# Analyzing the output impedance and open loop gain of a IC-741 Op-Amp

• Engineering
• PhysicsTruth
In summary: The open-loop gain is around 2x10^5 for the 741 Op-Amp. The open-loop gain is around 2x10^5 for the 741 Op-Amp.
PhysicsTruth
Homework Statement
Analyze qualitatively, the output impedance and the open loop gain of a IC 741 Op-Amp from its internal circuit.
Relevant Equations
##V_{out} = A_{o} ( V_{in1} - V_{in2})##
##A_{o}## is the open loop voltage gain.
Here is the internal circuit of the 741 Op-Amp -

Looking at the output stage, the output impedance seems to be the series combination of ##25 \Omega## and ##50 \Omega##, that is ##75 \Omega##. Is this a qualitative explanation enough to justify the output impedance, or am I wrong somewhere and need to take into account some other impedances as well.

Coming to the open-loop gain, the output Push-Pull amplifier has no voltage gain, only it's achieved at the Darlington pair and the initial input stage emitter followers. How do I go on to claim qualitatively that the open loop gain is significantly high, and nearing infinity?

PhysicsTruth said:
Looking at the output stage, the output impedance seems to be the series combination of 25Ω and 50Ω, that is 75Ω. Is this a qualitative explanation enough to justify the output impedance, or am I wrong somewhere and need to take into account some other impedances as well.
No. The output current flows either through one resistor or the other, depending on the output voltage and the way the load is connected. So it's basically 25 ohms or 50 ohms; there is no combination.

PhysicsTruth
DaveE said:
No. The output current flows either through one resistor or the other, depending on the output voltage and the way the load is connected. So it's basically 25 ohms or 50 ohms; there is no combination.
Okay, I realize that now. Coming to the open-loop gain, I guess only the Darlington pair, and the input stage emitter followers contribute to the small signal open loop voltage gain, and that should be the product of 4 transistors' current gain, and should be around the order of 10E+8, which is nearing infinity, as it should be, for an ideal op-amp. But, on doing some more extensive search, I found that the open-loop voltage gain is around ##2 \times 10^{5}## for the 741 Op-Amp. Where am I possibly going wrong, and does this happen due to the frequency response?

## 1. What is the purpose of analyzing the output impedance of an IC-741 Op-Amp?

The output impedance of an op-amp is an important parameter to consider because it affects the stability and accuracy of the amplifier's output. By analyzing the output impedance, we can determine the maximum load that the op-amp can drive without significant distortion or loss of accuracy.

## 2. How is the output impedance of an IC-741 Op-Amp calculated?

The output impedance of an op-amp can be calculated by measuring the voltage drop across a known load resistor while varying the load resistance. The output impedance is then equal to the slope of the voltage versus load resistance curve.

## 3. What factors affect the output impedance of an IC-741 Op-Amp?

The output impedance of an op-amp is affected by the internal circuitry and components of the op-amp, such as the transistor characteristics and feedback resistors. It is also affected by the frequency of the input signal, as well as the power supply voltage and temperature.

## 4. How does the open loop gain of an IC-741 Op-Amp impact its performance?

The open loop gain of an op-amp is the gain of the amplifier without any feedback. It determines the amplification factor of the op-amp and affects the accuracy and stability of the output. A higher open loop gain can result in better performance, but it also increases the risk of oscillations and other instabilities.

## 5. How can the open loop gain of an IC-741 Op-Amp be measured?

The open loop gain of an op-amp can be measured by applying a small input signal and measuring the output signal. The ratio of the output to input signals is equal to the open loop gain. This can be done using a multimeter or an oscilloscope.

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