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[SOLVED] Phototransistor + Op Amp Circuit Analysis

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PoS080

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Hello EE friends,

I need help solving this sample circuit for class. The photo transistor is responding to a 5khz photodiode not shown. I need help solving out for the output voltage.

My understanding is that the phototransistor will turn on and off at the same rate as the photodiode, and the signal created will go to the inverting op amp to get amplified.

This is how I normally solve for an op-amp, but I'm not sure if I have an R1, and how to solve for the transistor.

I1 = I2
(Vin-0)/R1 = (Vout - 0)/R2
(Vin)/R1 = (Vout)/R2
Vin(R2/R1) = V out
Screenshot_2.png
 

This is how I normally solve for an op-amp, but I'm not sure if I have an R1, and how to solve for the transistor.
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I guess you mean R1 is R2 in your schematic and R2 is R3 in the schematic.

(Vin-0)/R1 = (Vout - 0)/R2
Click to expand...
Wrong. Should have been: (VCC-0)/R2 = (0-Vout)/R3 (when transistor is ON).

Your circuit will invert the VCC voltage when ON and do nothing when OFF. U4 will never light.
 
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    PoS080

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You're right. I see it now, but can we assume that all of VCC will appear on the collector end? Shouldnt there be a drop Vce drop across the transistor?
 

PoS080 said:
but can we assume that all of VCC will appear on the collector end? Shouldnt there be a drop Vce drop across the transistor?
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All VCC will apear on the collector when the transistor is off.
When transistor is ON, to be more precise as you are saying, there will be a voltage drop VCESAT.
 
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