# Question about output impedance of output amplifier

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#### andover

##### Newbie level 4
Hi,
I got one question about the requirement of having small output impedance of the output amplifier to drive a low load resistor. I understand if we simply model the output amplifier as a voltage source in series with the output impedance(thevenin), so that most of the voltage signal is on the load, but if we use a current source in parallel with a resistor as the model(norton), we are expecting a large output resistance, so that most of the current is on the load, right?
So my question is why low output impedance is required for the output amplifier.

Thanks

Amplifiers are usually voltage feedback, you model the output as a voltage source in series with the output impedance (Thevenin).

low output impedance is required because the amplifier output is supposed to supply power to the load. You want to be able to put the same voltage on the load if the load is light or heavy.

The lower the output impedance of the amp, the more power can be delivered to the load.

Hi andover,

you gave the answer by yourself:

if we simply model the output amplifier as a voltage source in series with the output impedance(thevenin), so that most of the voltage signal is on the load

low output impedance to prevent loss of signal (gain), and to be able to handle the amount of current required by the load.

Amplifiers have an output impedance of 0.04 ohms or less to damp the resonances of speakers. The amplifier rating is called The Damping Factor.

LvW said:
Hi andover,

you gave the answer by yourself:

if we simply model the output amplifier as a voltage source in series with the output impedance(thevenin), so that most of the voltage signal is on the load

Thank you guys for the answers.

and I don't quite understand why do we model the amplifier as voltage source (thevenin) not current (norton)? any explanations?

Thanks again

The answer is relative to what kind of amplifier you mean. Thévenin and Norton are equivalent representations. But what if you idealize and have no output impedance?

An idealized opamp has voltage gain and no output impedance. The output has to be a dependent voltage source.

Likewise a transistor output is idealized as a dependent current source and infinite output impedance with the output current equal to gm x Vgs

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