# Simple question regarding Ideal Op Amps

#### enemias

Joined Sep 21, 2019
10

Can i assume the that the currrent going through is 0.03mA?

#### crutschow

Joined Mar 14, 2008
34,671
Can i assume the that the currrent going through is 0.03mA?
Yes.

Now you can continue through the feedback loop using only Ohms law to calculate all the voltages and currents.

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#### enemias

Joined Sep 21, 2019
10
thanks

#### enemias

Joined Sep 21, 2019
10
Yes.

Now you can continue through the feedback loop using only Ohms law to calculate all the voltages and currents.
So V_a would be 1.8, correct? assuming the V_n ( negative node) is 0?

#### crutschow

Joined Mar 14, 2008
34,671
So V_a would be 1.8, correct? assuming the V_n ( negative node) is 0?
Right value, but what's the polarity?

#### enemias

Joined Sep 21, 2019
10
Right value, but what's the polarity?
Why would it be negative, since PSC says says current entering a a resistor enters a + side, thus making the voltage +.

#### crutschow

Joined Mar 14, 2008
34,671
Why would it be negative, since PSC says says current entering a a resistor enters a + side, thus making the voltage +.
Which resistor? (You should label all the resistors).
You stated the negative node is zero, so what must the polarity be at the Va node for the current to go in the direction you show through the 60k resistor.

#### enemias

Joined Sep 21, 2019
10

Ok, so. since the negative node is 0, the current must flow to a lower voltage, meaning V_out will be negative? meaning at the node shared among the the resistors its -1.8V. So according so voltage V_a = -1.8V - Ground (0 V) = -1.8V? Sorry about not understanding this, i cant grasp negative voltage yet.

#### crutschow

Joined Mar 14, 2008
34,671
meaning V_out will be negative? meaning at the node shared among the the resistors its -1.8V. So according so voltage V_a = -1.8V - Ground (0 V) = -1.8V?
Yes, it is negative.
The circuit is a common op amp inverter circuit, meaning the polarity of the output is always opposite the input.

So now that you know Va with its polarity and the current through the 60k resistor, you can calculate the current through the 40k and 30k resistors.
From this you can calculate the output voltage.
i cant grasp negative voltage yet.
It's the yang of positive voltage.
Voltage is always between two nodes, so if one node is positive then the other node is negative with respect to that.
In this case you are measuring voltages with respect to ground, which can be considered the positive or negative node, depending upon the other nodes polarity.

#### enemias

Joined Sep 21, 2019
10
Yes, it is negative.
The circuit is a common op amp inverter circuit, meaning the polarity of the output is always opposite the input.

So now that you know Va with its polarity and the current through the 60k resistor, you can calculate the current through the 40k and 30k resistors.
From this you can calculate the output voltage.
It's the yang of positive voltage.
Voltage is always between two nodes, so if one node is positive then the other node is negative with respect to that.
In this case you are measuring voltages with respect to ground, which can be considered the positive or negative node, depending upon the other nodes polarity.
Ok, is it right to say that the current at the 40k resistor is flowing from ground to -1.8V? Since current always flows from Higher voltage to negative voltage? If so, how is that possible since there isnt any source to give current. Also, can i visualize all that grounds like if they were interconnected? im sorry im bombarding you with questions, hehehe (also if you have any videos that might help me visualize negative voltage lmk, i understand negative current)

#### crutschow

Joined Mar 14, 2008
34,671
is it right to say that the current at the 40k resistor is flowing from ground to -1.8V? Since current always flows from Higher voltage to negative voltage?
Yes, but it's not a "higher" voltage, it's the positive node (which here is ground).
Current always flows from the positive node to the negative node.
how is that possible since there isnt any source to give current.
But there is a source.
It's the negative node (which is being generated through the 30k resistor by the op amp output).
A source can be positive or negative.
Try to understand that there is no inherent difference between positive and negative other than to indicate the relative polarity between two nodes.
can i visualize all that grounds like if they were interconnected?
Yes, all ground symbols are connected together.
There is no voltage difference between them.
Ground is just an arbitrary point that all circuit voltages are referenced to.
It has no other significance.

#### enemias

Joined Sep 21, 2019
10
Yes, but it's not a "higher" voltage, it's the positive node (which here is ground).
Current always flows from the positive node to the negative node.
But there is a source.
It's the negative node (which is being generated through the 30k resistor by the op amp output).
A source can be positive or negative.
Try to understand that there is no inherent difference between positive and negative other than to indicate the relative polarity between two nodes.
Yes, all ground symbols are connected together.
There is no voltage difference between them.
Ground is just an arbitrary point that all circuit voltages are referenced to.
It has no other significance.
Thank you so much, you helped a lot!