Hi All, I have simple question about opamps:
If I use OPAMP to read cheapo condenser microphone on one end and generate boosted waveform on other end so I can see it with oscilloscope, what would be the power consumption in this case?
I want to have minimal power consumption because I will be using waveform purely for visual information, I do not need to send signal further to speaker or headphones so I only interested in voltage reads and as low current as possible so that battery drain in minimal, is there any specific OPAMP I need to be looking at or any OPAMP can be limited in how much current it outputs?
I just looked at the datasheet for the "classic" LM741 and it shows a maximum of 2.8mA. With an oscilloscope (at least 10 meghoms?) you'll have essentially no current into the load so that should be about it for the op-amp.
The condenser mic also needs a couple of milliamps (unless it has it's own battery).* And if you are using a single-ended power supply (i.e. one battery), you'll need some additional biasing resistors and those will draw some current.
- If the mic doesn't have it's own battery, make sure your circuit supplies DC power to the condenser. Condenser mics made for live performance use (usually) have their own battery. Computer mics get their power from the soundcard.
So basically if the load on output end of OPAMP is of high impedance (I hope I use correct terminology) then OPAMP's power consumption is it's own internal circuitry power consumption? So it is a matter of finding OPAMP with best power consumption characteristics in the datasheet?
You need to specify which current you are talking about. Oscilloscopes do not constitute a load so the op amp would have no load and you would be reading the no load output voltage.
The op amp input is high impedance and doesn't draw any current. The only current you can discuss is the op amp power supply current . There is nothing else.
is of high impedance (I hope I use correct terminology)
Impedance and resistance* are measured in Ohms. They both "resist" or "impede" the flow of current...
[u]Ohm's Law[/u] says the higher the resistance (or impedance) the lower the current. And, the higher the voltage the higher the current.
The impedance of an oscilloscope is so high that we can ignore it and assume (pretend) it's infinite. If the 'scope were the only load on the battery, we wouldn't ignore it because the battery will last longer on shelf with nothing connected than it will with a 'scope connected.
But, we usually don't talk about the resistance or impedance of the entire op-amp circuit because it can change with conditions. An op-amp with no signal will draw a certain amount of current but if we have headphones connected we are going to draw more current with a signal when the volume is turned-up.
- Resistance is one component of impedance. Inductive reactance and capacitive reactance, are the other components. In a circuit such as yours where there is no inductance or capacitance we can use the terms interchangeably. But, in AC amplifier circuits (such as yours) we usually say impedance.
I want to have minimal power consumption because I will be using waveform purely for visual information, I do not need to send signal further to speaker or headphones
OP specifically states there will be no headphones connected to op amp . Only connection will be the oscope
If I use OPAMP to read cheapo condenser microphone on one end and generate boosted waveform on other end so I can see it with oscilloscope, what would be the power consumption in this case?
The LT1215 is my op amp of choice and the datasheet says IS (supply current) is about 5 mA/per amplifier
(chip is a DUAL AMP package) so total current is 10 mA/per chip.
If you're just using this for driving a 'scope you presumably have mains available - why would super low power consumption be important??
But TI make some very low power consumption rail-rail opamps - see the TLV series
regards
Allan.
I think it's obvious from the nature of the OP's question that the reason he is asking is lack of electronics experience and more than likely power consumption is not an issue at all. He did mention a battery so maybe he just doesn't want to drain it.
At this point it is a mystery.
alexmg2:
Hi All, I have simple question about opamps:
If I use OPAMP to read cheapo condenser microphone on one end and generate boosted waveform on other end so I can see it with oscilloscope, what would be the power consumption in this case?
The power consumption would approximately be an internal DC consumption (due to DC biasing of the op-amp circuitry), plus whatever amount of power needed to drive the load - assuming no overload/saturation etc.
The OP clearly states there is NO LOAD because the oscilloscope does not constitute a load. Anyone who has used a scope knows that.
Thanks for reply everyone, well, right now it is a scope as I am trying to see what's really going on with signal and understand mechanisms behind the scene, difference between different wiring etc. Right now it is the oscilloscope and as a voltmeter I do understand that it has "infinite" resistance so it's not sinking any current thus not consuming power, that I understand now after first replies. I will be experimenting more, jsut ordered few different opamps from digikey, will break them out and start playing, but just thinking out loud more in that direction, let's say you have some kind if DAC in the output, like arduino for example, even in analogRead mode pin can sink current, right? So obviously then it will use some power? Can you limit power consumption by simply adding resistor? Please don't forget I'm a beginner so I can get some things totally wrong in such case don't kick me too hard 
Analog input current is 160 nA.
Can you handle that ?
I suppose I can live with that.
alexmg2:
I do understand that it has "infinite" resistance so it's not sinking any current thus not consuming power, that I understand now after first replies.
Ideal op amp inputs have infinite input impedance. Ideal oscilloscopes are like that too. Practical/real versions of these may have relatively large input impedance when compared with whatever source impedance we have connected. But then again....in some cases where the input impedance is not relatively large.....then it can be a matter for consideration. In your case the oscilloscope won't load down your op-amp much. Not much at all.
Ideal op amp inputs have infinite input impedance. Ideal oscilloscopes are like that too. Practical/real versions of these may have relatively large input impedance when compared with whatever source impedance we have connected. But then again....in some cases where the input impedance is not relatively large.....then it can be a matter for consideration. In your case the oscilloscope won't load down your op-amp much. Not much at all.
No kidding. Well , that was enlightening...
What is an "ideal oscilloscope" ?
I'm not familiar with that one.
Southpark:
Ideal op amp inputs have infinite input impedance.
There's more than that to an ideal op amp:
- Infinite input impedance
- Zero output impedance
- Infinite output amplitude
- Infinite bandwidth
- Infinite open-loop differential gain
Ideal components are impossible to construct with any actual material, so all actual op-amps will be designed with different trade offs of these values and many other non-ideal parameters (input offset voltage, current consumption, supply voltage limits, noise, etc.). That is the reason why there are so many different kinds. One type might be designed to have a high AC bandwidth at the expense of lower input impedance to amplify high speed video signals. Another may trade lower bandwidth in order to have a lower input offset voltage in order to amplify very low level DC signals.
Jiggy-Ninja:
There's more than that to an ideal op amp:
Yeah true.... although, we were only focusing on input impedance. An ideal op-amp does indeed have infinite input impedance (by basic circuit theory definition), just as an ideal oscilloscope has it too. ....click here.....
Hi Jiggy-Ninja..
you forgot:
zero offset voltage
infinite CMRR
infinite psu ripple rejection
zero power consumption
zero temperature coefficient
zero footprint
zero cost (?!)
any more?
In practice one chooses a real opamp as a set of compromises for a particular application..
Price is one of the considerations!
regards
Allan
Price did not seem to be a considerstion when they bought all the TEK scopes at work:
(MSO 4104,MSO 3054 (QTY-2), DPO 2024 (QTY-2), TPS 2024 , and some others plus a dozen HV DIFF probes ( $3000 - $5000 each) plus all the clamp on current probes and let's not forget tge logic analyzer.
allanhurst:
Hi Jiggy-Ninja..you forgot:
I did say "...and many other non-ideal parameters", and I meant many.
any more?
Output current limit (and short circuit survival)
supply voltage limits
How close the inputs and outputs are allowed to get to the rail voltages
Compensation and stability
Noise
Offset nulling inputs
Output disable inputs
And probably many more on top of that.