Op-Amp Current Analysis: Solving for Collector Currents with Base Resistors

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Discussion Overview

The discussion revolves around analyzing collector currents in an operational amplifier (op-amp) circuit, particularly focusing on the role of base resistors and the biasing of inputs. Participants explore the challenges of determining currents in the absence of explicit voltage or resistor values, with a specific mention of a National Semiconductor LH 0005 op-amp.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant expresses confusion about how to find the collector current in the first transistor, noting the addition of base resistors without clear guidance on biasing.
  • Another participant suggests that biasing inputs near ground is common for split power supply op-amps, which may help define the currents in the input differential stage.
  • There is a mention of given parameters such as Betas of 100 and Vbe of 0.7V, along with the power supply voltages of ±10V, but no clear biasing information is provided.
  • Participants discuss the number of transistors in the input stage, with one stating there are four, and question how to calculate the base current without voltage or resistors.
  • Concerns are raised about the lack of information regarding input voltage (vin) and how that affects the ability to solve for collector currents.

Areas of Agreement / Disagreement

Participants generally express uncertainty and confusion regarding the analysis of collector currents, with no consensus on how to proceed due to missing information about biasing and input voltages.

Contextual Notes

The discussion highlights limitations in the provided information, such as the absence of explicit biasing details, input voltage values, and resistor configurations, which are critical for analyzing the circuit effectively.

oddo
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Find all Collector currents

I don't even know where to start, I added base resistors but am still getting stuck how can I find current in the first transistor?
 

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oddo said:
Find all Collector currents

I don't even know where to start, I added base resistors but am still getting stuck how can I find current in the first transistor?

Welcome to the PF.

Were you told to bias the inputs near ground (as you have done with the two resistors that you've drawn)? If so, that starts to define the currents in the input differential stage, right? How does connecting the two differential inputs to ground define the collector currents of the input differential pair?
 
It doesn't state how it is biased, which is why I added those base resistors. all I am given is Betas are 100 and Vbe is 0.7V. Connected to +-10V. It says that it is a LH 0005 National Semiconductor
 
oddo said:
It doesn't state how it is biased, which is why I added those base resistors. all I am given is Betas are 100 and Vbe is 0.7V. Connected to +-10V. It says that it is a LH 0005 National Semiconductor

I think it's reasonable to assume that the inputs would be biased near ground. That's very common for split power supply opamps.

So you have the Vbe and current gain numbers -- look at the first stage and look for some constrained voltage across a resistor. That's often the first step in figuring out bias circuits...

Post you work so we can be sure you aren't missing any factors...
 
Q1 has no resistors so how can I find current at its base collector or emitter?
 
oddo said:
Q1 has no resistors so how can I find current at its base collector or emitter?

Expand what you are looking at. How many transistors are in the input stage?
 
berkeman said:
Expand what you are looking at. How many transistors are in the input stage?

I really have no idea how to start this. All it says is find all the collector currents. This is my first course in opamps and I'm blown away. Thanks for your help sir.
 
oddo said:
I really have no idea how to start this. All it says is find all the collector currents. This is my first course in opamps and I'm blown away. Thanks for your help sir.

Start by answering my latest question. How many transistors are involved in the input stage? And then, what other components are directly connected to the input stage transistors?
 
I think there's four. So do I calculate the impedance of them?
I don't know why it doesn't show vin, how can I figure out the base current without voltage or resistors to put into the fundamental equation to solve for Ic.
 
  • #10
oddo said:
I think there's four. So do I calculate the impedance of them?
I don't know why it doesn't show vin, how can I figure out the base current without voltage or resistors to put into the fundamental equation to solve for Ic.

Yes, there are 4 transistors in the input stage. What other components are there? What are the voltages at the top and bottom of the input stage? If the inputs are at ground, what can you say about the voltages and currents in the lower part of the input stage?
 

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