6.2 Ideal Amplifier Approximation

Chapter 6.2 Ideal Amplifier Approximation

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 6. OPERATIONAL AMPLIFIERS1076.2Ideal Amplifier ApproximationThe following are properties of an ideal amplifier, which to a good approximation are obeyedby an operational amplifier:1. large forward transfer function,2. virtually nonexistent reverse transfer function,3. large input impedance, Zin→∞ (any signal can be supplied to the op-amp withoutloading problems),4. small output impedance, Zout→ 0 (the power supplied by the op-amp is not limited),5. wide bandwidth, and6. infinite gain, A→∞.If these approximations are followed two rules can be used to analyze op-amp circuits:Rule 1: The input currents I+ and I− are zero, I+ = I− = 0 (Zin =∞).Rule 2: The voltages V+ and V− are equal, V+ = V− (A =∞).To apply these rules requires negative feedback.Feedback is used to control and stabilize the amplifier gain. The open-loop gain is toolarge to be useful since noise will causes the circuit to clip. Stabilization is obtained byfeeding the output back into the input (closed negative feedback loop). In this way theclosed-loop gain does not depend on the amplifier characteristics.6.2.1Non-inverting AmplifiersFigure 6.2 shows a non-inverting amplifier, sometimes referred to as a voltage follower.VinVoutFigure 6.2: Non-inverting, unity-gain amplifier.Applying our rules to this circuit we haveV+ = V− ⇒ Vin = Vout.(6.4)I+ = I− =0⇒ Rin =∞.(6.5)