Figure 5, which shows the Miller capacitance CM on the input side of the circuit. The size of CM is decided by equating the current in the input circuit of Figure 5 through the Miller capacitance, say iM, which is: , to the current drawn from the input by capacitor Cgd in Figure 4, namely jωCgd vGD. These two currents are the same, making the two circuits have the same input behavior, provided the Miller capacitance is given by: . Usually the frequency dependence of the gain vD / vG is unimportant for frequencies even somewhat above the corner frequency of the amplifier, which means a low-frequency hybrid-pi model is accurate for determining vD / vG. This evaluation is Miller's approximation and provides the estimate (just set the capacitances to zero in Figure 5): , so the Miller capacitance is . The gain gm (rO || RL) is large for large RL, so even a small parasitic capacitance Cgd can become a large influence in the frequency response of the amplifier, and many circuit tricks are used to counteract this effect. One trick is to add a common-gate (current-follower) stage to make a cascode circuit. The current-follower stage presents a load to the common-source stage that is very small, namely the input resistance of the current follower (RL ≈ 1 / gm ≈ Vov / (2ID) ; see common gate). Small RL reduces CM. The article on the common-emitter amplifier discusses other solutions to this problem. Returning to Figure 5, the gate voltage is related to the input signal by voltage division as: . The bandwidth (also called the 3 dB frequency) is the frequency where the signal drops to 1/ of its low-frequency value. (In decibels, dB() = 3.01 dB). A reduction to 1/ occurs when ωCM RA = 1, making the input signal at this value of ω (call this value ω3 dB, say) vG = VA / (1+j). The magnitude of (1+j) = . As a result, the 3 dB frequency f3 dB = ω3 dB / (2π) is: . If the parasitic gate-to-source capacitance Cgs is included in the analysis, it simply is parallel with CM, so . Notice that f3 dB becomes large if the source resistance RA is small, so the Miller amplification of the capacitance has little effect upon the bandwidth for small RA. This observation suggests another circuit trick to increase bandwidth: