What is the point in having a voltage follower?

In summary: If v+ - v- becomes slightly negative, the output will become much more negative, contributing additionally to the voltage at v+, making the output become even more negative, and so on until the rail is reached.
  • #1
theBEAST
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0
Solving for the voltage gain of an inverting op amp

Homework Statement


Find the gain for an inverting op amp.

3. The Attempt at a Solution

I tried to solve for the gain but I ended up getting a positive gain. I think I made a mistake with the current directions but I always thought that when doing nodal analysis, it doesn't matter what direction your current arrows are? Since the math should always work out? I am not too sure... What am I doing wrong?
VV1i3.jpg
 
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  • #2
It does matter what direction your current arrows are. KCL states the currents going into a node are zero. You show one current going into the node and the other going out from the node. Change the direction of the second one and you will have your inverter.
 
  • #3
lewando said:
It does matter what direction your current arrows are. KCL states the currents going into a node are zero. You show one current going into the node and the other going out from the node. Change the direction of the second one and you will have your inverter.

Thanks! Also what happens if I were to flip the op amp. In other words switch the terminals so that the - terminal is connected directly to the ground?
 
  • #4
You will be using the op amp in a way that does not use negative feedback then. The output will "rail" towards the positive supply voltage.
 
  • #5
lewando said:
You will be using the op amp in a way that does not use negative feedback then. The output will "rail" towards the positive supply voltage.

When you say rail, you mean that the output would be the same as the supply voltage right? As in it saturates? But I don't understand how this works, wouldn't the derivation be the same as what I did above with the voltage at node 1 to be 0V?
 
  • #6
theBEAST said:
When you say rail, you mean that the output would be the same as the supply voltage right? As in it saturates?
For an ideal op amp analysis, the output would be at the positive supply voltage. For real op amps, the output would not exactly get there but could get close if you were to use a "rail to rail" type op amp.
But I don't understand how this works, wouldn't the derivation be the same as what I did above with the voltage at node 1 to be 0V?
You asked what if you flipped the op amp (reverse the +/- terminals: - is at ground, + is at node 1). In this configuration, you are using positive feedback. Recall the open loop op amp gain equation:

vout = Gainopen_loop*(v+ - v-).

If v+ - v- becomes slightly positive, the output will become much more positive, contributing additionally to the voltage at v+, making the output become even more positive, and so on until the rail is reached.
 

1. What is a voltage follower?

A voltage follower, also known as a unity-gain amplifier, is an op-amp circuit that produces an output voltage that is equal to and in phase with the input voltage. This means that the output voltage follows the input voltage, hence the name "voltage follower".

2. What is the purpose of a voltage follower?

The main purpose of a voltage follower is to isolate the input signal from the output load. This means that the input signal does not have to provide any current to the load, which can be useful for sensitive or high impedance devices. Additionally, a voltage follower can provide a buffer between a signal source and the load, preventing any changes in the load from affecting the source.

3. How does a voltage follower work?

A voltage follower works by using an operational amplifier (op-amp) to amplify the input voltage. The op-amp has a very high input impedance and a low output impedance, allowing it to easily produce an output voltage that is equal to the input voltage. This is achieved through negative feedback, where a portion of the output voltage is fed back to the input, causing the op-amp to adjust its output to match the input.

4. When is a voltage follower used?

A voltage follower is commonly used in electronic circuits where a high input impedance and low output impedance are required. This includes applications such as impedance matching, signal buffering, and analog-to-digital conversion. Voltage followers are also often used in power supply circuits to provide a stable output voltage.

5. What are the advantages of using a voltage follower?

The main advantage of using a voltage follower is its ability to provide a high input impedance and low output impedance, which can be beneficial in many electronic circuits. Additionally, voltage followers have a simple design and are relatively inexpensive to implement. They also have a wide range of applications, making them a versatile component in electronic systems.

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