[SOLVED] Input and Output Impedance

[tahir4awan]

Input and Out impedance always confuse me. We read that for an amplifier input impedance must be large and output impedance must be very small.
The idea of low out impedance is that becase the ipedance of load like speaker is very small so we have to keep ipedance very low.
But what about input impedance why it must be so high what if I connect a low impedance source to the input of an amplifier then what happened.
What if I connect a high impedance source to the output of the amplifier.

 

[KJ6EAD]

Take a look at the article in Maximum power theorem - Wikipedia, the free encyclopedia and this section of All About Circuits: Maximum Power Transfer Theorem : DC NETWORK ANALYSIS.

All About Circuits is a pretty good online textbook to the extent that it's been completed: All About Circuits : Sitemap.

 

[keith1200rs]

If you are talking about audio amplifiers then the maximum power transfer theorem isn't really so relevant. It is more applicable to RF where you are amplifying power. In an audio amplifier you usually start out amplifying voltage. To amplify voltage you want to minimise the loss. So if you signal has 1k source impedance you wouldn't load it with 1k input impedance - you will lose half the voltage. You would try to have maybe 100k input impedance so the voltage loss in negligible.

Keith

 

[LvW]

Of course, I agree with Keith and his explanations which are 100% correct.
Nevertheless, I like to spend some words to answer the question.
At first, tahir4awan, you speak of an "amplifier". Please, realize that you have to discriminate between current and power and voltage amplifiers (the last being the most common types).
So, a voltage amplifier must amplify an input voltage that, normally, cannot be provided with a negligible low source impedance.
But when the amplifier draws a remarkable current (due to a comparable low input impedance) there will be a voltage drop across the source impedance resulting in an amplifier input voltage that is smaller than the original signal voltage to be amplified.
That`s the "loss" Keith has mentioned. Therefore, a good voltage amplifier needs very high input impedances.
Corresponding observations lead to the requirement of a rather low input impedance for a current amplifier.
LvW

 

[tahir4awan]

My question still exists why we distinguish between voltage and current. They are interrelated you cannot isolate voltage or current. For example if you say this is voltage amplifier, is it mean that it doesn't need current and when you say it is current amplifier is it mean it doesn't need voltage.Voltage is nothing but a force and it is current which does everything. We change voltage just for current to flow in a circuit not the voltage its self.
It is said the speaker is voltage dependent means it needs voltage amplification, does it mean it doesn't need current.If it needs current then it is proved that current is driving speaker. It is the motion of electrons in conductor that causes magnetic filed to be generated not the voltage.

 

[KJ6EAD]

You understand, so why get caught up in the semantics. Humans are air, water and food dependent devices, but to say we need air doesn't exclude the other requirements. We say that some things are voltage or current dependent to indicate which parameter must be controlled to achieve a desired performance. It's understood that Ohm's law is not invalidated by our desire to simplify our terminology.

 

[LvW]

Quote: For example if you say this is voltage amplifier, is it mean that it doesn't need current and when you say it is current amplifier is it mean it doesn't need voltage.

Nobody has claimed, that voltage or current are "not needed". For example, a voltage amplifier simply has the task to amplify voltages (as good as this can be achieved in hardware!). And, of course, there will be currents flowing.
Another good example is the BJT: Some people - and even some good and serious textbooks - state that the BJT is a current driven device because of Ic=hfe*Ib.
However, from the "cause and effect" point of view, cetainly this is wrong. A small current Ib never can be the cause for a larger current Ic. Instead, the BJT is voltage controlled (or even more exact: charge controlled) - even if an "input current" is applied (which, in fact, is nothing else than a voltage applied through a very large source resistance).
More than that, since several years the so called "current mode approach" in analog signal processing is discussed in a lot of papers and conference contributions. Here, all IC-internal signals are represented as currents - however, the cause of these currents always are voltages.

---------- Post added at 08:41 ---------- Previous post was at 08:09 ----------

As far as the question "current or voltage mode" is concerned, there are some very instructive contributions from a swiss author Hanspeter Schmid which can be found and identified via google.
As an example, here is one reference (magazine):
Analog Integrated Circuits and Signal processing (35), 2003,pp. 79-90. Kluwer Academic Publishers.

---------- Post added at 09:52 ---------- Previous post was at 08:41 ----------

You understand, so why get caught up in the semantics. Humans are air, water and food dependent devices, but to say we need air doesn't exclude the other requirements. We say that some things are voltage or current dependent to indicate which parameter must be controlled to achieve a desired performance. It's understood that Ohm's law is not invalidated by our desire to simplify our terminology.

KJ6EAD, I cannot agree with you. I assume you are an experienced engineer.
But can you really not understand when somebody (who most probably is a beginner in electronics) is getting "caught up in the semantics" ?
Of course, it is a common desire to "simplify our terminology".
But don`t forget that such a simplification is the second step!
As a first and most important step you have to understand what's going on in resistors, transistors and what is the physical cause of some effects!
And, of course, this touches the relations between voltages and currents.
For example: Is the solar cell a voltage or a current source - and is the answer to this question of any importance?
Or another question (mentioned already in my former answer): Is the BJT current or voltage controlled?
Simplification is good and necessary - otherwise the gain formula for an opamp with feedback would consist of more than 50 terms! However, at first you have to proove which pameter in which particular application can be neglected. But this can be done only if you have a sufficient understanding.
As you know - in electronics nothing is 100% correct; even not the simple voltage-current relationship (Ohms law) in a resistor. But when you know the cause and the amount of disturbances you can assess the influence of these effects which are called "parasitic". But at first, you have to understand!
Having this in mind I completely understand and appreciate the questions from TAHIR4AWAN.

Regards
LvW

 

[KJ6EAD]

LvW, your criticism is fair, and by the way, I am just a lowly technician.:smile: The OP may have tweeked one of my pet peeves, that of people who ask a question but have no patience for the answer or seek to argue with the answerer pointlessly, though the OP may have had no such intent.

 

[tahir4awan]

Thanks LvW