input and output impedance

Hi i know resistance is for DC and impedance is for AC. Impedance is a combined effect of resistance, inductance and capacitance. In short the systems overall opposing effect for the flow of current. But i don't understand the part where in op-amp we need high input impedance and low output impedance. Like if impedance is a blocking effect won't it block the input signal if it is really high?

I came across the following from some posts

"Infinite input impedance would allow one to feed any amount of voltage into a load without it absorbing any power. Zero input impedance would allow one to feed any amount of current into a load without it absorbing any power." and also

"To get all the voltage from a source to a target without loss.
you need high input impedance."

So like from what i got from this is if you want to supply voltage to the input without it drawing any current(or really low current) is the need for high input impedance. but i am confused. Isnt voltage and current inter-related. As in how can we apply voltage to a point if current is not going to flow and if really low current flows just for the circuit to work what is the need for just requiring high voltage. What is wrong if high current flows too??? Because from the first post if high i/p impedance decreases current flow shouldn't it also decrease voltage since V=IR. and why do we need low output impedance. Maybe this i can think of we want to retrieve our signal as full so we don't need much opposing effect. Can anyone explain to me in very very simple words. Really confused. Thank you!!!

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Also found this "Typically, a very high input impedance of op-amps is desirable because that means very little current is required from the source to make a voltage."

what does it mean by very little current required to make a voltage. I thought the required voltage is always applied directly from the power source to the input terminal.

I think what you are imagining when you see input impedance is an inline series resistance leading into the load. What its actually referring to is a equivalent resistance to ground. If you look at the picture below it shows an op amps input impedance (differential input impedance but it shows the general idea) Rin. If you imagine applying a voltage to the amplifier from a source with a given series source resistance you will create a voltage divider with the input impedance. The voltage at the node will be Vin*Rin/(Rin+Rs). If you had a low input impedance most of the input voltage would drop over the output resistance of your source and you would be left with nothing at the input of the amplifier. Alternatively if the input resistance is high or practically infinite your entire input voltage will appear at the amplifiers input.



As for the power portion of your question, P=IV. So if you can keep the current low by having a crazy high impedance you will be able to apply as high a potential as you want without dissipating any power through the load.

I thought the required voltage is always applied directly from the power source to the input terminal.

Click to expand...

Yes an ideal 5V power source will output 5V all the time but depending on the resistance you present to it the current draw will vary.