How to Determine Vd in an OP AMP Circuit with Diode?

In summary, the conversation is discussing the voltage drop across a diode in a circuit. The answer given is Vd = -5V, but the person is unsure how this was calculated due to the lack of information provided. It is determined that the current flow through the diode must be zero, making it reverse-biased and resulting in a voltage drop of -5V. The conversation also clarifies that the direction of the current flow shown in the diagram does not necessarily indicate the direction of the actual current flow.
  • #1
pyroknife
613
3
I have attached the circuit of interest.
The question is to find Vd (drop across diode as shown in the attached image).

Sorry about the handwriting on there (those can be ignored).
The answer was given to be Vd=-5V.

I can't figure out how they got that answer. I don't feel there is enough information given to be bale to determine the drop.

The voltage @ the left node of diode=+5V. You don't know the voltage @ the right node of the diode though. The voltage @ right node should be 10V-1KΩ*(I4).
If the answer is -5V, then I4 must be 0, but I don't see how you could have solved for I4 with the amount of info that was given.
 

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  • #2
I4 must be zero. Any current that flows through that resistor has to flow from left to right, as indicated, because otherwise it is flowing backwards through the diode, which it can't do. So the diode has 5V on the left side, and 10V + I4*1K on the right side (not -I4*1K as you wrote). So, the diode is reverse biased and no current is flowing in it, so I4 = 0.
 
  • #3
phyzguy said:
I4 must be zero. Any current that flows through that resistor has to flow from left to right, as indicated, because otherwise it is flowing backwards through the diode, which it can't do. So the diode has 5V on the left side, and 10V + I4*1K on the right side (not -I4*1K as you wrote). So, the diode is reverse biased and no current is flowing in it, so I4 = 0.

Oops, yeah it is +I4*1k.

Doesn't the left to right current flow as indicated mean the diode is 'forward' biased?
Right to left current flow for this diode should be reversed biased, no?
 
  • #4
In order for the current to flow left to right as shown, the diode would have to be forward biased, yes, but in fact it is reverse biased so there is no current flow. Just because I draw an arrow doesn't mean current is flowing.
 
  • #5
phyzguy is correct. The diode is reverse-biased so that there is no current flowing: I4 = 0. Therefore, the voltage across the resistor is 0 V. The voltage across the diode from left to right(by the shown polarities on the schematic) is 5 V - 10 V = -5 V.
 

1. How does a diode affect an op amp circuit?

A diode in an op amp circuit can be used to limit the output voltage and protect the circuit from overloading. It also allows the circuit to rectify signals, meaning it only allows current to flow in one direction.

2. What is the purpose of using a diode in an op amp circuit?

The main purpose of using a diode in an op amp circuit is to prevent the output voltage from exceeding a certain level. This is important for protecting both the circuit and any connected components.

3. Can a diode be used to amplify signals in an op amp circuit?

No, a diode cannot be used to amplify signals in an op amp circuit. Its main function is to limit and rectify the output voltage, not amplify it.

4. How do you choose the appropriate diode for an op amp circuit?

The appropriate diode for an op amp circuit depends on the specific application and required voltage and current ratings. It is important to choose a diode with a voltage rating higher than the maximum expected output voltage of the circuit.

5. Are there any limitations to using a diode in an op amp circuit?

Yes, there are some limitations to using a diode in an op amp circuit. It can introduce signal distortion and reduce the overall gain of the circuit. It is important to carefully consider the placement and type of diode used in the circuit to minimize these effects.

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