# Desirable output resistance

Discussion in 'Electronic Basics' started by Scale, Jun 15, 2007.

1. ### ScaleGuest

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

2. ### Bob PownallGuest

Scale wrote:
<Summary: When do you want an amplifier to have a high Rout vs. a low
Rout. Ditto for Rin.>

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

3. ### Lord GarthGuest

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:
http://www.irf.com/technical-info/appnotes/an-1071.pdf

4. ### EeyoreGuest

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

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

Graham

5. ### EeyoreGuest

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

6. ### Guest

Does anyone know how this company did this introduction on there
website. www.empero.us it is so awesome.

Ali

7. ### Bob PownallGuest

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
load, no matter whether the source and load are low or high impedance.

Bob Pownall

8. ### Lord GarthGuest

Sure, like this:

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9. ### EeyoreGuest

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

Graham

10. ### John O'FlahertyGuest

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.