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schematic

simulate this circuit – Schematic created using CircuitLab

I got a sine wave from an STM32 DAC.

Its voltage is 2.64 V. I wanted to amplify it. I used this schematic. Its gain is 4.3. I got 11.20V output of the op amp but the output current is 0.13 mA. I decided to drive a transformer with the output of op amp but the power is too low to do it.

Do you have an idea?

Some people say it is not possible to increase current with constant voltage but I don't know if that's true or not.

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  • \$\begingroup\$ What current do you require? At what voltage and frequency? 'Some people say' - now that's just speculation. Let's try to stick with facts. \$\endgroup\$
    – Kartman
    Apr 5, 2021 at 11:54
  • \$\begingroup\$ @Kartman I need 1 A at 11.20 V and 29 khz. My sine wave's frequency is 29khz. \$\endgroup\$
    – Bowman
    Apr 5, 2021 at 11:59
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    \$\begingroup\$ Have a look at the TL084's datasheet. What does it say about maximum output current and minimum load? \$\endgroup\$
    – Transistor
    Apr 5, 2021 at 12:00
  • \$\begingroup\$ for 1 A output current you will need two big transistors on the opamp output to build an additional class AB output stage. Or maybe there is an integrated class D stage available that can be run off an analog input voltage. \$\endgroup\$
    – tobalt
    Apr 5, 2021 at 12:01
  • \$\begingroup\$ An audio amp of > 12W might do the trick although 29kHz is at the upper end of the audio range. What is the load you want to drive? \$\endgroup\$
    – Kartman
    Apr 5, 2021 at 12:24

1 Answer 1

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A common way to increase drive output current is made with a fairly simple modification shown below: -

enter image description here

Resistor R is used to tie the BJT emitter to the negative rail and the output is taken directly from the emitter to drive a load connected to 0 volts. This class A amplifier will suffice in many cases but, depending on what the load actually is and what current is needed, more sophisticated drive circuits may be needed. The next step would be a class B amplifier: -

enter image description here

And, by the sounds of what load you are driving this might be sufficient. If you really were intent on keeping sinewave distortion to a minimum, you might choose the bias the two BJT bases and make a class AB amplifier.

enter image description here

Or, you may decide to go digital with a class D amplifier: -

enter image description here

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  • \$\begingroup\$ It looks schematically simple until you take the power management for 12 W output power into account. For a sine wave, this will waste ~30 W which might be already needing forced cooling. A class AB will be probably less hassle after all. \$\endgroup\$
    – tobalt
    Apr 5, 2021 at 12:30
  • \$\begingroup\$ @tobalt please feel free to leave an answer that is appropriate for the question's detailed specification in comments (that I was not privy to at the time of writing this answer) AND note that I suggested that more sophisticated drive circuits were needed depending what the load turns out to be. \$\endgroup\$
    – Andy aka
    Apr 5, 2021 at 13:45
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    \$\begingroup\$ @Andyaka sorry, I meant no offense and in fact upvoted since you included other power stage classes. I thought that there is no need for a separate answer when this one pretty much went already in the same direction. \$\endgroup\$
    – tobalt
    Apr 5, 2021 at 17:38

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