Re: OSC and unity gain buffer

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

Relevant Pages

  • Re: OSC and unity gain buffer
    ... gain bandwidth of the opamp is 1.5MHz. ... 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. ...
    (sci.electronics.design)
  • OSC and unity gain buffer
    ... positive input of the opamp LMC6484 and the negative terminal is ... connected to the output (unity gain buffer). ... from the oscillator and I should see one MHz ... gain bandwidth of the opamp is 1.5MHz. ...
    (sci.electronics.design)