Circuit Analysis Homework Help: Parts A & C

[KTiaam]

Homework Statement

Need help on part a.
and c.

The Attempt at a Solution

• Part A

Since Va = 4V then pin 2 and 3 also have 4v. Would that mean that Vin is = 4v as well?
Im just confused how to find I1.
Also I am not sure how to find Vc.

• Part C

As for part c, I am having trouble with it entirely.

Any help is appreciated!

 

[milesyoung]

Since Va = 4V then pin 2 and 3 also have 4v. Would that mean that Vin is = 4v as well?

Well, yes, but you'll have to argue that based on the characteristics of an opamp with negative feedback. If you think in terms of the two golden rules of ideal opamp analysis..

Im just confused how to find I1.

You know what the voltage is across R1. What about Ohm's law?

As for part c, I am having trouble with it entirely.

Consider again the behavior of an ideal opamp with the feedback shown. It desperately wants to turn on that BJT hard, so it can reduce the voltage across its terminals to zero, but that really only requires the BJT to just barely move out of its cutoff region. You usually assume something about V_BE then.

 

[KTiaam]

You know what the voltage is across R1. What about Ohm's law?

The thing that threw me off, is what voltage to use with r1,
But based off of the orientation of the current source, I use Vb?

 

[milesyoung]

The thing that threw me off, is what voltage to use with r1,
But based off of the orientation of the current source, I use Vb?

The voltage (with respect to ground) at Vb is 6 V, and you drop down to 4 V as you cross the resistor to Va.

Does that help?

 

[KTiaam]

The voltage (with respect to ground) at Vb is 6 V, and you drop down to 4 V as you cross the resistor to Va.

Does that help?

so that means the voltage drop across the resistor is 2v, then using V=IR
i get 2mA?

or do i use Nodal Analysis?

 

[milesyoung]

so that means the voltage drop across the resistor is 2v, then using V=IR
i get 2mA?

Yes.

Since Vb is at a higher potential than Va, what does that tell you about the direction of the current?

or do i use Nodal Analysis?

Nodal analysis is useful when you need to solve for unknown node voltages, but you're already given the node voltages on either side of the resistor, so all there's left to do is just apply Ohm's law.

 

[KTiaam]

Yes.

Since Vb is at a higher potential than Va, what does that tell you about the direction of the current?

Current goes from a high to low potential so it goes from right to left!

Thank you for all the help Miles! I might have more questions incoming lol.