Macro-transistor structures are disclosed. In some cases, the macro-transistor structures have the same number of terminals and properties similar to long-channel transistors, but are suitable for analog circuits in deep-submicron technologies at deep-submicron process nodes. The macro-transistor structures can be implemented, for instance, with a plurality of transistors constructed and arranged in series, and with their gates tied together, generally referred to herein as a transistor stack. One or more of the serial transistors within the stack can be implemented with a plurality of parallel transistors and/or can have a threshold voltage that is different from the threshold voltages of other transistors in the stack. Alternatively, or in addition, one or more of the serial transistors within the macro-transistor can be statically or dynamically controlled to tune the performance characteristics of the macro-transistor. The macro-transistors can be used in numerous circuits, such as varactors, VCOs, PLLs, and tunable circuits.