4.20 Phototransistor as a detector. Consider again Example 4.4, but now the photodiode is replaced with a phototransistor biased as shown in Figure 4.31. The phototransistor has a gain of 50. For a given input pulse train, show the output in relation to the input and calculate the voltage levels expected. Assume that all power that reaches the phototransistor is absorbed in the base- emitter junction