Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Imput Impedance and output Impedance

Status
Not open for further replies.

neo_star

New Member
wat is the practical use of having high input impedance and low output impedanace in an op-amp or in a transistor
 
Last edited:
In layman terms:
A high impedance input will distort the input signal less than a low impedance input. THis is due to the signal source having to drive the input as if it were a load- a heavier load is of a lower resistance/impedance which means it will draw more current.

A low output impedance means that it can output into a device with a crappier (aka lower) input impedance and have the signal be distorted less.

In technical terms:
Look at this diagram:
File:Source and load circuit Z.png - Wikipedia, the free encyclopedia

-Every signal source is like a voltage source (vs) that outputs the actual signal. Ideally, this is all a signal source would be, but in real life there is some resistance (Zs) in series with the input of the signal source. So a real signal source is more like a voltage source with a series resistor. The actual signal you are trying to transmit comes from the voltage source, but it must pass through the source impedance (Zs) first, and when it does, it will become distorted by a certain amount. How much it becomes distorted depends on how big Zs and Z_L are relative to each other since Zs and Z_L form resistive dividers.

-A signal input can be thought of as a simple resistor (Z_L) with one end tied to ground, and the other end is the input. The voltage, V_L, that appears across this resistor is the signal seen by the input.

Notice that Zs and Z_L are make a voltage divider. Can you see that as Zs gets larger relative to Z_L, the voltage divider steps down the input voltage/signal VL more and more? THis is not a good thing since the input signal VL just keeps getting more and more different from the actual output signal Vs. IN a perfect world, Zs would be equal to zero so whatever the output signal Vs would be exactly equal to the input signal V_L. In real life we can approximate this by making Zs very small and Z_L very large.

Resistances are used for simplicity in this example. The actual term is input IMPEDANCE, not resistance. THis is because there are also capacitors and inductors in parallel with the resistors forming capacitive and inductive dividers. So in real life, the signal does't just get "stepped-down". The capacitors and inductors behave differently for different frequencies in the signal and the signal becomes even more distorted.
 
Last edited:
impedance

Whoa, DK, excellent explanation!:eek::eek:Well done:)
 
In layman terms:
A high impedance input will distort the input signal less than a low impedance input.

Overall I understand your explanation, and it is good for the most part. But, I don't think it's correct to include distortion as part of the explanation. Partially because this seems to be a question of someone learning the basics, but also because much more detail is needed to address distortion.

Basically, the impedance matching of high to low creates maximum voltage gain, and when this is not met, the voltage gain is reduced. The issue of distortion is much more involved and depends on many things. With certain circuitry, poor matching could result in more distortion, but in other cases distortion would be less.

In case the OP finds formula and pictures enlightening, I've attached a PDF for relevant diagram and formula for voltage loading effects.

Note, I realize that it is possible that you are using the term distortion with a different meaning than I'm used to (you did qualify with "layman's terms"). However, generally distortion refers to nonlinear effects like harmonic distortion, so it's good if we clarify the difference so others don't misinterpret the answer.
 

Attachments

  • MultiStage.pdf
    17.5 KB · Views: 187
Hehe, yeah. By distort I simply meant "make different than what it should be".
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top