Solving Op-Amp Current Analysis: LH0005 National Semiconductor

In summary, the conversation discusses a homework problem involving an Op-amp and the LH0005 of National Semiconductor. The task is to determine the collector current through each transistor, given certain values for beta and VBE. The conversation includes hints and guidance on how to approach the problem, as well as a clarification on the input voltages and the use of ground. Finally, the conversation concludes with a request for further explanation on how to calculate voltage gain and Ri.
  • #1
txp2037
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Member advised to use the homework template for posts in the homework sections of PF.
Thank you for reading my post. I'm currently studying Op-amp, and there is a homework about LH 0005 of National Semiconductor for me to do: determine the collector current through each transistor

I'm trying to find the IC2 but don't know how. Please help me with this :)

Homework Statement


Given: β = 100 & VBE = 0.7

Homework Equations

The Attempt at a Solution


I tried to analyze but I'm stuck at the moment. Here are somethings that I've figured out so far:
VC2 = VCC - 10k*IC2
VE6 = VC2 - VBE6 = VC2 - 0.7
VC5 <<<
I2 = (VE2 + VEE) / 12k
I6 = (VE6 + VCC) / 2k

I'm trying to find the IC2 but don't know how. Please help me with this :)
a1d0e395e0.png

I'm looking forward to seeing your response soon :)
 
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  • #2
Txp2037, you have overlooked that Q6 is pnp (VBE=-0.7V).
 
  • #3
LvW said:
Txp2037, you have overlooked that Q6 is pnp (VBE=-0.7V).
Thank you for reminding me :) I'm struggling with the problem. By any chance you can give me some hints?
 
  • #4
txp2037 said:
Thank you for reminding me :) I'm struggling with the problem. By any chance you can give me some hints?

Question: Are you required to find the quiescent collector currents for the shown input voltages?
This will create problems because - assuming VBE=0.7V as mentioned - the base potentials for Q1 and Q3 are equal.
However, this is in contradiction to the given signal voltages.
It is more likely that Q3 will be completely off. However, in this case, we are not in linear mode anymore.
Perhaps you should review the task again (with the dc input voltages)?
 
  • #5
LvW said:
Question: Are you required to find the quiescent collector currents for the shown input voltages?
This will create problems because - assuming VBE=0.7V as mentioned - the base potentials for Q1 and Q3 are equal.
However, this is in contradiction to the given signal voltages.
It is more likely that Q3 will be completely off. However, in this case, we are not in linear mode anymore.
Perhaps you should review the task again (with the dc input voltages)?
Thank you for your reply. My requirement are (and I quote): Referring to the LH0005 op-amp equivalent circuit shown in figure, determine (a) the collector current through each transistor and (b) the de voltage at the output terminal. (Assume that each transistor has βdc = βac = 100 and VBE = 0.7 V.)
I think I can sort the (b) out, but I don't know how to find the collector current. I've written down some equations so when I know the IC2, I can solve the problem. About the input voltage, there are nothing given :)
 
  • #6
Oh sorry - I have misinterpreted the drawing. So "1" and "3" are node names and not two different input voltages.
Therefore, I now assume that both open base terminals are grounded, correct?

In this case, it is a simple task to find the voltage at the common emitter node (because of VBE=0.7V for each transistor). This allows to compute the current to the emitter resistor (12k) - and the current is equally splitted between both transistors Q2 and Q4.
 
  • #7
LvW said:
Oh sorry - I have misinterpreted the drawing. So "1" and "3" are node names and not two different input voltages.
Therefore, I now assume that both open base terminals are grounded, correct?

In this case, it is a simple task to find the voltage at the common emitter node (because of VBE=0.7V for each transistor). This allows to compute the current to the emitter resistor (12k) - and the current is equally splitted between both transistors Q2 and Q4.
You gave something awesome here. So as you said, the VB1 will be 0V. Then VE1 = VB1 - VBE1 = 0 - 0.7 = -0.7V.
VE1 = VB2 = -0.7V ==> VE2 = VB2 - VBE2 = -1.4V
Then I can find the I2 = 2IE2 = (VE2 + VEE) / 12k = (-1.4 + 10) / 12k =~ 0.717mA.
Am I currently on the right way?
 
  • #8
txp2037 said:
Then I can find the I2 = 2IE2
I rather think that I2=IE/2.
 
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  • #9
LvW said:
I rather think that I2=IE/2.
Thank you for your help. Under your guide, I've managed to solved the problem completely :) I was misunderstood about the "ground" voltage that should be 0V. With that information, everything was clear :) Thank you sir
 
  • #10
Could you further elaborate how voltage gain, Ri etc are calculated...
 

1. What is the LH0005 National Semiconductor Op-Amp?

The LH0005 National Semiconductor Op-Amp is a high-performance operational amplifier designed for use in current sensing and amplification applications. It is specifically designed for low-level current measurement and has a wide dynamic range and high accuracy.

2. How does the LH0005 Op-Amp work?

The LH0005 Op-Amp works by using a differential amplifier configuration to sense and amplify the difference between two input currents. It also has a high gain and low offset voltage, making it ideal for precise current measurements.

3. What is the purpose of solving Op-Amp current analysis?

The purpose of solving Op-Amp current analysis is to determine the output voltage of the Op-Amp based on the input currents and the circuit configuration. This is important in understanding the behavior of the Op-Amp and designing circuits that require accurate current sensing and amplification.

4. What are the key features of the LH0005 Op-Amp?

The key features of the LH0005 Op-Amp include a wide dynamic range of 10^-6 A to 10^3 A, high accuracy of 0.5% at full scale, low input offset voltage of 10 μV, high gain of 200,000, and low input bias current of 10 nA.

5. How do you solve Op-Amp current analysis using the LH0005 National Semiconductor?

To solve Op-Amp current analysis using the LH0005 National Semiconductor, you will need to understand the basic principles of Op-Amp circuits, including the concept of virtual ground and the use of Kirchhoff's Current Law. Then, you can use the LH0005's specifications and circuit configurations to determine the output voltage based on the input currents. You can also use simulation software or breadboard experiments to verify your calculations.

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