MHz opamp - RC filter in 0 . 18 p m CMOS

Opamp gain: The main difficulty in building a very high frequency opamp-RC filter is achieving sufficient opamp gain at the filter comer frequency. With the transfer function used here, 34 dB opamp gain results in 1 dB peaking and 30 dB opamp gain results in 2 dB peaking at the comer frequency. (This includes overcoming a 2 dB reduction in loop gain due to capacitance at the opamp input.) A conventional Miller-compensated opamp in this process can achieve a unity-gain frequency of around 1 GHz and hence 30 dB gain at only 30 MHz. Two-pole compensation can raise this by increasing the gain roll-off from -20dB/decade to around -30 &/decade. Two-pole compensation is normally implemented using Miller compensation [2], but feedfonvard allows greater bandwidth. The feedforward opamp used (Fig. 1) achieves a unity-gain frequency of 2.6 GHz (Fig. 2). The opamp phase margin is moderate (35“), but is very tightly controlled by the relative strengths of the main and feedfonvard paths.