Several techniques to approximate complex transfer functions are developed that can lead to physical insight useful for circuit design. The low-Q approximation provides simplified expressions for the salient features of second-order responses. These results are extended to higher-order systems. Graphical construction of impedance and transfer function asymptotes is introduced, leading to a physical interpretation of the origins of frequency response asymptotes, corner frequencies, and resonances. These techniques are applied to the design of an op-amp proportional-integral-derivative (PID) compensator circuit.