Dire and Major assitance needed for what should be simple, but poorly explained, problem with OpAmps

Discussion in 'Homework Help' started by WillMelnyk, May 4, 2016.

1. WillMelnyk Thread Starter New Member

May 4, 2016
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Good morning all

Great website and have perused the pages extensively over the last 3 years during my course but now reached an impasse with a problem that I cannot fathom.

An assignment focused on Op amps has a question that all my research garners is an explanation that amounts to nothing more than "because".

The problem.

A design based on ideal characteristics show the 741 op-amp as a suitable oscillator at a frequency of 800kHz.

The question is that; a 741 op-amp is not suitable as an oscillator within a Wein bridge of 3 section ladder circuit at a frequency of 400kHz. Why?

Any ideas or pointers on this would be greatly appreciated, as the last 2 days of reading and research have produced nothing.

Thanks in advance.

Will

2. Marley Member

Apr 4, 2016
171
46
This is due to the frequency you want this oscillator to work at. Basically, you need a certain amount of gain at the working frequency. Probably about 3 for a Wein Bridge oscillator. Also, the phase shift at this frequency need to be taken into account.

Somewhere in the specification of the op-amp there should be a graph of open-loop gain and phase shift verses frequency. This is known as the gain-bandwidth or Bode plot.

Should look like something like the attachment (the attachment is not for a 741 but is similar). Google for a Bode plot for a 741. You will find that a 741 is not a high frequency device.

Can you identify a component inside the 741 that might cause this problem?

• frequencyresponse.PNG
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Last edited: May 4, 2016
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3. WillMelnyk Thread Starter New Member

May 4, 2016
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The corrective capacitor that is used and the rate that it takes to charge and discharge to maintain a stable oscillation?

4. dannyf Well-Known Member

Sep 13, 2015
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417
800khz/3 < 400khz

5. WillMelnyk Thread Starter New Member

May 4, 2016
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What does that equation relate to specifically please? Is that an actual equation for ideal frequency / gain?

6. WillMelnyk Thread Starter New Member

May 4, 2016
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Thanks for your reply, greatly appreciate your time and knowledge with this.

The corrective capacitor that is used and the rate that it takes to charge and discharge to maintain a stable oscillation?

7. Marley Member

Apr 4, 2016
171
46
There is a compensation capacitor inside the LM741. This is added to make the amplifier stable at unity gain (when an external feed back loop is used to set the loop gain to 1). This capacitor causes a reduction of gain with frequency of 6dB per octave (or 20dB per decade). It also causes a phase shift that will reach 90deg. This is also known as the Miller Effect.

It is possible to get op-amps where the compensation capacitor can be external and therefore selected for the application. But all op-amps have some internal miller capacitance and therefore have high frequency gain and phase effects.

You need to research "Bode plots" and "Stability Criterion".

Your amplifier needs to have a phase shift that is not far away from 0deg at the operating frequency. A Wein Bridge is a type of phase shift oscillator. It depends for its operation on having a known phase shift (created by the resistors and capacitors) at the oscillation frequency. Some phase shift in the amplifier will move the oscillating frequency away from the calculated value. Too much will prevent it from working at all. That's assuming you can get sufficient gain at the working frequency. Hope this helps!

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8. WillMelnyk Thread Starter New Member

May 4, 2016
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Massively. Absolutely amazing.

Thank you very much sincerely.

Are you student, or are you an engineer?