Electrical Circuit
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SECTION 1
INTRODUCTION
Abstract:
Many scientists are studying the effects of electromagnetic radiation on the body. In order to do this we have to understand the tissue and its related components. After many experiments scientists have learned that the skin acts as an parallel RC circuit. Due to skin and layers of fat the electricity that flows through the body will simulate the RC circuit and it will produce a certain capacitance.
Lab Introduction:
Reference: Appendix A, page 1,2,3,
In this experience we will design a Wein -bridge oscilator.
That will create a frequency of 25KHZ and then use the signal to determine the value of the resistance and capacitance of the biceps. This sinusoidal output will later be compared as a phase shift so that we can derive equation for determining the appropriate evaluated capacitance and skin resistance.
SECTION 2
DESIGN OSCILLATOR:
2.1 -Frequency-Domain Circuit:
The Wien-bridge oscillator, Fig 3 Appendix A, consists of an op-amp and two voltage dividers. One voltage divider will consist of : R1, C1, R2, C2, and is also located in the left side of the circuit.
The other voltage divider will be made of only two resistors on the right side of the circuit: R3 and R4. These two voltage dividers will provide us with our output voltage that is supplied by our Op-amps 9V and -9V. Due to the circuits positive and negative feedback the positive and negative inputs will try to always balance each other out and thus creating a sinusoidal wave. Therefore we cal this type of circuit an Oscillator.
2.2 – Balanced Bridge
Using the Appendix A fig. 3, we can show how the bridge can be balance so that it can produce a sinusoidal wave. In order to make the bridge balanced we will need for the voltages at node a and node b, to be equal. This will happen only when Z1/Z2= R3/R4, where Z1 is the combined impedance of R1 in series with C1 and Z2 is the impedance of R2 in parallel with C2.
2.3 – Oscilation
By adding a noise voltage to the circuit between node a and node b we will derive an equation for the oscillation relationship between node a and b.
So it will be:
R3/R4 = R1/R2 + C2/C1.
2.4 – Oscillation Frequency
By using the first equation, R3/R4 = Z1/Z2 we will derive an equation for the frequency () of the Oscillator.
= 2* Freq.
= 1/(2* Sqrt(R1*R2*C1*C2))
This will determine our frequency due to the Resistance and capacitance in our circuit.
2.5 – Component Values for Oscillation
Using C1=C2= .5 nF with the equations derived from above we found that:
R3= 3K R4= 1.5K R1= 13K