Re: Function Generator Output Impedance

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Virtually all function generators will have 50 ohms as the lowest output
impedance (600 and 1000 ohms are also popular...)

Bear in mind the following:

The actual output impedance may not be 50 ohms, but it may just be that
when you set the output amplitude to, say, 1 volt, that it will produce a 1
volt signal across a 50 ohm load (allowing for whatever the real output
impedance of the generator actually is...).  If the output really IS 50
ohms, then, with no load (i.e. 1Meg scope input impedance), the generator
will put out 2 volts.

Careful reading of the manual will usually tell what is what.



Now... if you want a low output impedance.. you'll need some sort of buffer
amplifier.  If the power requirements are low, an opamp unity gain buffer
will do nicely, depending on the frequency (don't go trying to put 1 MHz
through a 741 type op-amp...)

If you need some serious power, and the frequency is low enough, a standard
audio amplifier will work (although.. if your load is reactive, the amp
might be unstable).  McIntosh tube amps were (are?) popular for this kind
of thing since they are transformer output and are pretty tolerant of weird
output impedances.

There are a number of fast, moderately high power op amps available (from
Apex, among others), and, one could always build a suitable buffer.


Just a standard emitter follower (source follower if FET) can work pretty
well, although you have to watch bias points, and it will be Class A.



Tesla list wrote:
> 
> Original poster: "Barton B. Anderson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>
> 
> Hi All,
> 
> Question:
> While looking for low output impedance generators, I keep running across 50
> ohms which appears to be typical. Are there lower output impedance
> generators? Are there methods to reduce a typical 50ohm output impedance
> generator down to near zero ohms?
> 
> Thanks,
> Bart