# OSC and unity gain buffer

• posted

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.

• posted

This sounds a lot like a homework problem, but I'll help anyway...

-- The slew rate of your LMC6484 there is specified at 1.4V/us

-- What slew rate is required to run a 1MHz sine wave at 3V? (Hint: It's rather more than 1.4V/us... a 1V sine wave at 1Hz has a maximum slope of 2*pi volts/second...)

• posted

s

f 2*pi

Hi Joel,

Thank you for your hint but this is not a homework problem. I was just trying to learn about opamp myself. Are you saying that its slew rate must be greater than 1.4V/us?

Thanks, John.

• posted

"eeaj2002" "Joel Koltner"

Hi Joel,

Thank you for your hint but this is not a homework problem. I was just trying to learn about opamp myself. Are you saying that its slew rate must be greater than 1.4V/us?

** He did.

The maximum slew rate of a sine wave occurs around the zero crossings & is given by the formula:

SR = 2 x pi x Vpk x F

when F is in MHz, then SR is in V / uS.

Your voltage follower must have a SR equal to or greater than the signal it is passing.

If you simply reduce the level or frequency of the sine wave sufficiently, the output wave will become sine.

..... Phil

• posted

The slew rate of the amplifier must be greater than the slew rate of the signal coming out -- otherwise you'll have an output that tries to be a sine wave, but has these straight sections where the amplifier can't slew fast enough.

```--
Tim Wescott
Wescott Design Services```
• posted

OK... besides the usual suspects such as The Art of Electronics, you might want to read something like, "Op Amps for Everyone" by Ron Mancini. It's available as a printed book from Amazon, although it's also available freely for download at

. "Intuitive IC Op Amps" by Frederiksen is also quite good, although it's out of print, having been published in 1984. (I keep meaning to scan it in one of these days and hope that the fine folks at National Semiconductor don't mind if I post it...)

"Are you saying that its slew rate must be greater than 1.4V/us?"

Yep! The "hint" was to suggest how you get the necessary slew rate... at 1V,

1Hz you need an op-amp with a 6.28 (2*pi) V/s slew rate, so at 3V, 1MHz you're up to 2*pi*3*1e6 = 18.8V/us -- well beyond what your LMC6484 can achieve.

---Joel

• posted

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.
```--
Tim Wescott
Wescott Design Services```
• posted

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. :-)

• posted

Thank you all for your help. I need further help please. I wrote a simple program that generate a monochrome NTSC signal. I connected this signal to one of the eye glasses which take the NTSC signal but nothing was displayed. When I connected my signal to the DVD player and I fed the output of DVD player into the eye glasses then it displayed fine. I am assuming the reason for not displaying is the color burst that is missing (When I compared my signal and the output of the DVD player signal, the color burst was missing). This why I was trying to see if I can sum the 3.472MHz signal with my NTSC signal and see if I can display my signal on the eye glasses. By the way the eye glasses have VGA color display that takes the NTSC or PAL signal.

Thank you all,

John.

• posted

More of your generalistic non-informational crap anyone could say. Why don't you explain to us the buffer performance characteristics required to meet the EIA timing error. If you can't do that, you're not useful.

• posted

This is highly unlikely to be the problem; I don't think I've ever encountered a monitor that couldn't handle a regular old (RS-170) black and white signal.

This won't work...

1) The colorburst frequency is 3.579545MHz or somesuch -- but definitely not 3.472MHz. 2) It's a color *burst*, not a color *sub-carrier*. Take a look at the timing of a color video signal and you'll see it's only present during one of the "porch" after the sync pulse on each line. (Perhaps you know to do this and I just misunderstood what you wrote?)

Have you tried feeding your signal into a regular old TV with a composite video input?

Can you take a picture of a 'scope shot to show us what your timing looks like? Last time I was writing software to directly generate video, getting all the timing correct took plenty of time. To begin with you might want to just generate lines of video (containing horizontal sync) but skip vertical sync -- if you generate each line the same, the fact that the frame is "rolling" won't matter.

Note that unfortunately the newer and hence "smarter" a TV is, the less useful it'll tend to be for debugging since it'll want to, e.g., just give you a blue screen when it doesn't "like" your video signal.

---Joel

• posted

than

Oh? You're quite the ignoramus today...thanx for more ammo:-)

• posted

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."

• posted

When you find the answer to the first question, perhaps you can give us a tutorial on differential gain and phase specifications for video amplification and tie this into the application...

• posted

What the F is taking you so goddammed long to answer some simple questions!!! And all this delay from the 12-hour crisis manager, expediter , and miracle worker...LOL...

• posted

Keep it up guys. Soon I'll be able to extract a filterable commonality ;-)

...Jim Thompson

```--
|  James E.Thompson, P.E.                           |    mens     |
• posted

A triple post! Dimbulb, is that you?!

```--
Keith```
• posted

Well, the question is still unanswered, ex-pert...

• posted

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

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?

• posted

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

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.