In an electronic circuit, a transistor is used for switching, relaying, and amplifying various signals.  In the hydraulic world, this would be equivalent to a valve that can be opened and closed with a separate pressure input, with a second one-way check valve on the output side of the assembly.  As more pressure is applied onto the control line, more fluid is allowed to flow through the valves.  The one-way check valve simulates the one-way nature of a transistor's output.
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Vanessaezekowitz