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1 Electronic Circuits OP AMPs

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2 Electronic Circuits Operational amplifiers are convenient building blocks that can be used to build amplifiers and filters. Op-amps are a special type of high gain DC amplifiers that are integrated circuits composed of many transistors & resistors. Operational amplifiers are convenient building blocks that can be used to build amplifiers and filters. Op-amps are a special type of high gain DC amplifiers that are integrated circuits composed of many transistors & resistors.

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3 Electronic Circuits Before an amplifier can be classified as an operational amplifier, it must have certain characteristics, three of the most important are; 1.VERY HIGH GAIN 2.VERY HIGH INPUT IMPEDANCE 3.VERY HIGH OUTPUT IMPEDANCE Before an amplifier can be classified as an operational amplifier, it must have certain characteristics, three of the most important are; 1.VERY HIGH GAIN 2.VERY HIGH INPUT IMPEDANCE 3.VERY HIGH OUTPUT IMPEDANCE

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4 Electronic Circuits BASIC OP AMP

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5 Electronic Circuits There are two input pins (non-inverting and inverting), an output pin, and two power pins. The ideal op-amp has infinite gain. It amplifies the voltage difference between the two inputs and that voltage appears at the output. There are two input pins (non-inverting and inverting), an output pin, and two power pins. The ideal op-amp has infinite gain. It amplifies the voltage difference between the two inputs and that voltage appears at the output.

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6 Electronic Circuits Without feedback this op-amp would act like a comparator (i.e. when the non-inverting input is at a higher voltage than the inverting input the output will be high, when the inputs are reversed the output will be low).

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7 Electronic Circuits Two rules will let you figure out what most simple op-amp circuits do: 1.No current flows into the input pins (i.e. infinite input impedance) 2.The output voltage will adjust to try and bring the input pins to the same voltage. Two rules will let you figure out what most simple op-amp circuits do: 1.No current flows into the input pins (i.e. infinite input impedance) 2.The output voltage will adjust to try and bring the input pins to the same voltage.

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8 Electronic Circuits The most common type of op-amp is the voltage feedback type (voltage follower).

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9 Electronic Circuits Voltage Follower: No current flows into the input, Rin = ∞ The output is fed back to the inverting input. Since the output adjusts to make the inputs the same voltage. Voltage Follower: No current flows into the input, Rin = ∞ The output is fed back to the inverting input. Since the output adjusts to make the inputs the same voltage.

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10 Electronic Circuits Vout = Vin (i.e. a voltage follower, gain = 1). This circuit is used to buffer a high impedance source (note: the op-amp has low output impedance 10- 100Ω). Vout = Vin (i.e. a voltage follower, gain = 1). This circuit is used to buffer a high impedance source (note: the op-amp has low output impedance 10- 100Ω).

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11 Electronic Circuits

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12 Electronic Circuits Inverting amplifier;

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13 Electronic Circuits Inverting amplifier; Because no current flows into the input pins there can’t be a voltage drop across R1/R2. Vin+ is therefore at 0V (this is called a virtual ground). The output will adjust such that Vin- is at zero volts. Inverting amplifier; Because no current flows into the input pins there can’t be a voltage drop across R1/R2. Vin+ is therefore at 0V (this is called a virtual ground). The output will adjust such that Vin- is at zero volts.

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14 Electronic Circuits This makes Rin = R1 (not ∞). The current through R1 & R2 have to be the same since no current goes into the input pins. Therefore I = Vin/R1. Vout = Vin+ - IR2 = 0 - (Vin/R1)R2. Therefore Vout = -Vin(R2/R1) This makes Rin = R1 (not ∞). The current through R1 & R2 have to be the same since no current goes into the input pins. Therefore I = Vin/R1. Vout = Vin+ - IR2 = 0 - (Vin/R1)R2. Therefore Vout = -Vin(R2/R1)

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15 Electronic Circuits Non-inverting Amplifier:

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16 Electronic Circuits Non-inverting Amplifier: No current flows into the input, Rin = ∞. The output adjusts to bring Vin- to the same voltage as Vin+. Non-inverting Amplifier: No current flows into the input, Rin = ∞. The output adjusts to bring Vin- to the same voltage as Vin+.

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17 Electronic Circuits Therefore Vin- = Vin and since no current flows into Vin- the same current must flow through R1 & R2. Vout is therefore VR1 + VR2 = Vin- + IR2 = Vin- + (Vin/R1)R2 Vout = Vin (1 + R2/R1) Therefore Vin- = Vin and since no current flows into Vin- the same current must flow through R1 & R2. Vout is therefore VR1 + VR2 = Vin- + IR2 = Vin- + (Vin/R1)R2 Vout = Vin (1 + R2/R1)

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18 Electronic Circuits Side by side on inverting and non inverting amps;

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19 Electronic Circuits Summing amplifier Each input contributes to the output using the following equation: Vout = -V1(R4/R1) - V2(R4/R2) - V3(R4/R3). Summing amplifier Each input contributes to the output using the following equation: Vout = -V1(R4/R1) - V2(R4/R2) - V3(R4/R3). R4

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20 Electronic Circuits Difference amplifier Vout = 2(V2-V1)*(R2/R1). Also, Rin(-) = R1, Rin(+) = R1 + R2. Difference amplifier Vout = 2(V2-V1)*(R2/R1). Also, Rin(-) = R1, Rin(+) = R1 + R2.

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Operational Amplifiers Instructor: Chia-Ming Tsai Electronics Engineering National Chiao Tung University Hsinchu, Taiwan, R.O.C.

Operational Amplifiers Instructor: Chia-Ming Tsai Electronics Engineering National Chiao Tung University Hsinchu, Taiwan, R.O.C.

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