Intuitive way to understand ideal current and voltage amplifiers

In summary, an ideal current amplifier should have 0 input impedance and infinite output impedance, while an ideal voltage amplifier should have an infinite input impedance and 0 output impedance. This is because an ideal voltage amplifier needs to have a high input impedance to prevent loading of the output signal from the previous stage, while a low or zero output impedance ensures that the output voltage is not affected by the load impedance. For an ideal current amplifier, the infinite input impedance prevents the current from "looping back" in the circuit, while the high output impedance ensures that the current flows towards the load. This concept can be better understood through the analogy of a voltage divider circuit and the use of an infinite resistor to prevent loading. Additionally, having a background in
  • #1
Saptarshi Sarkar
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TL;DR Summary
Need an intuitive explanation as to why input and output impedances of ideal current and voltage amplifiers are what they are.
I was reading about ideal current and voltage amplifiers and the book says that an ideal current amplifier should have 0 input impedance and infinite output impedance while an ideal voltage amplifier should have an infinite input impedance and 0 output impedance.

I am not quite sure I understand why this is. Can anyone give me an intuitive explanation as to why it is that way?
 
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  • #2
Start with the ideal voltage amplifier: You don't want its input impedance to load the output signal from the previous stage, so the infinite input impedance keeps that from happening. Think about practical DMM and oscilloscope measuring devices. Do you see why you want them to have very high input impedances to keep from loading/affecting the signals that you are trying to measure?

And to the extent that the output impedance is very low or zero, even if there is some finite load impedance, that will not affect the value of the output voltage from that ideal voltage amp. Does that help?

The analogous things apply to an ideal current amp. Knowing the "why" for an ideal voltage amp now, can you say a bit about why you think the ideal current amp will have zero Zin and infinite/high Zout? :smile:
 
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  • #3
berkeman said:
Start with the ideal voltage amplifier: You don't want its input impedance to load the output signal from the previous stage, so the infinite input impedance keeps that from happening.

Maybe it's because I am not from an engineering background but from a Physics background, but I don't understand what you mean by 'loading the output signal'.

What I think I understood from your explanation is that due to the infinite input impedance of the next amplifier circuit, the signal from the previous circuit would be free to enter the next circuit (which would not be the case if there was a short between the two input impedances).

As for an ideal current amplifier, having infinite input impedance would make sense as it would prevent the current from coming back in the circuit by making a U turn as current tries to follow the path of least resistance. So, whatever the load may be, the load resistance will always be less than infinity and the current will thus flow towards the load.

Is my reasoning correct? Please point out if there are any mistakes.

Thanks
 
  • #4
Saptarshi Sarkar said:
Maybe it's because I am not from an engineering background but from a Physics background, but I don't understand what you mean by 'loading the output signal'.
Are you familiar with the circuit concept of a "voltage divider"?

Have a look at this tutorial diagram to see if it makes sense how connecting a finite load to the output of a voltage divider "loads down" the voltage you are trying to sense (unless the input impedance of the load you are connecting is infinite):

https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/resistor-res15.gif

1580064372469.png
 
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The ideal current amplifier is a bit more complicated in terms of the Zin and Zout. After we can make you comfortable with the ideal voltage amplifier concepts, we can address the "why" for the ideal current amplifier.

Have you had any basic circuits as part of your physics classes so far? Any transistor circuits? Knowing your background will help us to tune our answers to make them more understandable for you. :smile:
 
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berkeman said:
Have a look at this tutorial diagram to see if it makes sense how connecting a finite load to the output of a voltage divider "loads down" the voltage you are trying to sense (unless the input impedance of the load you are connecting is infinite)

Thanks! This was a great explanation. I get what loading means now and I understand how adding an infinite resistor in parallel to the output impedance of the previous amplifier can help in maintaining the signal voltage.

But, I am also a bit confused by the circuit, if I add a 0 output impedance , I am basically shorting the X and Y terminals in the voltage divider output, won't that heavily load down the output voltage? Why then should an ideal voltage amplifier have 0 output impedance?

berkeman said:
Have you had any basic circuits as part of your physics classes so far? Any transistor circuits?

Yes, I had transistors and their CB, CE and CC configurations in our course. I also had basic Operational Amplifiers circuits like the Inverting and Non-Inverting Amplifier.
 
  • #7
Saptarshi Sarkar said:
I get what loading means now and I understand how adding an infinite resistor in parallel to the output impedance of the previous amplifier can help in maintaining the signal voltage.
Great!
Saptarshi Sarkar said:
But, I am also a bit confused by the circuit, if I add a 0 output impedance , I am basically shorting the X and Y terminals in the voltage divider output, won't that heavily load down the output voltage? Why then should an ideal voltage amplifier have 0 output impedance?
The output of the voltage amp does not drive the output of the previous stage, it drives the input of the next stage.

https://www.sanfoundry.com/wp-conte...ear-integrated-circuits-questions-bank-q3.png

1580071020718.png
 
  • #8
Saptarshi Sarkar said:
But, I am also a bit confused by the circuit, if I add a 0 output impedance , I am basically shorting the X and Y terminals in the voltage divider output, won't that heavily load down the output voltage? Why then should an ideal voltage amplifier have 0 output impedance?
You're confusing the simplified example of loading effects in a circuit with how amplifiers are built. That resistor circuit is very different from an amplifier. Ideal amplifiers have their outputs essentially "disconnected" from their inputs, except of course for the signal being amplified. That is why ideal amplifiers are a really useful approximation, they greatly simplify analysis.
 

Related to Intuitive way to understand ideal current and voltage amplifiers

1. What is an ideal current amplifier?

An ideal current amplifier is a device that amplifies an input current signal without changing its voltage. In other words, the output current is an exact replica of the input current, but with a larger magnitude. This type of amplifier has an infinite input impedance and zero output impedance, meaning it does not draw any current from the input source and can supply an unlimited amount of current to the load.

2. How does an ideal current amplifier work?

An ideal current amplifier works by using a high-gain transistor or operational amplifier to amplify the input current signal. The input current is converted into a voltage signal, amplified, and then converted back into an output current signal. This process is known as current-to-voltage conversion and voltage-to-current conversion.

3. What is an ideal voltage amplifier?

An ideal voltage amplifier is a device that amplifies an input voltage signal without changing its current. This means that the output voltage is an exact replica of the input voltage, but with a larger magnitude. Similar to an ideal current amplifier, an ideal voltage amplifier has an infinite input impedance and zero output impedance.

4. What is the difference between an ideal current amplifier and an ideal voltage amplifier?

The main difference between an ideal current amplifier and an ideal voltage amplifier is the type of input signal they amplify. An ideal current amplifier amplifies an input current signal, while an ideal voltage amplifier amplifies an input voltage signal. Additionally, the input and output impedances of these amplifiers are different, with an ideal current amplifier having infinite input impedance and zero output impedance, and an ideal voltage amplifier having zero input impedance and infinite output impedance.

5. How can I use the concept of ideal current and voltage amplifiers in practical applications?

The concept of ideal current and voltage amplifiers is used in various practical applications, such as audio amplifiers, power supplies, and communication systems. In these applications, the input signal needs to be amplified without being distorted, and ideal amplifiers provide this functionality. However, in reality, no amplifier can be completely ideal, so engineers must consider the limitations of real-world amplifiers when designing circuits.

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