Output voltage and output current of an Opamp

In summary: Charles - please, can you explain ? Why do you think, that (c) is correct?...an assumption (ideal opamp) that is NOT in accordance with the task description (A=200).Charles - please, can you explain ? Why do you think, that (c) is correct?Pushoam has started his calculation with an assumption (ideal opamp) that is NOT in accordance with the task description (A=200). Perhaps an oversight on my part. I spotted the ## A=200 ## also, but I think (maybe incorrectly) that that is a typo. Usually Op-Amps have very high gain, e.
  • #1
Pushoam
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Homework Statement


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Homework Equations

The Attempt at a Solution



Output voltage and output current of an OP-Amp

Taking the OP-Amp as an ideal OP-Amp, the voltage at terminal 1 is 0 with respect to ground.

The voltage at B is ## V_B = - 0.5 V ##

## i_{in} ## is from A to B.

There is another 5 mA current going from C to B.

I think both of these current should go towards D. Then, no current is coming out of the OP-Amp, i.e. the direction of ## i_o ## is opposite to what is given in the question.Since, ## i_in ## is very very less than 5mA, approximately 5 mA goes through 50 ## \Omega ## resistor. So, the voltage drop across this resistor is 0.25 V. Since, the current is towards D, ## V_o = -0.5V -0.25V = -0.75 V ##So, the correct option is (c).

Is this correct?
 

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  • #3
Thanks.
 
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  • #4
Charles Link said:
Yes, this is correct. :)

Charles - please, can you explain ? Why do you think, that (c) is correct?
Pushoam has started his calculation with an assumption (ideal opamp) that is NOT in accordance with the task description (A=200).
 
  • #5
LvW said:
Charles - please, can you explain ? Why do you think, that (c) is correct?
Pushoam has started his calculation with an assumption (ideal opamp) that is NOT in accordance with the task description (A=200).
Perhaps an oversight on my part. I spotted the ## A=200 ## also, but I think (maybe incorrectly) that that is a typo. Usually Op-Amps have very high gain, e.g. ## A=200,000 ## and higher. The assumption is made that the inverting terminal is a virtual ground. A gain of only 200 would likely make that assumption somewhat inaccurate. ## \\ ## Additional comment: I believe the purpose of the exercise is to teach students to calculate ideal op-amp configurations by hand. A hand calculation of the non-ideal A=200 would be somewhat difficult. I'm not sure what is presently in use for that=when I was in school in 1980, a program called SPICE would have computed the result.
 
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  • #6
Yes - without any doubt, a gain calculation based on an open-loop gain of only A=200 would be somewhat more involved.
However, based on Black`s classical feedback formula, it is not a big problem.
On the other hand, perhaps you are right and there was a typing error - and A=200k would be the envisaged number,
But - who knows?
 
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  • #7
A by-hand analysis would not be too difficult. The op-amp is replaced by a suitable input resistance and a controlled voltage source with a gain of 200. Two essential nodes means that nodal analysis will be easy enough to do by hand.
 
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  • #8
gneill said:
A by-hand analysis would not be too difficult. The op-amp is replaced by a suitable input resistance and a controlled voltage source with a gain of 200. Two essential nodes means that nodal analysis will be easy enough to do by hand.
I do think though, in the multiple choice question, they were simply looking for the answer for an ideal op-amp. I do think the gain of ## A=200 ## was mislabeled.
 
  • #9
But, in the question, input impedance is also given.
But, I don't know any other way to calculate it.
But, from the given options, doesn't it become clear whether one has to idea op - amp approximation or not?
 
  • #10
Pushoam said:
But, in the question, input impedance is also given.
But, I don't know any other way to calculate it.
But, from the given options, doesn't it become clear whether one has to idea op - amp approximation or not?
In general, an ideal op-amp is well known to require a very large gain factor. ## A=10^6 ## is not uncommon for a commercial op-amp. I do think this was probably a misprint, because learning how to compute the case of the ideal op-amp is much more important than doing the non-ideal case with a low gain such as ## A=200 ##.
 
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  • #11
I doubt that it was a misprint as both the gain and input impedance of the amplifier were given, and both values were much less that typical of a modern, garden variety op-amp. The question has the benefit of demonstrating how close to ideal performance you can get with even modest gain and input impedance, thanks to the feedback loop. The early, discrete component op-amps (both tube and transistor based) had characteristics akin to these specs.

Recognizing that you can use an ideal approximation to choose amongst the given multiple choice answers is commendable. Being able to justify that approximation is an even better thing :smile:
 
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  • #12
I believe there is a bigger problem than Ideal vs Non-ideal op-amp. The 50KΩ feedback resistor needs 5V -0.5V across it to balance the ID (+5% for the input current). That puts 5V -0.5V volts at the junction of the 100Ω and the 50Ω resistors. However the problem asks for the voltage at VO, not the voltage at the 100Ω resistor
 
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  • #13
Tom.G said:
I believe there is a bigger problem than Ideal vs Non-ideal op-amp. The 50KΩ feedback resistor needs 5V across it to balance the ID (+5% for the input current). That puts 5V voltage at the junction of the 100Ω and the 50Ω resistors. However the problem asks for the voltage at VO, not the voltage at the 100Ω resistor
Please check your arithmetic. It's -.5 volts, not 5 Volts. I believe the OP got a reasonably accurate answer.
 
  • #14
Charles Link said:
Please check your arithmetic. It's -.5 volts, not 5 Volts.
Oops! You're right. Thanks. Am correcting the post. (That's what happens when I don't write things down.) :H
 
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Related to Output voltage and output current of an Opamp

1. What is an Opamp?

An Opamp, short for operational amplifier, is an electronic circuit component that amplifies the difference between two input signals. It is commonly used in various electronic devices, such as audio amplifiers and signal processing circuits.

2. What is the output voltage of an Opamp?

The output voltage of an Opamp depends on the values of the input voltages and the gain of the amplifier. It can range from near 0 volts to the maximum output voltage of the Opamp, which is typically around 10 volts for most Opamps.

3. How is the output voltage of an Opamp calculated?

The output voltage of an Opamp is calculated using the formula Vout = (V+ - V-) x A, where V+ and V- are the input voltages and A is the gain of the amplifier. The gain can be set by external resistors connected to the Opamp.

4. What is the output current of an Opamp?

The output current of an Opamp is the amount of current that can flow through the output pin of the amplifier. It is usually limited by the maximum output current rating of the Opamp, which can vary depending on the model and manufacturer.

5. How does the output current of an Opamp affect its performance?

The output current of an Opamp is an important factor in determining its performance and capabilities. A higher output current allows the Opamp to drive larger loads, while a lower output current may result in distortion or other issues. It is important to choose an Opamp with an appropriate output current rating for the intended application.

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