Possible webpage title: What is the Maximum Gain of an Op-Amp?

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    Gain Max Opamp
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Discussion Overview

The discussion revolves around determining the maximum gain of an operational amplifier (op-amp) circuit based on given constraints, including output voltage limits and current specifications. Participants explore the implications of resistor values and circuit configurations in relation to the op-amp's performance.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about the value of R1 and its impact on their calculations.
  • Another participant highlights the importance of the output voltage and current limits, suggesting these constraints are crucial for determining maximum gain.
  • A participant questions whether the output current limit of -200 mA applies at the op-amp's saturation point and its relevance to KCL equations.
  • One participant notes that the gain is determined by the resistor values, which they initially misunderstood, and suggests that the maximum gain could be infinite under certain conditions.
  • Another participant clarifies that the sum of the resistors is 90k, not each resistor, and emphasizes the need to consider output current and voltage limits.
  • A participant calculates a gain of 8 based on the maximum current and load resistor value, proposing specific resistor values to achieve this gain.
  • There is a mention of feedback current and its potential effect on the gain calculation, indicating a nuanced understanding of the circuit dynamics.

Areas of Agreement / Disagreement

Participants express differing views on the maximum gain of the op-amp, with some suggesting it could be infinite while others calculate specific values based on current and resistor configurations. The discussion remains unresolved regarding the exact maximum gain and the implications of the constraints provided.

Contextual Notes

Participants note missing assumptions regarding the op-amp's output voltage range and the lack of a datasheet, which could affect their calculations. There is also uncertainty about how the output current limits influence KCL equations.

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Homework Statement



http://img440.imageshack.us/img440/9996/untitledbq0.png

Homework Equations



Please see picture.

The Attempt at a Solution



Please see picture.

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I have a strong feeling this is incorrect. I think I'm missing something important here... I don't know what R1 is, so I'm a little lost.
 
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You haven't used two facts you were given in the question:

1. The output voltage can't exceed the power supply voltages, i.e. =15 <= V0 <= +15
2. The output current from V0 must be less than 200 mA.

Those two conditions limit the values that V0 can have. When you know the maximum value of V0, you can find the the maximum gain of the circuit.

When you know the gain, you could then find R1 and R2 to give that gain, but the question doesn't ask you to do that.
 
AlephZero said:
You haven't used two facts you were given in the question:

1. The output voltage can't exceed the power supply voltages, i.e. =15 <= V0 <= +15
2. The output current from V0 must be less than 200 mA.

Those two conditions limit the values that V0 can have. When you know the maximum value of V0, you can find the the maximum gain of the circuit.

When you know the gain, you could then find R1 and R2 to give that gain, but the question doesn't ask you to do that.

"2. The output current from V0 must be less than 200 mA."

Does that mean that I_o = -200mA when the op-amp is operating at it's saturation point?

Also, should those two facts I overlook affect my KCL equations? I'm believing not, but I already made an error so I'm not quite sure.
 
I'm missing something here (it wouldn't be the first time, however). The gain of the overall amplifier is set by the values of the two resistors, which you indicate as given in the problem statement and equal to each other.

Given that the overall gain of the circuit is set by the external resistors as +2, and given that 2V across the load resistor of 40 Ohms demands less than 200mA max that the opamp can source, then the max gain of the opamp itself could be infinite, it would seem.

Is that really the whole problem statement word for word?
 
Oh, that's what I missed. The sum of the resistors is 90k, they are not 90k each. Okay, so you need to figure out whether you run out of output current or output voltage range first.

Except, they didn't give you a datasheet showing the output voltage range of the opamp, so I guess you need to assume the ideal, which is the +/- rails.
 
DefaultName said:
"2. The output current from V0 must be less than 200 mA."

Does that mean that I_o = -200mA when the op-amp is operating at it's saturation point?

Also, should those two facts I overlook affect my KCL equations? I'm believing not, but I already made an error so I'm not quite sure.

I would take it to mean the limits on the output current are +/- 200 mA. You need to do a KCL at the output to find the limits for V0. (For practical purposes R1 and R2 don't affect this much becase R1+R2 = 90k ohm and RL = 40 ohm).

If you think about the directions of the current in the resistors (or as Berkeman implied, if you recognise the circuit is a non-inverting amplifier) then you can decide whether the output voltage is positive or negative when VS = +1V. If you decide wrong, you will probably finish up with negative values for R1 and R2 which doesn't make sense.

You need two equations connecting the two unknowns R1 and R2. When you know the V0 corresponding to VS = +1V, you can get one equation from KCL at the input (node V1). The other equation is R1 + R2 = 90k.
 
looks like a gain of 8 to me.. if you understand the maximum current drawn is 200mA then I x R = 200mA x 40 Ohms = 8 volts therefore gain of 8 is known. then r1 and r2 ratio can be worked out. 10k 80k respectively. this is not taking into account the feedback current which is probably around 77uA. R2/R1 = 80k/10k = Gain of 8
 
Last edited:
trevie1970 said:
looks like a gain of ___ to me.. if you understand the maximum current drawn is 200mA then . . .

Hello Trevie,

Welcome to Physics Forums. Actually, the goal was to get DefaultName to figure that part out.
 
yea, sorry, I didn't realize until I had posted the answer?
 
  • #10
Never mind!

This thread is over a year old ... don't worry, no harm done :smile:
 

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