Electronics > Beginners

What's the use of OpAmps?

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David Hess:

--- Quote from: Darkwing on May 01, 2019, 08:23:03 pm ---But what I don't understand is: can they drive loads? Are they suitable for "the power stage" of a project? It doesn't seem so. I would be interested, if someone could point me to a part what I would then call an "OpAmp Driver", that can handle bigger loads.
--- End quote ---

Operational amplifiers are designed two drive their expected load, which is often the input network to another operational amplifier, *and* their feedback network.  So general purpose parts are usually specified for loads of 2 to 10 kilohms.  Low voltage noise parts can drive much heavier loads down to hundreds of ohms because they must use low impedance network to preserve their low voltage noise.  Audio parts might be expected to work with 32 ohm headphone loads.

Video parts can usually drive 150 ohms from a double terminated 75 ohm transmission line plus their own feedback network.  RF parts of course will do the same for 100 ohms from double terminated 50 ohm transmission line.  Some Video and RF parts can drive multiple double terminated transmission lines in parallel.

High output current comes at a price.  The larger transistors take up more space making the die more expensive.  The large transistors also will not fit in the smallest packages.  Higher output current requires higher power dissipation.  Higher power dissipation creates temperature gradients across the semiconductor die which compromises precision.  Thermal feedback from the output transistors to the input transistors actually limits open loop gain among other things so high precision (and low distortion) designs benefit from using an external buffer if they must drive heavy loads like the low impedance feedback network required for low noise.

Back in the 70s, just about everybody who made a 741 had a power version of the 741 like the uA791.


--- Quote ---(Maybe that's why I never saw a use for them in any of my projects so far, because I understand 'amplification' as being able to 'amplify voltage AND current'. [A MOSFET with 1.4mΩ Rdson is not even slightly impressed, when there's a current of 5A or so ... would an OpAmp be?])
--- End quote ---

A very common audio power amplifier topology is just an operational amplifier scaled up to provide hundreds of watts; it has the same 3 or sometimes 4 stages doing the same things.

tggzzz:
It, of course, entirely possible to have an opamp that doesn't fit on a single die. The first opamps were all like that.

Compound opamps avid the weaknesses of integrated opamps, e.g. higher voltage or power etc.

dmills:
You can also get opamp boosters that are typically more or less unity gain POWER stages that are fast enough to place within the feedback loop after the opamp proper and could provide significant output current. The trick is to use a booster that is MUCH faster then the opamp that it is boosting so it does not eat too far into available phase margin.

See for example the BUF634 from TI.

BTW, that 1.4mR mosfet will absolutely notice 5A is you run it in the linear region with say 5V across it (25W will be dissipated), those things are generally designed as switches not linear components. You can get mosfets for linear use, but they tend to be packaged to dissipate the inevitable heat  and have much less emphasis on low RDS(on) as a figure of merit.

Regards, Dan.

MrAl:
Hello there,

The op amp is most basically a negative feedback amplifier with high gain.  This combination means we can get very low error at the output.
The basic operation in most pure analog op amp circuits is the output of the op amp tries to force the two inputs to be the same value.  As it does this, the side effect is that it provides some function like amplification of the input signal.

An interesting side effect is that the output is the inverse of whatever we put in the negative feedback path.  For example if we put a high pass filter in the feedback path then the output becomes a low pass filter for the input signal.  If we put a squaring circuit in the negative feedback path then we get the square root of the input at the output.

Probably the reason it is called an Operational Amplifier is because it is basically an amplifier that enjoys very widespread applications.  From amplifiers to filters to math operations to even logic.  The limiting factor comes from the specifications of the specific op amp being considered.

The LM358 is a very popular and low cost op amp very suitable for experimentation.  It has limitations but the cost is very low.
By contrast the LM741 that used to be popular is much harder to use.

Jan Audio:
If you take rail to rail input and output opamp you can have voltages at input and/or output at the supply voltages, else you have a rail offset with normal opamps wich limits your dynamic range.
These TLV272IP are nice opamps ( someone gave me the hint in this forum and i tryd ), they have input to negative rail, cost around 1 euro, if you use only 5 volt those MCP are cheaper.

For the rest i go with transistors to make things smaller if possible and it wont sound bad, these opamps cost more power.

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