helpfull ideas for small signal amplifier please. 1

[ElectroNerd]

i would like to amplify a 50Hz ac signal with a variable amplitude of anything from aprox 1mV to 25mV peak...why?... i am currently using a hall effect CT to monitor the output current from an ac generator. i get a dc mV signal from the CT that acts as the input to my prototype engine speed controller. the speed of the generator engine is increased at 4 preset increments depending upon the actual load current. i would like to replace the CT. i have been experimenting with a hall effect transistor. when the tr is placed within the magnetic field of the current carrying conductor an ac signal is imposed onto a 2.5V dc carrier voltage. when i decouple i get an ac signal of 2mV output (rms)for 1 amp of load current. the responce is linear and the max load current expected is 10 amps, giving 20mV AC signal. as i need a dc input of around 1 to 10 volts for my controler i want to amplify the ac mV signal to Volts and rectify / smooth as necissary. i know i can use an opamp to give a gain of up to 1000 but i do not want to add a dual polarity power supply to the circuitry. can i use an opamp on a single rail supply to solve my problem, and if so, how? i have spent a few hours tinkering but have not realy got anywhere. any help or suggestions would be greatly appreciated. thanks everyone.

 

[pebe]

As you're only dealing with AC, you can use an op-amp with a single supply rail, like this:

Use a MOS i/p op-amp like a CA3140 because its input current is lower than a bipolar such as a 741. Connect 2 x 470K resistors in series from Vdd to Vss, and connect their junction to the non-inverting i/p. Couple your i/p to that point with a 100nF capacitor. You will then have your 50Hz signal 'sitting' there at half rail.

Connect a 1M resistor from op-amp o/p to its inverting i/p, and from there connect a 1K resistor in series with a 100uF capacitor to Vss. The 1M/1K gives x 1000 gain and the 100uF gives DC isolation.

At the o/p you will then have your signal x 1000, again with its mean value sitting at half rail. You can then couple it with a 1uF capacitor and fit a resistor (100K?) from that point to Vss to give DC restoration. You can get the peak AC level using a diode and capacitor in the usual way.

 

[nbucska]

For speed control I would use optocoupler. (series R= 100 ohm/V, P=20*V mWatt. Connect the LED into the diagonal of a
diode bridge. Generates 2* freq.
Want sketch? send fax #.
<nbucska@pcperipherals.com>

 

[ElectroNerd]

thanks everyone for your ideas. i have now used the idea suggested by pebe and the problem is solved. thanks pebe. thanks eng-tips, great forum. i will be back quite soon i expect.

 

[BottomTime]

Hi Pebe,

You mentioned DC restoration in your post. Can you suggest a good book that describes what it is and when it's good to use?

I'm looking for a good book or two that goes beyond the basic op-amp circuit fundamentals and delves into the workings of larger circuits.

Thank you,

Randy

 

[pebe]

I'll have a look with Google and see if I can come up with something

 

[BottomTime]

Thank you! I too have been searching the net but lack the experience to know if the books I see cover the topic. Some years ago I used to belong to an engineering book club, but can't remember the name of it. I'd probably re-subscribe if I could find it or another good one.

Thank you again,

Randy

 

[felixc]

Not sure if the "IC Opamp Cookbook" by a Mr Jung still exists but it's been my reference for years. Not just recipes. All the circuit principles were in it.

 

[pebe]

I found this tuition series that includes op-amps.

http://www.allaboutcircuits.com

 

[WorkedVYSV8]

2mV of signal is pretty small, i would have thought that would have just been conducted noise... ?

 

[pebe]

Here's another link for tuition on op-amps.

 

[pebe]

..and another for general tuition.

 

[pebe]

Sorry, missed out the link. Here it is.

http://users.telenet.be/educypedia/electronics/electronicaopening.htm

 

[pebe]

..and the previous one.

http://www.web-ee.com/primers/files/slod006b.pdf

 

[BottomTime]

pebe,

Thank you for your help. I too used to have a copy of the Op-amp cookbook but must have loaned it to someone and haven't gotten it back. I now have the new IC Op-amp cookbook and active filter cookbook on the way from Amazon.

I think I found the answer I was looking for in an old book about television circuits. I couldn't remember why DC re-insertion was required for some video signals but think I now know why. Boy oh boy, use it or lose it!

Thank you for your help, I appreciate it.

Randy