Chapter 10-1: Positive/Negative Feedback Op Amp
Recommended Article : 【Circuit Theory】 Chapter 5. Capacitors, Inductors. 【Circuit Theory】 Chapter 10. Op Amp
1. Meaning of Floating Capacitor
⑴ When two arbitrary metals are connected within a circuit, they perform a function similar to a capacitor.
⑵ Inside an Op Amp, there are countless transistors that create a virtual capacitor.
⑶ These are called stray capacitors: capacitors that seem to be floating, hence the name.
⑷ The capacitance of these stray capacitors is referred to as stray capacitance.
2. Feedback Direction of Op Amp
⑴ Negative feedback ( good )
Figure 1. Negative Feedback Op Amp
⑵ Positive feedback ( bad )
Figure 2. Positive Feedback Op Amp
⑶ Issues with Positive Feedback Op Amp
Figure 3. Issues with Positive Feedback Op Amp
① Explanation of the above diagram
○ Essentially represents the equivalent circuit of an Op Amp.
○ A virtual capacitor with capacitance C is represented in blue.
○ The output of the Op Amp is shown as a dependent voltage source.
② Negative feedback Op Amp: Corresponds to case 1
○ Ideal Op Amp: vo = A(v2 - v1)
○ Real Op Amp: vo = A(v2 - v1)
③ Positive feedback Op Amp: Corresponds to case 2
○ Ideal Op Amp: vo = A(v2 - v1)
○ Real Op Amp: vo = v+ or v- (Op Amp’s supply voltage)
3. Understanding through Thevenin Equivalent Circuit
⑴ Existence of Thevenin Equivalent Circuit
Figure 4. Existence of Thevenin Equivalent Circuit
① Assumption: Ri = ∞, Ro = 0, A ＜ ∞
② Formulation: Apply KCL (Kirchhoff’s Current Law) to the super node marked in blue.
⑵ Interpretation of Thevenin Equivalent Circuit
Figure 5. Interpretation of Thevenin Equivalent Circuit
① Formulation
② Negative feedback Op Amp: Circuit Design where A ＞ 0
○ For all A, vth and Rth are positive.
○ Q converges to a constant value (stable).
③ Positive feedback Op Amp: Circuit Design where A ＜ 0
○ For some A, vth and Rth are negative.
○ Q diverges: Output voltage reaches the maximum voltage, the supply voltage, that the Op Amp can reach.
④ Conclusion: Unlike the ideal Op Amp, the real Op Amp introduces a virtual capacitor, leading to different results based on the feedback direction.
Input : 2016.01.12 00:40
Modified : 2020.09.13 23:54