Multi-Path Amplifier Expertise
We have leading expertise in designing Multi-Path-Amplifiers:
Multi-Path Programmable Gain Amplifiers
PGA 30dB Gain, 1dB Steps, BW = 20MHz, THD < -80dB
PGA 40dB Gain, 1dB Steps, BW = 20kHz , THD <-120dB
For applications with large signal bandwidth and high linearity requirements (e.g. signal bandwidth fs= 0-20MHz & THD <-80dB), a conventional two-stage operational amplifiers with very high transit frequency (ft > 20MHz * 80dB (THD) > 200GHz), consuming a lot of power is necessary. It may even be that the required transit frequency is infeasible in the target technology.
To reduce the required transit frequency, the conventional two-stage Miller operational amplifiers are replaced by multistage operational amplifiers with feedforward Gm-C compensation. The concept of multistage operational amplifiers is shown below. The shown Multi-Path Operational Amplifier consists of three signal paths, with different transit frequencies and different numbers of gain stages. At the overall transit frequency, only the first-order path —consisting of the third integrator gm3 plus output stage— has unity gain. All other paths show a gain that is considerably smaller than unity at this frequency. Thus, stability is determined only by the two-stage system consisting of the third integrator plus the output stage.
The Bode Plot shown below compares the bode plot of a conventional Miller operational amplifier with a plot of a four-stage operational amplifier. Both have a transit frequency of 2 GHz. As the gain of the four-stage amplifier near the transit frequency is mainly determined by the third integrator, the phase and gain margins are similar to the Miller operational amplifier. For lower frequencies, the other two paths contribute to the overall gain, and gain slopes with 40 dB per decade in the case of two stages and 60 dB per decade in the case of three stages are achieved. Therefore, the four-stage operational amplifier achieves much higher gain within the signal band as well as at the loop filter integrator bandwidth.
Because high gain is achieved by cascading amplifier stages, minimal device lengths can be used, which makes low power consumption and/or high bandwidth possible. Very low supply voltages are possible as there is no need for cascode transistors.
To overcome the conditional stability of this multistage structure, the feed-forward paths are designed such that the lowest order path saturates last. Thus, when the operational amplifier is overdriven, it reduces to an unconditionally stable system. To drive the resistive loads a feed-forward class-AB output stage is used. To achieve sufficient power supply rejection, the design is fully differential throughout.
Architecture of Multi-Path Operational Amplifier
Open Loop Bode Plot