OpAmp Non-standard Feedback

In summary: Again, the circuit designer has the duty of surrounding his amplifier with a feedback circuit that allows it to hold its inputs equal. If he doesn't do this, the amplifier won't "operate".
  • #1
Rudinhoob
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0
OpAmp "Non-standard" Feedback

Hello,

This is my first post here. Please excuse my poor English and grammar. I will try to make my questions clear.

As I understand, with a negative feedback, where the output connected to the inverting input and in the ideal case, the Vn and Vp will be very close in a sense that Vn tracks the voltage of Vp so that the difference is almost zero.
However, when changing the feedback configuration, for example connecting the Vn input to other parts than the output, or using a positive feedback, how does this affect the ideal rules in analysis?

Thanks.
 
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  • #2


That's a hard concept to wrap your brain around when first starting out.

The little integrated circuit we call an "op-amp" is just a high gain amplifier.

When we surround an "op-amp" with a feedback circuit that allows it to hold its inputs equal,
we now have an electronic circuit that behaves in accordance with some mathematical "operation" defined by the feedback network.
So we call it an 'operational amplifier' circuit.
Perhaps we could have done better with the naming comvention, perhaps not.

The circuit you describe , signal applied to +in and Vout connected to -in, performs the math operation Vout = Vin - 0.
More precisely, Vout = Vin(1-1/(amplifier's open loop gain)),
and since open loop gain is around a million that reduces to ~Vin - Vin/(106), whhich rounds to Vin - 0.

Remember - it is the duty of the circuit designer to surround his amplifier with a feedback circuit that allows it to hold its inputs equal.
Else it won't operate.


See if this is any help in your studies:
http://www.analog.com/static/import... Amp Applications Book (PDF)/P2 ChH_final.pdf

This site is a treasure trove:
http://www.philbrickarchive.org/

old jim
 
Last edited:
  • #3


When we surround an "op-amp" with a feedback circuit that allows it to hold its inputs equal, we now have an electronic circuit that behaves in accordance with some mathematical "operation" defined by the feedback network.

How can we tell that a certain feedback configuration makes the two input voltages almost equal, and hence assume Vn = Vp?
 
  • #4


Rudinhoob said:
How can we tell that a certain feedback configuration makes the two input voltages almost equal, and hence assume Vn = Vp?

What i do is write Kirchoff's voltage law solutions for voltage at each input, then set those equal and solve for Vout.
If Vout doesn't fall between the supply rails, the circuit won't "operate".
 
  • #5


Hello,

Thank you for your question. As you correctly stated, in a standard negative feedback configuration, the output voltage is fed back to the inverting input of the op-amp, resulting in a very small difference between the inverting and non-inverting input voltages. This is known as the virtual short concept and allows for accurate amplification and linear operation of the op-amp.

When using non-standard feedback configurations, such as connecting the Vn input to other parts of the circuit or using positive feedback, the ideal rules for analysis may no longer apply. This is because the virtual short concept may not hold true in these cases and the op-amp may not behave in a linear manner.

In such cases, it is important to carefully analyze the circuit and take into account the effects of the non-standard feedback on the op-amp's behavior. This may require using more complex analysis techniques, such as using the op-amp's transfer function or considering the effects of non-idealities in the op-amp itself.

I hope this helps clarify your question. Please let me know if you have any further inquiries.
 

1. What is OpAmp Non-standard Feedback?

OpAmp Non-standard Feedback is a circuit design technique used in operational amplifiers to modify the traditional negative feedback loop. It involves using additional components and connections to alter the amount or direction of feedback, resulting in improved performance and functionality.

2. How does OpAmp Non-standard Feedback differ from traditional feedback?

In traditional negative feedback, the output of an operational amplifier is fed back to the input in an inverted manner. However, in OpAmp Non-standard Feedback, this feedback loop is modified using additional components to achieve a desired change in the amplifier's behavior.

3. What are the benefits of using OpAmp Non-standard Feedback?

OpAmp Non-standard Feedback allows for more precise control over the amplifier's gain, bandwidth, and distortion. It also enables the creation of specialized circuits, such as active filters and oscillators, that are not possible with traditional feedback.

4. What are some common applications of OpAmp Non-standard Feedback?

OpAmp Non-standard Feedback is commonly used in audio and instrumentation circuits, as well as in active filters, oscillators, and signal generators. It is also used in precision measurement and control systems, where high accuracy and stability are required.

5. Are there any drawbacks to using OpAmp Non-standard Feedback?

One potential drawback of OpAmp Non-standard Feedback is that it can be more complex and challenging to design and troubleshoot compared to traditional feedback. It also requires careful consideration of component tolerances and stability to ensure the desired performance is achieved.

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