[sdiy] Output impedance check...

[Stewart Pye]

Hi Justin,

In this case, for digital output, I'd just drive the LED via a resistor 
from the op amp as Phil has said. If you use a high efficiency LED the 
resistor could be quite a high value.

Regards,
Stewart.


On 2/8/2013 10:58 PM, Justin Owen wrote:
> Thanks Phil,
>
> Sorry - I should have clarified - inputs/outputs are digital (0V or +10V only). This is an output for a logic device and all negative or AC voltages are dealt with elsewhere.
>
> I'd adjust the brightness of the LED via R8 - it is quite low at the moment I might adjust that at some point but I'm not a huge fan of setting LEDs at retina-burning levels.
>
> Interesting that R10 (and possibly the transistor?) is redundant in this case - being digital. Since I started using LED drivers I've kind of become wary of not using them. There's a lot of RTL in this circuit so load/impedance is important.
>
> Thanks for the 1K confirmation.
>
> Ta!
>
> J
>
>
> -----Original Message-----
> From: Phil Macphail [phil.macphail at liivatera.com]
> Received: 08.02.2013 12:45:19
> To: Justin Owen
> Cc: SDIY List
> Subject: Re: [sdiy] Output impedance check...
>
> 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.
>
>
> Phil.
>
>
> 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!
>>
>> J
>>
>>
>>
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