[sdiy] transformers, power amps, and op amps.

Nils Pipenbrinck n.pipenbrinck at hilbert-space.de
Wed Aug 19 12:13:12 CEST 2015


On 08/19/2015 09:15 AM, alfred.pear at gmail.com wrote:

> i have the following questions: 
> 1. to drive a speaker, the current is scaled up considerably - is it
> safe to send it back into an op amp? i know that operational amplifiers
> theoretically reject current, but how do i determine their limits? and
> does high current impact offset voltage?

The opamp input only takes as much current as it needs, not as much as
the power source could provide.

So yes, it is safe.


> 2. if i use a transformer at the output, i could potentially be stepping
> up the voltage considerably - is that safe to send that back into an op
> amp? 

If you use a transformer to step up the voltage and feed-back the signal
from the secondary side of the transformer back to the opamp inverting
input, then the gain would not change at all. The voltage gain provided
by the transformer just gets compensated by the opamp doing less
amplification.

So you don't get any benefit from doing so. You even loose power because
of the loss in the transformer.

You could feed-back only a fraction of the transformer output voltage
back to the input. In this case you can generate output voltages higher
than the supply voltage. The signal would be less beefy though (aka it
can source less current).

In general you should never have a reason to use transformer with
transistor audio amplifier circuits. Output transformers have their own
problems. For example there is an effect called inductive kickback that
can destroy your driver stage if you unplug the load from the secondary.

Output transformers are used in circuits where you operate at high
voltages and low currents (tube amplifiers). The output transformer is
used in such circuits to step down the high voltage, high impedance
signal a lower voltage, lower impedance signal.

Best,
  Nils



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