When to use an op-amp?

Discussion in 'General Electronics Chat' started by jaygatsby, Dec 18, 2011.

  1. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
    I am new to the subject. When is it good to use an op-amp over a transistor for amplification?

    Thank you
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    All the time. A bit of overstatement. In some applications the op amp may be overkill. Limitations of op amp.

    * hard to get frequency response over 100 k hz ( but there are some video op amps that go much faster.

    * output current in range of 10's of ma. Use transistor for boost.


    * differential input
    * high input imped. ( for most common types )
    * low parts count
    * simple equations for predicting output
    * useful in wide variety of circuits.
    * response from 0 hz.
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    High frequency tuned amplifiers also more often use transistors, although the vast bulk of circuits are integrated nowadays. Op-amps as such are not common above perhaps some MHz (this limit has been moving up over time - I may be out of date). Some higher frequency amplifiers have specialised designs, not necessarily with differential inputs.

    At really high frequencies, from hundreds of MHz into GHz, amplifier structures tend to become simpler. It is difficult to have a feedback loop extending over many stages at these frequencies without encountering problems with phase lag.
  4. Wendy


    Mar 24, 2008
    Op amps tend to have idealized characteristics. Folks have already mentioned where it isn't used, but in many cases it is fun to play with where it can be used, understanding that sometimes it just doesn't work.

    Overall it is a bit like digital, when it works as expected it can be very satisfying.
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    When you want to build an analog computer*

    *additional parts required.
  6. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
    Thanks! Why is differential input considered such an advantage?
  7. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
    I assume the parts list includes an etch-a-sketch?
  8. Audioguru

    New Member

    Dec 20, 2007
    A differential input can show the voltage difference between two signals. One can be the input and the other can be negative feedback that reduces the extremely high gain to a useable amount and then it reduced the distortion to almost nothing.

    A transistor produces very high distortion.
    jaygatsby likes this.