# increasing voltage using operational amplifier

I want to build own circuit using operational amplifier and resistors. I have 12V power source (car accumulator) and 6V input voltage (Vin). I want to get 12V Vout and 25W at the output. So that is about 2A current(25/12=2).

I heart that operational amplifiers (OA) work only in low currents. Therefore I am not sure that it is appropriate to use OA to implement my aim.

My aim is to get 12V voltage and 25W power at the ouput from input voltage Vin=6V and 12V power source(car accumulator) using following circuit

So is this appropriate circuit? If, no can someone provide alternative curuits? Thanks!

• It sounds like you want a DC-DC converter instead. But is your input 6V or 12V? If you have a 12V power source, why not just take your 25W directly from that? – pjc50 Aug 2 '13 at 9:53
• I wrote that input is 6V and power source is 12V. I can't take directly from that because siren must sound when input is 6V, and when input is 0 Vout must be also 0. – Nurlan Aug 2 '13 at 9:57
• Do you want a switch (in which case a relay is probably the easiest solution) or do you want to modulate the siren with the 6V signal? – pjc50 Aug 2 '13 at 10:15
• Please edit your question to be more about your problem, and less about your solution. It's very unclear exactly what you are trying to accomplish, though it is pretty clear your solution is wrong. – Phil Frost Aug 2 '13 at 11:11
• I suggest a circuit or block diagram showing what you intend. At the moment I think you have a 12V battery with a 6VAC input signal that needs to be amplified to drive a speaker at 12V, 25 Watts. The speaker is for a siren.... very loud siren. – Spoon Aug 2 '13 at 12:06

I'm assuming that you are wanting to amplify a 6V ac signal into a 12V ac signal. Here's the problem; using a power op-amp from a 12V supply is not the problem but the amount of output voltage it can produce will be somewhat less than 12Vpeak to peak.

The output transistors will not be able to turn-on hard enough to produce 12Vp-p out. You might find a device that may achieve 10Vp-p however.

Moving on (and assuming you can produce 12Vp-p out of a power op-amp), the next problem is the current - you have calculated that the current will be $\frac{25W}{12V}$ and concluded that it will be about 2A - this is correct for DC output voltages but some way off the mark if you want an ac signal.

A 12Vp-p sinewave will have an RMS value of $\frac{12}{2 . \sqrt{2}} = 4.24V_{RMS}$

And if you want a power of 25W then the current will be about $5.9A_{RMS}$

You may want to rethink your requirements based on the above. Maybe you are more concerned about having a squarewave output and if so then the problem is finding a power op-amp that can produce 12Vp-p from a 12Vdc supply.

One option is to use a H bridge amplifier if the load you want to drive is suitable for this type of output connection. Another option is to use a transformer output to step up by (say) 25% to accomodate the power op-amp's inability to produce 12Vp-p.