Power amplifiers and opamp circuits have negative feedback that keeps the output voltage constant even if the load current is zero or is a high current (an opamp cannot have a high output current). Then by definition it is a voltage source and its output impedance is very low.

not necessarily.

you can configure opamps (with negative feedback) in such a way that it has very high output impedance - think about all the opamp-based constant current sources.

 

HUH!!

Rifaa

think about it this way: all an ideal voltage source does is to maintain the same output voltage that doesn't change with the load -> aka it has zero output impedance.

think your amp as a variable resistor. let's say that it is connected in serial with an 8ohm speaker to a 9v battery. you want a constant output voltage on the load of 4.5v. so you adjust the variable resistor / amp so that it drops 4.5v current (its resistance = 8ohm).

let's say that you replace the 8ohm speaker with a 4ohm speaker. but you still want to output 4.5v on that 4ohm speaker. you can just readjust the variable resistor's / amp's resistance to 4ohm, and you have an output of 4.5v on the new load.

from the load's perspective, you have created a constant current source of 4.5v -> aka a variable resistor (with a feedback mechanism in you) has an output impedance of 0ohm.

that in essence is what negative feedback is all about.

 

as to the 2nd part of my comments: an amp's gain typically goes down with frequencies. as such, the amount of negative feedback goes down with it. so an amp with negative feedback appears less and less as an ideal voltage source.

As a matter of fact, when the negative feedback goes away and positive feedback sets in, the amp appears to have "negative" output impedance: its output voltage (its amplitude more precisely) goes UP as the load gets TOUGHER / LOWER.

that is the basis for load-invariant amplifiers.

as to high impedance at low frequencies, that's mostly caused by input or output capacitors' inability to pass dc or near dc signals.

hope that helps.

 

So this means no one is sure about impedance matching thingy ???


Rifaa

 

Old vacuum tube audio amplifiers matched the output impedance of the amplifier to the impedance of the speaker. Then the speaker had very poor damping of its resonances and was a "boom box".

Solid state amplifiers are usually DC-coupled to the speaker and have very high negative feedback for an extremely low output impedance that provides excellent damping of the resonances of a speaker. The drive to the speaker is "tight".

So with DC-coupling the output impedance is still very low at very low frequencies.
At 20kHz the amplifier still has plenty of open-loop gain and plenty of negative feedback for a very low output impedance.
At radio frequencies the output impedance rises but who cares?

 

So this means no one is sure about impedance matching thingy ???


Rifaa

not really.

impedance matching serves primarily three purposes (two really): 1) maximization of power transfer; 2) minimizes noise; and 3) minimizes reflection. #3) is important at high frequencies only.

#1 and #2 are related.

#1 is particularly important when you want to transfer energy (think of a power amplifier) from a high output impedance source (a tube amp) to a low input impedance load (a speaker). that's why you use a step-down amplifier to lower the output impedance of the tube amp, or from the tube amp's point of view, increase the effective impedance of the speaker, so as to maximize the output power transfer from the amplifier to the speaker.

that is important because tubes are low gain devices so they cannot afford to apply much negative feedback to them. However, solid state devices are high gain devices, and with negative feedback, they have very low output impedance, so low such that the benefits of impedance matching aren't significant, nor practical: you just cannot produce a speaker that has just a few mohm of impedance.

however, for preamplifiers, it is important to match the impedance. and the same applies to high frequency applications: xdsl modems for example routinely has output resistors of 75ohm to match the impedance in the transmission line / cable - more for reduce reflection though.

 

Quote:
Originally Posted by howartthou

　

Still confused about how an amp has an output impendance with no load. And why the output impendance is meant to match the impedance of a load once connected?
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I rescued this from your thread, it got lost way out there,
hope this could help you understand output impedance better.

To understand output impedance look at these diagrams
The output impedance is the resistor marked RC.
the resistor in the box represents a transistor.

1. shows the output voltage across the transistor which becomes the output voltage fed to a load. This is at no load voltage.

output impedance no load.jpg



2. shows voltage output to a load resistance equal to RC.
output impedance equal load.jpg


3. shows volt. out when load resistance is much smaller than RC.
output impedance small resistance load.jpg


4. finally voltage to the load when you change output impedance RC, to be much smaller than the load resistance.
output impedance small impedance out.jpg


From this you can see why it is important to try to keep the RC (output impedance of amp stage) as low as possible with respect to the load it's driving.

 

Beduin
Thanks for the video links, yes they will help, need to get over the "cirkewit" pronounciation the material is really good.

Audioguru
Thanks for the diagrams and your explanation. Its really helpful. I am getting there.

Jony
Always a good help, thanks.

And thanks to anyone I missed and to all for your patience. Yup, I am still a noob and will keep chipping away until I can one day hold a decent discussion with you people