op-amp circuit help

NascentOxygen

You may be under a misunderstanding that the only voltage source in the circuit is Vcc. That would be quite wrong. Not shown in many schematic diagrams are the + and - DC supplies powering the op-amp itself.

The Vcc shown serves to supply current to the zener diode. What voltage do you expect to find at the junction of the two 10k resistors and the zener diode?

Bearing in mind what you said about the horizontal 10k resistor, deduce the voltage that you would expect to measure on the right hand side of that horizontal 10k resistor (i.e., as measured at the inverting input (-) of the op-amp).

moe619

V1 and V2 my lecturer kind of gave us an idea that there both 15 V and Vcc is 10V now im more confused

NascentOxygen

The voltages powering the op-amp do not dictate the output voltage of the op-amp, meaning you can largely ignore those voltages---which is why they are not even shown on some circuit diagrams. What is important is that the output voltage of the op-amp is controlled by the input voltages of the op-amp.

moe619

then lets calculate the current..?

NascentOxygen

I framed some leading questions in post #18. Still waiting to see your answers.

I'll be away, may not be back on for 3 or 4 days. 

moe619

5 V? we said the horizontal 10k would be 0 V

NascentOxygen

Right. So how to determine the op-amp's output voltage? Remember, it's nothing to do with Vcc, providing Vcc is sufficiently high.

moe619

the output voltage of the non-inverting amplifier is in phase with the input voltage.
Vout=Vin

NascentOxygen

Meaning Vout would be how many volts?

moe619

5V? more or less?

NascentOxygen

Let's suppose the op-amp has an open loop low frequency differential gain of 10,000. Can you determine Vout exactly?

moe619

approximately V out can be determined
there is a formula but its for a summing/difference amplifier
Vout= -(V1 + V2) (for unity gain mode)

Vout= -(Rf/R) (V1 + V2) (where R1=R2=R)

......................
Vout= V2 - V1 (for a unity gain amplifier)

Vout= (Rf/R1) (V2 - V1) (for an amplifier with gain where R1=R2 and Rf=R3

NascentOxygen

At seeing the word exactly I thought you may have applied this formula:

Vout = A (V₊  ̶ V₋)

But 5V is close enough. So with an output voltage of 5V, how much current is flowing in the load resistor? I believe that is what you are asked find?

moe619

Vout = A (V₊  ̶ V₋) never seen this formula is it the same as Vout=Av*Vin

V=IR
5=I x 10k
I= 0.0005 A

NascentOxygen

The op-amp triangle shows two inputs. What is Vin?
That looks right.