Need to BOOST CURRENT at op-amp output

Discussion in 'The Projects Forum' started by alexjamesa, Apr 15, 2009.

  1. alexjamesa

    Thread Starter New Member

    Feb 25, 2009
    Overview. The vibration motor in the circuit I'm working on "tracks" the energy in a particular frequency band of an audio signal. For example, in the filter circuit below, the central frequency is about 2kHz, so when the audio signal has lots of power around this frequency, the motor vibrates a lot; otherwise, it doesn't. The problem I'm having involves getting enough current (200mA) to the little motors.

    Signal Path. Specifically, an audio signal from an iPod (~1V) is inputted to a bandpass filter (attached), which has a gain of 4 and central frequency of 2kHz. So, the filter outputs ~4V when the audio signal has a large 2kHz component; however, the current isn't high enough to drive the motors—it needs to be 200mA. (The motor's impedance is about 50 ohms, and it vibrates with increasing amplitude from 3 to 5 V; there's a bridge rectifier preceding it in the actual circuit.) I've tried a power amplifier (LM386N4) between the filter and rectifier which has worked, but it's not ideal, because it needs a fair number of external components and its minimum gain is 20; I need a gain of 1.

    The Problem: What is a straightforward way to boost the available output current of my filter from 20 mA to 200 mA (without changing the voltage)?

    (I've looked into using an emitter follower, though I'm not sure whether a one transistor ( or two transistor ( design is suitable, and perhaps more importantly, what transistor model to use...)

    Suggestions and advice would be very welcome. Thank you.
  2. bertus


    Apr 5, 2008

    There are power opamps (even upto 10A like the LM12C see datasheet).

  3. -SK-


    Apr 14, 2009
    You could make a simple transistor amplifier; after all, you want to send increased current through the motor, not amplify the output signal of the filter. An NPN or PNP transistor's basic function, when biased correctly, is as a current amplifying device. You will need a few external resistors to bias it, though you can really simplify it if you don't need precisely, say, 200mA at 4V filter output.

    Check this out :

    As far as parts go, there are lots of transistors commonly available that can handle that kind of current. Your location would determine suppliers. As a quick example, I searched and found a 2N6517 from ON Semi, that can handle up to 500mA of current.
  4. SgtWookie


    Jul 17, 2007
    An L272 or L2722 power opamp would solve the whole problem rather neatly, and they're cheap. Just swap out your opamp for one of those, and you're done.
  5. alexjamesa

    Thread Starter New Member

    Feb 25, 2009
    Thanks for the tips. The variety of power amplifiers offered is quite limited where I've been shopping (no L272, L2722 or anything similar), but we'll see what other suppliers in town can offer.

    Regarding op-amp power supply terminals, for an op-amp that has Vcc/GND terminals for the power supply, does this mean that the ground reference for the rest of the circuit is the same as the GND terminal, as opposed to the center tap used for op-amps with V+/V- terminals?