[sdiy] Outputs and Impedances

[Tony Kalomiris]

Matthew and OP,

The OP said he was driving a reactive load (transformer) and "2 voltage
dividers".
>From studying the datasheets of several common op-amps and Jung's book,
along with my engineering textbooks, output voltage swing is definitely
affected by load resistance - probably no big news for you practicing
engineers (I've been out of the game for over 35 years so it's not exactly
fresh in my mind). For example if the load falls below 2K or lower the
"drive" or output voltage swing drops and anything lower and you are
looking at a significant drop. I don't claim to be brilliant , but if I
were driving passive loads, and one reactive one I would use those extra
op-amps in the quad package of that TL074 to buffer the signal. Jung and
manufacturers recommend 200 ohm for reactive loads, and or for output
short circuit protection (only 709 - yes I am reading 1974's first
edition, but he mentions it's a good idea to even with 741 etc which have
indefinite short circuit protection. The plots typically show two loads,
2K and 10 K. The OP can measure or calculate the total load resistance (or
impedance) and check that it's within the recommended range to meet the
typical characteristics. Slew rate is also affected by load so this might
be one the "improvements" the OP was after. Then again none of this might
not matter in non-critical circuits where a few volts drop in peak-to-peak
output is affected. Also why not scope the output, if one is available, to
see if the output had indeed been affected by those various loads?

I realize this is elementary for you local experts but I thought I would
put this out there in an effort to redeem myself and hopefully contribute
something useful the OP may not have considered.
Shouldn't we be encouraging discussion and experimentation and not
spoon-feeding 'expert' answers? I for one lurk most of the time because it
is very intimidating to ask things in here.
I will refrain from advising anyone from now on but instead keep an open
mind and use the question to actually learn or relearn something.

Humbly,

Tony K


On 18-10-25 10:16 PM, "mskala at ansuz.sooke.bc.ca"
<mskala at ansuz.sooke.bc.ca> wrote:

>On Thu, 25 Oct 2018, Youssef Menebhi wrote:
>> I have the output of an op-amp in a non-inverting configuration - and
>> I would like to spread its output to three different inputs.  One
>> input is a voltage divider feeding into a transformer, the other two
>> are just voltage dividers.  If I take three equal-sized resistors
>> (let's say 1k) out of the initial op-amp, will that evenly distribute
>> the signal? Or do I need additional buffer stages for each output?
>
>You shouldn't need the resistors.  Just connect the op amp output to all
>three inputs in parallel.  "Evenly distributing the signal" shouldn't be
>an issue; the op amp's output is intended to act as a pure voltage source
>(very low impedance) and as long as it's able to do that, the presence of
>other inputs in parallel should not affect the voltage seen by each input.
>
>Exceptions would be if the inputs have such low impedances that when
>connected in parallel they require more current than the op amp can
>supply; or if there's an issue of interfacing to the outside world, where
>someone might plug in something weird that screws up both the op amp
>output and the other inputs.  In those cases, you'd need additional
>buffering or protective circuitry.
>
>-- 
>Matthew Skala
>mskala at ansuz.sooke.bc.ca                 People before tribes.
>http://ansuz.sooke.bc.ca/
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