Driving 8 Ohm Electromagnetic Vibrator With 741 Op Amp Oscillator Output

I've had to design and build an oscillator (which can have its frequency and amplitude altered) capable of driving an electromagnetic vibrating unit. I've created a square wave oscillator using a 741 IC, with an operating frequency from around 20 Hz through too 400 Hz. When hooked up to an oscilloscope the output is a nice and clear 5V ac (10V p-p) square wave. The problem arises when I feed this input to try and drive load, which is the electromagnetic vibrator. The data sheet specifies the vibrators resistance is 8 ohm. What I noticed is that the previously uniform 5V ac square wave collapses to nearly negligible values (500mV) and in turn isn't able to even begin to drive the unit. I've tried using the signal to drive a npn transistor in saturation, so it is acting as a switch, however I don't think the 741 is outputting enough current (max 20mA) to drive the transistor into saturation. My plan was to use the output of the 741 to switch the transistor on and off, which will be directly shorting a 9v battery to the input terminals of the vibrator, as from tests completed a 9v battery provides enough power to drive the unit. I've tried feeding the signal into a LM317 operating as a current regulator to try and increase the current, however the same problem still occurs when the output is used to drive the load, being the voltage collapses and distorts and I believe the frequency range shoots up into the kHz region. Any idea's, thoughts our helpful hints would be very much appreciated.

Ouch, you need 1 amp to drive the transducer. Use an audio power amplifier (Texas Instruments have a range, as I am sure a number of other companies do) Need a modest heat sink, and the load can be placed inside the feedback loop (the amplifier acts as a current output device), which keeps the fidelity of the drive waveform at lower frequencies. What is the corner frequency of the transducer, you need L as well as R, should be in the data sheet.
A single sided switch is usually useless, I don't know what your configuration was, but they can sink but not source current, or vice versa, so have poor performance on one edge of the drive signal as the frequency increases.
An LM317 is not designed for this type of operation.
I have used high power op amps like the Texas parts to drive the coils on YAG oscillators very successfully.
Be careful using spice, sound engineering has to be done first,
Cheers,
Richard

A max of 1A, I've tested it in the lab and it will run off as little as 150 mA. I've sourced a LM386 which is an audio amplifier capable of delivering 3/4 of a watt which may be enough to drive the transducer, however I don't know whether it would be best to use the 386 as the oscillator or feed the output from the 741 into the 386 to boost the signal. The data sheet is fairly vague it states the operating frequency of the device (0.1Hz - 5 kHz), the resistance (8 ohm), max current (1A) and Amplitude (7mm @ 1Hz, decreasing with frequency).
The purpose of the whole system is to be able to demonstrate standing waves and various harmonics, which has been achieved with a laboratory function generator, however creating my own function generator capable of driving the device has proved far more tricky than I predicted.

I was thinking of ordering an OPA547 High Current Op amp, and simply replacing the 741 with this device (of coarse changing any respective pins)? Do you believe this would work

Also I did the simulation for a NPN and PNP transistor in a push pull configuration, but from the simulation it gave a ridiculous value of 1 KA across the 8 ohm load

You can add a NPN/PNP push pull stage using discrete devices, but you may have stability issues to sort out. Did you put the transistors inside the feedback loop of the 741.
The power amp solution has the least components. You are still pushing it with the LM386, I would go for a slightly better part in terms of power, give yourself some headroom. As I said, be wary of spice, you have to do the engineering work on paper first, from data sheets and design info, then put it into spice (bearing in mind that you have no ideas of how complete the models are for the components you are using unless you look at them in the libraries) , then bread board it. A bread board is the only way in the end to verify that the thing is working to spec.
In my opinion, build a separate oscillator using your 741, which you know works, and feed the power amp.
That keeps the function separate. That way you can use a commercial low power function generator if you need to.You will need to sort out drive levels, which are dependent on the gain of the power amp stage,
cheers,
Richard

Further to your comments, the OPA device may well do it (741 separate as oscillator).
For the transducer to follow the signal at lower frequencies, it should be put inside the feedback loop. This complicates the design, however. You have to consider the power dissipation more carefully, the minimum gain the device has, and the power supply headroom. If you are not able to do that, then put the transducer directly on the output as you would a loud speaker,
cheers,
Richard

something similar too this kind of arrangement?

or do i simply send the output of the 741 into the non- inverting terminal of the 386 and ground the inverting terminal... or something similar?

Hi Gene,
re the transducer driver, I am not sure what your experience is in doing this sort of work.I live and work on a small farm just outside Christchurch in New Zealand, so obviously cannot drop in to help. Trying to solve this problem through the forum is going to take some time I suspect.
I suggest you look at this video on using the LM386.
https://www.youtube.com/watch?v=XHkKIEAo1vc
and let me know if is too simple or the right level. If it is OK, then build the circuit as described, and see how it goes.
Note that the signal from your oscillator will need to be attenuated by twenty or so, and AC coupled, before it goes into the pot on the input of the LM386. I find spice is of little help if your design experience is not good enough to understand the pitfalls of spice, but if you don't want to build first, then by all means use it. The schematic supplied in the video will be on the data sheet of the LM386. In fact, the data sheet has enough examples of its use that it should be all you need. Plug those values into your spice model for the LM386 and in theory it should work. Because the LM386 is usually used between ground and a positive supply, that explains the need for AC coupling on input and output. You could run your oscillator the same way, but you have to bias the op-amp inputs up to half rail, are you happy about that?
Let me know how it goes,
cheers,
Richard

No worries I'll check it out now, Thanks for the tip mate. I'm just in the process of messing about with the gain of the LM 386, I was thinking of adding a diode to the output of the 741 to give only the positive half cycle, then feeding that into the LM386, because as you stated the LM386 is designed to operate between (+V and Ground). I'll watch this video first then test the my circuit out.

I'll keep you posted, Thanks for the help Richard!

Gene

Re biasing the op amp inputs upto half rail, that would by simply adjusting the dc offset of the device??

See this link on how to run 741 op amp on a single rail
http://www.learnabout-electronics.org/Oscillators/osc42.php
You will need to put a series resistor and capacitor on the output of the oscillator, the resistor acts as an attenuator, since the LM386 has a min gain of 20.
Don't use 1M on a 741, drop the positive feedback resistors by a factor of 10, offset and bias currents are high on a 741.
One of the disadvantages of capacitive coupling is you can get a big spike due to the coupling capacitors, at the output of your power amp on turn on, which will thump the transducer.

Gene,
see if you can get a copy by Horowitz and Hill of the "art of electronics". It is fairly old now, I believe a new edition is in the pipe line, but the second edition is worth a read,
cheers,
Richard

ohhh k, thanks for all the tips Richard!

I'll be giving these a go today, ill keep you posted.

Regards,

Gene