[sdiy] Output impedance check...

[Justin Owen]

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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