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Desirable output resistance

Hello. I have a simple question.

I am currently taking an electronics course and I'm confused by one thing: When is output resistance desirable?

We are studying various typed of amplifiers and transister based circuits (source followers, current sinks, differential amplifiers, etc, pretty common stuff I guess), and we're always finding Rin, Rout, Av, etc of the entire amplifier. Av is a given, but what exactly is the significance of Rout? I would have assumed that a low Rout would be desirable for the same reason that a low Rout is desirable in a voltage source (er... right?). Am I right? Are there times when a high Rout is desirable? Is it always desirable?

Also, on that note, a high Rin IS a good thing, right? I assumed that a lower Rin means that connecting the amplifier has a stronger effect on the circuit and alters the results.

Any help greatly appreciated! Thanks a lot!

--MDL

Reply to
Scale
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Basically, it depends on whether you're building a voltage amp or a current amp.

Voltage amp: High Rin good (doesn't load down whatever's driving it), low Rout good (think of a voltage divider and where you want most of the voltage to drop)

Current amp: Low Rin good (think current divider and where you want most of the current to go), high Rout good (Ditto, but from the other point of view).

If I'm wrong, I'm sure someone will flame me...

Bob Pownall

Reply to
Bob Pownall

Consider that an audio amplifier supplies a typical speaker with a large current to create the loud sound you hear. An audio preamplifier would boost the voltage of its input in order to drive the power amp.

Here's a link to a class D amplifier app note:

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Reply to
Lord Garth

For any circuit using voltage transfer it's never desirable.

However for impedance matched (transmission line) circuits it's essential.

Graham

Reply to
Eeyore

Yes, a high Rin is generally a good thing.

I can however think of an example in audio where you may not want the highest possible Rin.

Dynamic microphones (moving coil) have complex electro-mechanical behaviour. In some cases a very high load impedance will result in undesirable under-damped behaviour which can result in frequency response anomalies.

To take example, 'professional' microphones used in broadcasting, recording and live sound have typical source impedances in the 150-200 ohm region (but with

500-600 ohms also considered 'normal'). The usual load impedance of a mic amp is 2kohms (the 10:1 loading gives good voltage transfer for optimum signal to noise ratio) but a few simply 'sound better' with a classic 600 ohm load.

Graham

Reply to
Eeyore

is

Does anyone know how this company did this introduction on there website.

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it is so awesome. Please contact me if you can help!

Ali

Reply to
Imperiumsbay

One not-so-minor correction to my earlier post.

If you're concerned about maximizing power transfer (for example, if you're working with very small power levels), you'll want to match your input impedance to your source and/or your output impedance to your load, no matter whether the source and load are low or high impedance.

Bob Pownall

Reply to
Bob Pownall

Sure, like this:

IEG

 

:-)

Reply to
Lord Garth

Power transfer is mostly irrelevant to good signal-to-noise ratio.

Graham

Reply to
Eeyore

An Rout lower than necessary might be a problem if the output could be shorted. It might also create instability when driving a capacitive load. If Rin was very high in a circuit driven from a low impedance source, but through a long cable, it might be more sensitive to noise. There are generalizations you can make, but you have to look at the whole circuit.

-- John

Reply to
John O'Flaherty

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