Re: Video Buffers

From: Mac (foo_at_bar.net)
Date: 01/17/05 

Date: Mon, 17 Jan 2005 01:44:15 GMT

On Sun, 16 Jan 2005 16:02:31 -0700, uvcceet wrote:

>
>>The first example has a gain defined by the resistor ratio. The second
>>one is a follower without voltage gain, mostly used to provide an output
>>drive with a low enough impedance.
>
>>In the video world you normally need voltage gain whenever there is a
>>source termination, and they use that kind of termination a lot. IOW,
>>when there is a 75 Ohm resistor in series to an output to make sure there
>>will be no reflections on a coax cable even if the termination at the far
>>end isn't so stellar. This 75Ohm will then form a voltage divider with
>>the coax, assuming it also has 75Ohm impedance. This causes the signal to
>>drop by 6dB in amplitude (to half its voltage) which can be made up by an
>>amp that offers 6dB gain.
>
> Joerg,
>
> Thanks for the explanation. The buffer with the two resistors is a 2:1
> amp, where both resistors are 1K, so that on the final stage, in series
> with each output line, is the required 75 ohm resistor you mentioned.
>
> Where can I learn how to know what the value of the single feedback
> resistor ought to be on the output stage that uses the follwer, to arrive
> at the required impedance?

The single feedback resistor doesn't set the impedance. It may affect
the offset voltage error or the frequency characteristics of the feedback
(when you consider that the input has some parasitic capacitance). In
classic op-amp theory, when you are dealing with ideal op-amps, the
single feedback resistor does nothing and could be replaced with a short
circuit.

My best guess in this case is that it somehow provides balance for the
DC offset currents flowing in or out of the inverting and non-inverting
terminals, thereby minimising offset voltage. Hopefully someone will
provide you with a better answer. ;-)

Usually, I look to the datasheets and app notes for this kind of
information but I didn't see any for this particular part.

>
> The circuit I am looking at works, but I would prefer to understand why
> the values are what they are, as opposed to just going through life always
> using a 680 ohm feedback resistor, and one day, having a problem and not
> being able to understand it :-)
>
> This probably shows that I am not an expert in OpAmp theory <g>
>
> The amp in question is an EL2244, if that makes any difference.
>
> Much obliged for you sharing your wisdom.
>
> John

--Mac

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