[sdiy] Output impedance check...
chris at chrismusic.de
Fri Feb 8 14:22:47 CET 2013
I'm not so sure about R10 being redundant - what if the 072 drives the
transistor into saturation? What would protect the LED from being zapped
Ok, 100k may be little high, but without the base current being
controllable, I wouldn't leave it out completely.
On Fri, 8 Feb 2013 14:45:10 +0200 Phil Macphail
<phil.macphail at liivatera.com> wrote:
> Hi Justin,
> R10 is redundant in this circuit, unless you wan to limit the current through the LED to around 100uA, which seems a little low
And the LED won't light at all until the signal exceeds Vbe plus Vforward of the LED, which is probably 2.5-3 volts total. For negative signal swings Q3 and the LED are reverse-biased which is probably OK for small signals, but you need to check the breakdown voltage for the transistor.
> So to answer your questions, the output impedance will still be 1k but it would be much better to use an op-amp if the output signal isn't digital. If the signal is digital, just drive the LED via a resistor from the op-amp without the transistor.
> On 8 Feb 2013, at 14:30, Justin Owen <juzowen at gmail.com> wrote:
> > Was hoping someone could take a quick look at this to double check my assumptions?
> > Standard op-amp buffer as output. Input is from 'in-circuit', output is to the outside world - yes, where dragons be.
> > http://www.sdiy.org/juz/out_impedance_01.pdf
> > Is the impedance of the jack output still 1K even though there is an LED driver also hanging off the same op-amp output?
> > Any real reason why not to do this under 'normal' use?
> > Thanks!
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