DC coupling with a differential amp-is this right? - diyAudio
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Old 1st September 2012, 06:32 AM   #1
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Default DC coupling with a differential amp-is this right?

Hello everyone,
I am in the proccess of designing a simple, one mosfet (per channel) class A amp, and I want to incorporate DC coupling in all stages.
For the input I used an opamp as a buffer, and according to LTspice it does block the DC from reaching my source (yay), so good bye input cap.
For the output I used a differential amplifier. inverting input takes the signal+DC and non inverting is essenssially a low pass filter with a cutoff frequency as close to 0 as possible. Gain is 1. Now bear with me, I am new to electronics and know very little about op amps. But this should work. DC+signal-DC=signal. and according to LT spice the dc offset at the output is in the region of 1-2 microvolts.
So this is correct? and with real components and a few hours of continuous operation how is this affected? it should keep working correctly, no matter what the dc offset in the opamp input becomes.
Now my next step is to make the buffer and differential step discrete, I wouldn't like to have to deal with elements in IC that I cannot really control. The designs I have find (simple 2 transistor differential amps) dont seem to work correctly in the simulation.). Any ideas?
I dont have the time to upload the schematic, plus it looks a bit messy right now. I'd appreciate some enlightment. thanks.

Last edited by spyrusthevirus; 1st September 2012 at 06:50 AM.
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Old 1st September 2012, 10:47 AM   #2
DF96 is offline DF96  England
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Real opamps have bigger offset than the perfect opamps used in simulations. LTspice is probably just calculating the input offset caused by input current passing through input resistors. An opamp on its own is most unlikely to block DC, so I suspect you are misunderstanding what LTspice is telling you.

Why do you want DC coupling in all stages? As you are new to electronics much better to use AC coupling - less to go wrong.
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Old 1st September 2012, 12:09 PM   #3
AndrewT is online now AndrewT  Scotland
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If you don't understand the circuit, you cannot ask LT Spice the correct questions.
Without the correct questions you cannot get the answers you need to determine if the circuit is working properly.
See the problem. LT Spice pumps out an answer and because you don't understand the circuit you can't tell if the answer is right or wrong.

You must understand before you can ask.

There is absolutely no point in asking LT Spice the wrong question !
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regards Andrew T.
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Old 1st September 2012, 01:51 PM   #4
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My "problem" is that I want to avoid the decoupling capacitors or an output transformer. The transformers will be a bit expensive for me (I think?) and capacitors will reduce the performance on the low end. Instead of scouting the internet for a ready solution I'd like to work it out on my own.
So maybe I should redifine my question. Through what principle can one achieve DC coupling? despite the implementation used.

I'm new to making electronics with my hands. I am studying electronic engineering and have the course foundations for circuits and electronics (discrete ones at least). But we haven';t seen op amps in class yet, so yes they are new to me. But practice is very diffrent from theoretical analysis most of the times, so I am asking for some help. So please bear with me :P

So why wouldn't the DC coupling in the output stage not work? Because of the non-idealities of op amps or something in their principle of operation that I am missing?
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Old 1st September 2012, 05:10 PM   #5
DF96 is offline DF96  England
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Opamps work down to DC, therefore they pass DC. Practice is not very different from theoretical analysis most of the time, unless the theory is hopelessly misapplied or overly simplistic. People sometimes apply the wrong theory, or apply the right theory in the wrong way, then find that practice is different; instead of concluding that they have done it wrong, they conclude that 'theory' doesn't work in practice. The wrong theory rarely works!

The correct capacitors do not reduce performance, especially for the simple circuits used by beginners. If you want to work it out on your own why are you asking questions here? Much better to 'scout' the internet - there is a lot of useful information out there.

To achieve DC coupling you ensure that the DC output of one stage provides the correct bias for the next stage. It adds complexity to circuit design, even when the final circuit looks simpler than one using AC coupling, because you can't design one stage at a time.
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Old 1st September 2012, 05:39 PM   #6
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Quote:
Originally Posted by DF96 View Post
If you want to work it out on your own why are you asking questions here? Much better to 'scout' the internet - there is a lot of useful information out there.

To achieve DC coupling you ensure that the DC output of one stage provides the correct bias for the next stage. It adds complexity to circuit design, even when the final circuit looks simpler than one using AC coupling, because you can't design one stage at a time.
I was wondering if what I thought off made any sense. With my limited knowledge it did make sense, so if I did not ask I'd think I had found a valid, working solution. Which apparently is not the case, so I'd say I did the right thing.

So when we use op amps we ignore the DC offsets they output because of the bias like we do in transistor small signal analysis? But they are there, correct? How would I theoretically calculate that offset for an opamp implementation, like for example the Cmoy?
I admit I had a look after my second post at a solution by Benny Jørgensen available here HeadWize - Project Addendum: A Class A MOSFET Headphone Driver by Greg J. Szekeres
I had a "damn Im an idiot moment". But I'm slowly getting the idea I think. I worked out how this works on that particullar amp, tomorrow I'll try to work out something "similar" for other amp topologies.
How can I verify that my solution is working properly?
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Old 1st September 2012, 06:02 PM   #7
DF96 is offline DF96  England
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You can't calculate the offset. The most you can do is calculate the maximum offset, assuming that the opamp meets its spec. So you might end up with a calculation saying that the output offset will be somewhere between -10mV and +25mV. This may or may not be sufficient, depending on how sensitive the next stage is. Remember, with DC coupling the DC stage gain may be the same as the AC stage gain (or it may not be the same - depends on the circuit).
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Old 1st September 2012, 06:12 PM   #8
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I see. So should I need to find out that offset I'd have to take the actual component and measure it. And stil I would not be totally sure if that measurement will remain the same as the circuit works and warms up, drifts and so on. So the most suitable thing is to go with the worst case scenario, that there is DC offset and it is more than you could handle in your implementation (which could not be the case at all but hey, worse case it is...), do i understand this right?
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Old 1st September 2012, 06:46 PM   #9
DF96 is offline DF96  England
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Yes, you have to assume there will be a DC offset. To cope with this there are various techniques:
1. AC coupling using capacitors or transformers
2. DC offset adjustment using preset pots
3. DC servos
4. capacitors in feedback loops to reduce DC stage gain below AC stage gain - but then you have capacitors again

By far the simplest option is AC coupling, which is why it is used so much especially in basic circuits.
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