Basic electricity part 1 chapter 5 Op-amp.pptx

laecture note part1

1. Chapter five An Op-Amp An Op-Amp contains several Transistors, Resistors, and a few Capacitors and Diodes. For simplicity, an Op-Amp is often depicted as follow: We consider the op-amp as a single component with input and output characteristics. Two signal inputs: 1. Inverting 2. Non-inverting Two dc power supply leads (+ve & −ve) One output lead

2. Op-Amp Packages

3. To analyze an op-amp feedback circuit: Assume no current flows into either input terminal Assume no current flows out of the output terminal Constrain: V+ = V- Ideal Op-Amp

4. Operation of Op-amps The input stage of an op-amp is a differential amplifier. The op-amp amplifies the difference between the two input terminal voltages.

5. Op-Amp Output The output of the amplifier is determined by  The gain of the amplifier.  The polarity relationship between V1 and V2.  The values of the supply voltages, +V and -V.  The load resistance Input/Output Polarity • The output polarity follows the sign of Vdiff . • If V2 – V1 > 0 the output polarity will be positive. • If V2 – V1 < 0 the output polarity will be negative.

6. • As the open loop gain of most op-amps is extremely large the output of an open-loop circuit is either the maximum positive or negative voltage.

7. General Rules The following two conditions define the rules that follow: Amplifier amplifies the difference between its input voltages(ΔVin) 1: Vout = A ΔVin where ΔVin ≡ V+ - V- “A” is the “open loop gain” of the amplifier Output voltages can never exceed its supply voltages, V-- and V++. 2: V-- < Vout < V++ Typical supply voltages are V-- = -15V and V++ = +15V . Applying these two conditions leads to 3 different cases: a. If A ΔVin > V++ then Vout = V++ b. If A ΔVin < V-- then Vout = V-- c. If V-- < A ΔVin < V++ then Vout = A ΔVin

8. Feedback The most important step in analyzing an op-amp circuit is to determine first the type of feedback that is being employed. Feedback refers to connecting the output of the op-amp to its input, usually through resistors, and there are three basic ways. No Feedback, Positive Feedback & Negative Feedback.

9. Inverting (Negative Feedback) Amplifier The current through R1 must be identical to the current through R2 leads as immediately to the application of Ohm’s law, namely: (Vin – V-)/R1 = (V- - Vout )/ R2 Golden rule of op-amps state that V+ = V-. Since V+=0, then V- = 0 . Vout = Vin (-R2/R1 )

10. Non-Inverting (Positive Feedback) Amplifier For the particular circuit above, ΔV = V+ since V- = 0. So, • if V+ > 0, Vout = V++ • if V+ < 0, Vout = V-- Since Vout change its state whenever V+ crosses 0V, we need to find what value of Vin results in V+ = 0. Since V+ is a voltage divider formed by R1 and R2 between Vin and Vout it follows that: • V+ = (Vin R2 + Vout R1 )/(R1 + R2)

11. Comparator Op-amp: If Δ Vin > 0, Vout = V++ If Δ Vin < 0, Vout = V--. Its output will change its state when ΔVin changes its sign and it will do so at ΔVin = 0. The circuit to switch its output when Vin crosses a certain preset voltage level, often called the threshold voltage, Vth .

12. Op-Amp Summing Amplifier

13. Op-Amp Differentiator

14. Op-Amp Integrator

15. Op-Amp Differential Amplifier

16. THE END!