How would I draw this non-inverting amplifier?

  • Thread starter marcadams267
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In summary, a practical op amp should have a gain of 10, zero offset voltage, and a feedback resistor of 100K.
  • #1
marcadams267
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Summary: So far, I have only dealt with ideal operational amplifiers, so I am kind of lost trying to visualize how a practical op amp should look like.

A non-inverting amplifier with a gain of 10 uses 100K as its feedback resistor. It gets its input from a signal source whose source resistance is 5K. The amplifier has zero offset voltage. However, it has an input bias current of 2 µA and an input offset current of 0.2 µA.
Draw the op-amp circuit.

Is this right?
Also, is R2 = 11.11k? I used A = 1 + RF/R2 to get this value

circuit.png
 
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  • #2
Seems no issues with equivalent circuit schematic above.
Regarding G=10 non-inverting opamp, i prefer to use 91k and 10k, these values are closest approximation to G=10 using E24 preferred numbers series.
 
  • #3
You have significant input current so you need to match the source impedance of the (–) and (+) inputs. The op-amp output can be seen as having a low impedance.
So with Rf = 100k, and R = 11k11, the source impedance on the (–) input will be Rf in parallel with
R, giving 10k0. To avoid offset voltage due to input currents you need to give the (+) input the same source impedance as the (–) input. The driver has Rsource = 5k0 now, so you need to add an external Rspecial = 5k in series to match the (–) input.

Draw your potential divider so it is obviously a potential divider by keeping the current flowing in a straight line.
Don't make a hedgehog with currents radiating out from the op-amp, keep parallel currents parallel.

gainx10.png
 
  • #4
Post 2 is not showing the offset current.
 
  • #5
rude man said:
Post 2 is not showing the offset current.
Or post #3? The offset current was left for later. I judged that matching the source resistance was more important and enough for a first step.
I expected @marcadams267 to have noticed the missing offset current from the way it was drawn. Maybe Ios needs to be drawn as a separate current source between the two input terminals.
 
  • #6
Baluncore said:
Or post #3? The offset current was left for later. I judged that matching the source resistance was more important and enough for a first step.
I expected @marcadams267 to have noticed the missing offset current from the way it was drawn. Maybe Ios needs to be drawn as a separate current source between the two input terminals.
I couldn't see post#2 until now.
Offset current should be depicted exactly as the OP did in post 1, i.e. as part of bias current.
 
  • #7
rude man said:
Offset current should be depicted exactly as the OP did in post 1, i.e. as part of bias current.
Then how can you know that the inverting input would be 1.9 uA and the non-inv input 2.1 uA? Could it not be the other way about?

A model should generate two identical 2u0 bias currents, with a sinusoidal offset current of 1u0 between the two inputs. With zero amplifier input that offset would show up on the output of the amplifier, revealing the sensitivity to the Ios max = 2u0 A.
Supply voltage and temperature dependence of bias current can be applied to the two identical current sources. Sweeping the offset current of the model is then independent of the bias.
 
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Likes Tom.G

1. What is a non-inverting amplifier?

A non-inverting amplifier is a type of electronic circuit that amplifies a signal without changing its polarity. It is commonly used in audio and instrumentation applications to increase the strength of a signal without introducing any distortion.

2. What components are needed to build a non-inverting amplifier?

To build a non-inverting amplifier, you will need an operational amplifier (op-amp), two resistors, and a power supply. The op-amp is the main component that amplifies the signal, while the resistors are used to set the gain of the amplifier.

3. How do I calculate the gain of a non-inverting amplifier?

The gain of a non-inverting amplifier can be calculated using the formula: Gain = (1 + R2/R1), where R1 is the resistor connected between the input and the op-amp's inverting input, and R2 is the resistor connected between the op-amp's output and its inverting input. For example, if R1 = 10kΩ and R2 = 20kΩ, the gain will be (1 + 20/10) = 3.

4. What is the purpose of the input and feedback resistors in a non-inverting amplifier?

The input resistor is used to provide a path for the input signal to reach the op-amp's inverting input. The feedback resistor is connected between the op-amp's output and inverting input and helps to stabilize the amplifier's gain. It also plays a crucial role in preventing oscillations in the circuit.

5. How can I ensure the stability of a non-inverting amplifier?

To ensure the stability of a non-inverting amplifier, it is important to choose appropriate resistor values for the input and feedback resistors. The resistors should also have low tolerances and be matched to avoid any imbalances in the circuit. Additionally, using high-quality components and minimizing any sources of noise can also help improve the stability of the amplifier.

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