Electronics > Beginners
Op amp not working out as planned
StillTrying:
--- Quote from: netdudeuk on April 04, 2018, 04:42:22 pm ---I've replaced the NJM5532 with a MCP602 (http://ww1.microchip.com/downloads/en/DeviceDoc/21314g.pdf) running at 5V with the non-inverting input biased to 2.5V with two 10k resistors.
--- End quote ---
Biasing the +Ve input to 2.5V only makes sense if the signal is AC coupled to the 4k7 by adding a cap in series with it, at 100kHz a 0.1uF will do.
netdudeuk:
Thanks for all the replies so far.
My original plan was to take the AD9833 DDS Signal Generator module and make a basic lab instrument with it.
I believe that I need to boost the output to a more useful level so decided to use an op amp to do that.
I'm now using the MCP602 op amp to do that at the moment but I'd probably like to end up with a range up to about 10Vdc. So, that would need a different single supply rail to rail op amp.
The has an output frequency range of 0 MHz to 12.5 MHz and I'd want to be able to get all or most of that range out of it.
Paul Moir:
You won't be able to get a 10v signal out of an opamp with a 6v max supply like the MCP602. You would need more than a 10V supply because even "rail to rail" opamps get a little flaky in their performance near their supply rails.
Start by making a nice +15v/-15v supply for your opamp and use ones that will work with it. Just an LM7815 + LM7915 will work very well. Then start experimenting with opamps that work within it. Your project is ambitious and it will grow a bit (eg, you'll want an offset control eventually) but you stand to learn quite a lot from it.
EDIT: Maybe just a +12v/-12v would be better and give you a wider range of opamps. Other's thoughts?
netdudeuk:
--- Quote from: Paul Moir on April 05, 2018, 08:50:58 am ---You won't be able to get a 10v signal out of an opamp with a 6v max supply like the MCP602. You would need more than a 10V supply because even "rail to rail" opamps get a little flaky in their performance near their supply rails.
Start by making a nice +15v/-15v supply for your opamp and use ones that will work with it. Just an LM7815 + LM7915 will work very well. Then start experimenting with opamps that work within it. Your project is ambitious and it will grow a bit (eg, you'll want an offset control eventually) but you stand to learn quite a lot from it.
EDIT: Maybe just a +12v/-12v would be better and give you a wider range of opamps. Other's thoughts?
--- End quote ---
Thanks for the reply.
I did mention in the previous post that the MCP602 would be no good for 10V and that I would need a different op amp. Any suggestions please ?
I have a DP832 so I could supply the +15V/-15V with no issues although that would be two out of the three rails leaving me with only a 5V rail.
Single rail still seems easier, especially as I could get up to 15V from the DP832 as a starting point.
Learning is an objective. I've already learned plenty from the project.
Zero999:
A bipolar power supply is probably easier, as there's no AC coupling to worry about.
Start from the beginning.
What peak input voltage and output voltages do you require? That will let you work out the gain, required bandwidth and slew rate.
What load is this going to be driving? Is it going to be via a long cable? If so, the load impedance, will have to match, the characteristic impedance of the cable.
https://en.wikipedia.org/wiki/Characteristic_impedance
Going from what you've said, you need a fairly fast op-amp. A 12.5MHz sine wave, with a peak voltage of 5V (10V peak to peak) is a slew rate of 392.75V/µs (see link below), and if you want a gain of two, you need a gain bandwidth product of more than double the maximum frequency.
http://www.radio-electronics.com/info/circuits/opamp_basics/operational-amplifier-slew-rate.php
I'd suggest a video amplifier IC, as a suitable op-amp won't be cheap. This is not a simple project. Careful attention needs to be given to layout, otherwise there will be problems with instability and oscillation.
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