Understanding Op Amp Node Voltage Analysis: Equation Differences at Node D

[seang]

Hey, I think I need a little help understanding node voltage analysis with ideal op amps. For the circuit I've attached, at node D, I wrote the equation

((1/1000) + (1/1000))Vd - (1/50000)Vc - (1/1000)Ve = 0

However my prof's equation for node D is:

((1/1000) + (1/1000))Vd - (1/1000)Ve = 0

I don't understand why he didn't include the Vc like I did. I understand that Va = Vc, but you can't just say Va = 0, can you?

thanks much, sean

 

[berkeman]

Where do the "1000" numbers come from? Can you label the resistors or show their values? And no, Va will not be zero unless the current source has Iout = 0.

 

[ravenprp]

Hi Sean,

Node voltage analysis is a powerful tool for analyzing circuits with op amps. It allows us to find unknown voltages at different nodes in the circuit by using Kirchhoff's Current Law (KCL) and Ohm's Law.

In your equation, you correctly included the voltage at node C (Vc) in the KCL equation for node D. However, in your professor's equation, they have already taken into account the relationship between Vc and Va (which is Vc = Va), so there is no need to include Vc in the equation. This is because Va is already known to be zero, as it is connected to ground. So, in your professor's equation, they have simplified it by substituting Vc for Va and eliminating it from the equation.

In summary, both equations are correct, but your professor's equation is a simplified version that takes into account the known relationship between Va and Vc. I hope this helps clarify the differences in the equations for you. Keep practicing and you'll become more comfortable with node voltage analysis in no time.

Best of luck,