OSC and unity gain buffer [Design]

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From: Fred Bloggs on 10 Apr 2008 19:38

From: Fred Bloggs on 10 Apr 2008 19:54

Michael A. Terrell wrote:
> Fred Bloggs wrote:
>
>>Michael A. Terrell wrote:
>>
>>>Fred Bloggs wrote:
>>>
>>>
>>>>Oh? You're quite the ignoramus today...thanx for more ammo:-)
>>>
>>>
>>>
>>>
>>> If you know so damn much about video amplifiers why didn't you tell
>>>the OP that a unity gain buffer is the wrong configuration, or that you
>>>need to use double termination?
>>>
>>> So much for your 'ammo'. OTOH, you ARE always shooting blanks.
>>>
>>>
>>
>>A unity gain buffer would not be the wrong configuration for a series
>>terminated drive into a high impedance termination of the line, such as
>>into a scope. Can you tell me why you would want to use the x2 series
>>termination with a video amplifier? I don't think you mentioned anything
>>more than matching line/termination impedances which means as usual your
>>post is misinformational and/or incomplete...Your statement:"Without the
>>extra gain and resistor you have a sever mismatch which causes no end of
>>problems."
>
>
>
> Fred. double terminated video amps have been the standard for over 50
> years. I can't help it if you don't understand. Look at a few of the
> Linear, Analog or Maxim data sheets for video amplifiers and educate
> yourself.
>
> In the early days the video amp was a cathode follower circuit.
> Early solid state were emitter follower circuits. The amp needs as low
> of an output impedance as practical, then has to be matched to both ends
> of the 75 ohm coax. I apologize if you can't grasp this simple concept.
> Look at simple video distribution designs, too.
>
> The OP said that he is using National's LMC6484 with a 1.5 MHz
> badwidth which isn't a good choice. He didn't show a sample of the
> circuit, or provide a link to it.

> Unity gain amplifiers generally don't
> like high capacitance loads. The series resistance eliminates that
> problem, and provides an excellent match to the cable's characteristic
> impedance.

That is the answer to the question, and the driver does not to be
matched to the impedance, it is only for the reflection...

> Without more information about his design, it is impossible
> to give exact details. He didn't tell us if it is DC or capacitor
> coupled. If it is DC coupled is there any DC offset. That was why I
> asked if he had looked at it with a scope. Th last video amp design I
> worked on had a 0 to -63 dB output control, in .1 dB steps. It also had
> a DC to 40 MHz bandwidth. It is under a NDA, and I had to leave all the
> documentation behind when the job was completed. The gain control used
> an 18 bit serial interfaced D/A converter, and a four quadrant
> multiplier to give a -24 to -63 dB range. For the upper range a set of
> analog switches switched in a 24 dB gain stage. This method allowed the
> full range, while keeping the noise figure at an acceptable level. The
> -3 dB point varried from 49 to 73 MHz in the production boards. The
> embedded controller measured the DC offset and zeroed it, as well as
> setting the gain and switching the extra stage. This was followed by 16
> seperate Sallen Key filters for the desired bandwidth. They were
> selected by a pair of 16 to one analog mux chips.
>
>
> The worst video amplifier I ever had to work with was an RCA design
> with 17 6146 tubes. It was used in a TTU-25B TV transmitter, and
> probably other TV transmitters. One tube drove the other 16 in a
> distributed amplifier configuration, and all 16 had to be matched. It
> modulated the 1 KW stage of the TTU-1 transmitter which was the driver
> for the larger transmitter design.
>
>

What the hell- is that a data dump on everything you know about video
amps? Can't put your finger on anything specifically, you have to dump
the whole fairy tale?

From: Phil Hobbs on 10 Apr 2008 20:11

Tim Wescott wrote:
> eeaj2002 wrote:
>>
>> Hi all,
>>
>> I need help with my circuit please. I have a one MHz osc fed into the
>> positive input of the opamp LMC6484 and the negative terminal is
>> connected to the output (unity gain buffer). At the positive input
>> terminal I have the 1MHz. from the oscillator and I should see one MHz
>> at the output as well but I see distorted sine wave signal. If the
>> gain bandwidth of the opamp is 1.5MHz. should not I see one MHz. at
>> the output of the opamp as well. I did run the oscillator at 3.0
>> Volts and the opamp at 15V.
>>
>> Thanks for you help in advance,
>>
>> John.
>
> You really don't want to run an op-amp right up against it's
> gain-bandwidth product. The design philosophy of an op-amp is that you
> use a crappy amplifier with tons of excess gain, and fix all of the
> amplifier problems with tons of feedback. When you get close to the
> loop bandwidth, you no longer have tons of excess gain with which to
> generate tons of feedback, and all you're left with is a crappy amplifier.
>
> I'd be looking for an op-amp with a G-B product of 10MHz or more (and
> 10x is still wimpy), or if my circuit only needed to be AC coupled I'd
> seriously consider using an emitter- or source-follower to buffer my
> oscillator.
>

Not all op amps are glorified 324s, though some certainly are.

Many of those crappy amplifiers are actually pretty good nowadays,
unless you expect them to be nice and linear up to the supply rails on
both sides, put up with your relatives, cook your dinner, and so on.
I'd get a bit cranky with that sort of treatment myself.

Cheers,

Phil Hobbs

From: Tom Del Rosso on 11 Apr 2008 04:32

"Tim Wescott" <tim(a)seemywebsite.com> wrote in message
news:6rydnbZMeMTA2WPanZ2dnUVZ_g2dnZ2d(a)web-ster.com
>
> You really don't want to run an op-amp right up against it's
> gain-bandwidth product. The design philosophy of an op-amp is that
> you use a crappy amplifier with tons of excess gain, and fix all of
> the amplifier problems with tons of feedback. When you get close to
> the
> loop bandwidth, you no longer have tons of excess gain with which to
> generate tons of feedback, and all you're left with is a crappy
> amplifier.

That paragraph should be in the 3rd edition of AoE.

--

Reply in group, but if emailing add another
zero, and remove the last word.

From: Tim Wescott on 11 Apr 2008 10:54

On Thu, 10 Apr 2008 09:57:21 -0700, Joel Koltner wrote:

> "Tim Wescott" <tim(a)seemywebsite.com> wrote in message
> news:6rydnbZMeMTA2WPanZ2dnUVZ_g2dnZ2d(a)web-ster.com...
>> The design philosophy of an op-amp is that you use a crappy amplifier
>> with tons of excess gain, and fix all of the amplifier problems with
>> tons of feedback.
>
> And here I was about to give you $100k to build me a class A,
> zero-feedback, tube-based audio amplifier because it will OBVIOUSLY
> sound so much better than a traditional design with your approach. :-)

I didn't say that that's the _correct_ design philosophy, just that it's
_the_ design philosophy for op-amp (circuit) designs.

If I say that I believe that tube-based amplifiers are the only way to
go, will you send me that contract for $100K? For another $50K I'll
believe that it needs an output transformer with nickel laminations and
all-silver windings, plus a choke feed with similar construction. If you
want I'll even specify that all the current-carrying components must be
dipped in LN2 before assembly, to align their crystalline structure for
better sound.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

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