# Phototransistor modeled as a Thevenin equivalent circuit

1. Mar 22, 2008

### Winzer

1. The problem statement, all variables and given/known data
you have a phototransistor that you have modeled as a Thevenin equivalent circuit. It has a output impedence(Thevenin Impedence) of 10kohm and produces a peak voltage o2 2V at a frequency of 80 MHz. The input impedence of the oscilloscope as this simple combination of input impedences.
a.draw the osilloscope/photodetector circuit
b. calculate the half power frequency
c. a 50 ohm resistor is added in parallel to the 1M ohm load. draw the new circuit.
d. calculate the new break frequency.

2. Relevant equations
$$f_b=\frac{1}{2\Pi R C}$$

3. The attempt at a solution
1.The photodetector in parallel with the capacitor and 1m ohm resistor
2. I just took $$f_b=\frac{1}{2*\Pi*10^3 ohms*20^-9F}=81.40MHz$$
3.The photodetector in parallel with the capacitor,1m ohm resistor, and 50 ohm resistor
4. not too sure

2. Mar 23, 2008

### Phrak

Could you clear-up this typo?

"The input impedence of the oscilloscope as this simple combination of input impedences."

3. Mar 24, 2008

### Winzer

Sorry,
"Model the oscilloscope as this simple combination of input impedences."

4. Mar 24, 2008

### Winzer

Actually for b. I get 7.87 Hz.

5. Mar 25, 2008

### Winzer

Sooo......is that right?