Electronic – More equations than unkowns in amplifier circuit

active-filtercircuit analysisfrequency responsekirchhoffs-lawsoperational-amplifier

I am trying to find the Magnitude Response of the gain of this amplifier circuit.

The gain formula is: $$ H(\omega) = \frac{\tilde{V_{out}}}{\tilde{V_{in}}}$$

My amplifier circuit is as follows:
butterworth_bandpass_amplifier_circuit

I am trying to find the magnitude response:
$$ |H(\omega)| = \frac{|\tilde{V_{out}}|}{|\tilde{V_{in}}|}$$

The end goal: to get both V_out and V_in as functions of ω (with the resistor and capacitor values treated as constants). I will then use a tool (i.e. MATLAB, Maple, or other graphing software) to plot the magnitude response as a function of ω, and I will keep adjusting the values for the resistors and capacitors until the plot shows that the cutoff frequencies at both sides of the pass band are right where I want them.

How I am trying to get the equation: Before working with the absolute value, I am trying to get the equation V_out/V_in as one fraction with the only variable being ω and the constants being the impedances of the resistors and capacitors (ZR1, ZR2, ZR3, ZC1, ZC2).

The problem: I have way more equations than unknowns! The circuit is way over-defined. I have tried to use substitution to solve the problem, and was taken in circles. I tried to plug the equations into a matrix, but the calculator returned an error. How can I solve this over-defined system of equations? For now, please treat the impedances ZR1, ZR2, ZR3, ZC1, and ZC2 as constants (i.e. don't plug in the capacitor formula ZC=1/jωC or the resistor formula ZR=R just yet, I'd like to get an expression with just Z's first to keep things simple).

What I'm stuck trying to get: An expression V_out/V_in = [expression with only Z's]. This means that Vm, I1, I2, I3, and I4 have all been substituted out.

Equations:
$$\tilde{V_{out}} – 0V = (\tilde{I_{1}})(Z_{R2})$$
$$\tilde{I_{1}} + \tilde{I_{2}} – \tilde{I_{3}} – \tilde{I_{4}} = 0$$
$$\tilde{V_{out}} – \tilde{V_{m}} = (\tilde{I_{2}})(Z_{C2})$$
$$\tilde{V_{m}} – \tilde{V_{in}} = (\tilde{I_{3}})(Z_{R1})$$
$$\tilde{V_{m}} = (\tilde{I_{4}})(Z_{R3})$$
$$0V – \tilde{V_{m}} = (\tilde{I_{1}})(Z_{C1})$$

To reiterate: I want to find ( V_out / V_in ) = [expression with only Z's]. All Vm, I1, I2, I3, and I4 have been substituted out. Then I can finally plug in the capacitor and resistor impedance equations and get an expression with R (resistance) and C (capacitance) constants as a function of ω. But this hasn't been working (6 equations, only 5 unknowns: Vm, I1, I2, I3, and I4). V_out and V_in are not unknowns since they will be shown as a fraction on the left hand side of the equation.

Thanks in advance.

Best Answer

If you are familiar with the function could you please explain in an answer instead of simply saying that solutions might "exist somewhere on the internet"? Thank you!

Not "might exist" but "do exist". Try this site's simulator: -

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The end goal: to get both V_out and V_in as functions of ω (with the resistor and capacitor values treated as constants). I will then use a tool (i.e. MATLAB, Maple, or other graphing software) to plot the magnitude response as a function of ω, and I will keep adjusting the values for the resistors and capacitors until the plot shows that the cutoff frequencies at both sides of the pass band are right where I want them.

Looks like you need a tool to keep plugging in values to get the response you want i.e. that is your end goal. The Okawa electric tool is just that.