This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common applications.

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From the course by Georgia Institute of Technology

Introduction to Electronics

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This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common applications.

From the lesson

Op Amps Part 1

Learning Objectives: 1. Develop an understanding of the operational amplifier and its applications. 2. Develop an ability to analyze op amp circuits.

- Dr. Bonnie H. FerriProfessor

Electrical and Computer Engineering - Dr. Robert Allen Robinson, Jr.Academic Professional

School of Electrical and Computer Engineering

Welcome back to Electronics, this is Dr. Robinson.

Â In this lesson, I want to look at the relationship between the inverting and

Â non-inverting op-amp amplifier configurations.

Â Let me start out by drawing this schematic for an inverting op-amp amplifier.

Â You have the ampere resistor, R1.

Â A feedback resistor, RF.

Â Here is our output voltage.

Â And here is the input voltage.

Â And remember, when we have this type of configuration,

Â we have a ratio of output voltage to input voltage, or the gain.

Â Is equal to negative R F over R 1.

Â Now a common mistake that I see students make is informing

Â the schematic for a non-inverting op amp amplifier from the schematic for

Â the inverting amplifier like this.

Â If we have an input resistor, again, R1.

Â Here's Vn.

Â Our op-amp feedback resistor, Rf.

Â Here is V out.

Â And here is our ground.

Â So the belief seems to be that if this is an inverting amplifier

Â with the input voltage connected to the inverting terminal, then we can form

Â a noninverting amplifier by simply interchanging the noninverting and

Â inverting terminals like this.

Â But this is not the correct way of forming a noninverting amplifier.

Â We actually have positive feedback in the circuit where the output voltage is

Â applied back to the non-inverting terminal through the resistor RF.

Â Remember, a real op amp is powered with DC power supplies.

Â Say, V plus and V minus.

Â If you built this circuit, what would you,

Â what you would find is that the output voltage is in one of two states.

Â It's either equal to.

Â The DC V plus voltage or the DC V minus voltage are voltages near

Â these voltages because of internal voltage drops inside the op-amp.

Â So, the circuit is not a noninverting amplifier.

Â It's not an amplifier.

Â Let's look at how the inverting and non-inverting configurations are related.

Â Let me draw another schematic.

Â Here's our op-amp with the inverting terminal,

Â the non-inverting terminal.

Â Here's the output voltage.

Â v out.

Â R f.

Â R 1.

Â In this case, let me leave these two terminals as just

Â open terminals that I'm going to label A and B.

Â Now, we can see that if I made terminal A the input voltage, and

Â I made terminal B ground.

Â Then we would have the inverting configuration here and

Â we would have a gain of negative Rf over R1.

Â And I can tabulate.

Â The me.

Â Draw this table.

Â If terminal A were the input voltage VN.

Â And terminal B were ground.

Â Then we would have amp type.

Â An inverting amplifier.

Â However, if I interchange the location of input voltage and ground.

Â In other words if I make A ground, and B the input voltage.

Â We would have the non-inverting configuration.

Â So the correct way to form a non-inverting amplifier from the inverted

Â amplifier is not to exchange the non-inverting and inverting terminals.

Â But to exchange the positions of the input voltage and ground.

Â So, this, this topology here can actually be used to create both.

Â The inverting amplifier.

Â Input voltage here, here's R1, here's our feedback resistor, RF.

Â The output voltage,

Â the inverting configuration, and if we interchange.

Â This VN and this ground, we get this circuit.

Â Feedback resistor RF.

Â Output voltage V out.

Â This is still the inverting terminal.

Â This is the non-inverting terminal, where we apply the input VN.

Â So we have a non-inverting amplifier.

Â Now, you can see that, in both of these configurations, we have negative feedback.

Â The output voltage is applied to the inverting terminal

Â through the feedback resistor, Rf.

Â But what differs is the location of the input voltage and the ground positions.

Â So let's go ahead and derive the gain expressions.

Â For each of these two configurations.

Â Now remember if we consider this to be an ideal op-amp the inverting

Â the voltage at the inverting terminal and

Â the voltage at the non-inverting terminal must be equal to each other.

Â Now we also know that there's no current into the input terminals of the op-amp.

Â So if we write a note equation at this node.

Â We have zero volts here which means that the voltage here must be zero.

Â This current plus this current

Â must be equal to zero because we know there's no current in this branch.

Â So, I can write that VN over R1 which would be the current here.

Â Must be equal to negative V out over RF or

Â the V out over V in is equal

Â to negative Rf over R1.

Â An inverting configuration because of this minus sign.

Â Now, for this circuit, again it's true that the voltage at the inverting terminal

Â must equal the voltage at the non-inverting terminal.

Â But in this case, the non-inverting voltage is V in.

Â So the voltage at this node is V in.

Â So we can obtain the voltage V out

Â by starting with this known node voltage here, V in.

Â And adding to that one IR drop across RF.

Â Now we know that the current through R1 would

Â be equal to the voltage here at the inverting terminal.

Â VN divided by R1 and

Â we know that the current through R1 must be the same current that flows through RF

Â because no current can flow into the op-amp input terminal.

Â So we can write that the output voltage, Vout, is equal, equal to our known node

Â voltage V-in, plus the IR drop across RF.

Â The current through RF would be equal to V-in divided by R1.

Â And then we multiply by R F to get the voltage across R f.

Â We can then factor out V N and bring it to the side to get V out

Â over V in is equal to 1 plus Rf over R1.

Â A non-inverting amplifier configuration.

Â Compared with the inverting gain here because of this negative sign.

Â Now in summary,

Â remember, to form a non-inverting amplifier from a inverting amplifier.

Â You do not exchange the, the non inverting and inverting terminals in the op-amp,

Â but you change the location of the input voltage and ground to the circuit.

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