How about?
When Vin goes low, at some point the FET turns off. When the FET turns off, the only current flowing is the IB current. That gives you the the 0.1mA times 50ohms = 5mV that you properly noted in your original post.
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How about?
When Vin goes low, at some point the FET turns off. When the FET turns off, the only current flowing is the IB current. That gives you the the (-)0.1mA times 50ohms = -5mV that you properly noted in your original post.