Voltage Summing Circuit

The AC input and output voltages to the op amp circuit below are:

V 1 ( t ) = cos ( ω t + π ) V 2 ( t ) = cos ( ω t π / 2 ) V o u t ( t ) = M cos ( ω t + θ ) V_1(t) = \cos(\omega t + \pi) \\ V_2(t) = \cos(\omega t - \pi/2) \\ V_{out}(t) = M \cos(\omega t + \theta)

What is the sum of M M and θ \theta ?

Details and Assumptions:
1) R 1 = 1 R_1 = 1
2) R 2 = 2 R_2 = 2
3) R 3 = 3 R_3 = 3
4) The voltages at the + + and - terminals of the op amp are the same
5) The currents into the + + and - terminals of the op amp are zero
6) The units of θ \theta are radians


The answer is 3.818.

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1 solution

Karan Chatrath
Mar 15, 2020

From the diagram above, applying Ohm's law gives:

V 1 V P = I 1 R 1 V_1 - V_P = I_1R_1 V 2 V P = I 2 R 2 V_2 - V_P = I_2R_2 V P V o u t = I R 3 V_P-V_{\mathrm{out}} = IR_3

Applying Kirchoff's current law and the current condition for an ideal Op-Amp gives:

I = I 1 + I 2 I = I_1 + I_2

It is assumed that V G = 0 V_G = 0 . Using the ideal Op-Amp voltage condition:

V P = V G = 0 V_P = V_G = 0

Using all expressions to solve for V o u t V_{\mathrm{out}} gives:

V o u t = ( V 1 R 1 + V 2 R 2 ) R 3 V_{\mathrm{out}} = -\left(\frac{V_1}{R_1}+\frac{V_2}{R_2}\right)R_3

Given that:

V 1 = cos ( ω t ) V_1 = -\cos\left(\omega t\right) V 2 = sin ( ω t ) V_2 = \sin\left(\omega t\right)

Leads to:

V o u t = 3 ( cos ( ω t ) sin ( ω t ) 2 ) V_{\mathrm{out}}= 3\left(\cos\left(\omega t\right)-\frac{\sin\left(\omega t\right)}{2}\right) V o u t = 3 2 ( 2 cos ( ω t ) sin ( ω t ) ) \implies V_{\mathrm{out}}= \frac{3}{2}\left(2\cos\left(\omega t\right)-\sin\left(\omega t\right)\right) V o u t = 3 5 2 ( 2 5 cos ( ω t ) 1 5 sin ( ω t ) ) \implies V_{\mathrm{out}}= \frac{3\sqrt{5}}{2}\left(\frac{2}{\sqrt{5}}\cos\left(\omega t\right)-\frac{1}{\sqrt{5}}\sin\left(\omega t\right)\right) V o u t = 3 5 2 cos ( ω t + arccos ( 2 5 ) ) \implies V_{\mathrm{out}}= \frac{3\sqrt{5}}{2}\cos\left(\omega t + \arccos\left(\frac{2}{\sqrt{5}}\right)\right)

Which means that:

M = 3 5 2 ; θ = arccos ( 2 5 ) \boxed{M = \frac{3\sqrt{5}}{2} \ ; \ \theta = \arccos\left(\frac{2}{\sqrt{5}}\right)}

The answer is:

M + θ 3.81775 \boxed{M + \theta \approx 3.81775}

Where θ \theta is entered in radians. It would be useful to mention the units of angle to be entered in the problem statement.

@Karan Chatrath If V G V_{G} is zero then how V P V_{P} will be zero. I can't able to understand.

A Former Brilliant Member - 1 year, 2 months ago

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Refer to the fourth point under 'Details and Assumptions' in the problem statement.

Karan Chatrath - 1 year, 2 months ago

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