Re: OSC and unity gain buffer

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
.