$ Yes that's exactly how it is. It is a project. We are allowed power regulators and op-amps..the rest allowed are BJT Mosfets, caps and resistors.. \$\endgroup\$ – Raaj Mar 22 '13 at 14:19
  • \$\begingroup\$ You could build a basic oscillator & voltage-doubler circuit and decouple the output. \$\endgroup\$ – John U Mar 22 '13 at 14:22

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