Design techniques for improving the HF response of a monolithic JFET operational amplifier

Bandwidth limitations of an operational amplifier are explored in relation to the second-order frequency response caused by the output follower´s inductive emitter impedance when driving capacitive loads or by the local AC feedback associated with the Miller integration function. The substrate p-n-p follower is shown to seriously degrade the bandwidth in the sink mode due to its low ω/SUB τ/. New design techniques are discussed which use the integration capacitor to provide local AC feedback around a new all-n-p-n transistor output stage. This reduces both the open-loop distortion and the quiescent output resistance for a given quiescent current. This also maximizes the lower limit of the inductive frequency range in the sink mode to improve the second-order frequency responses when driving capacitive loads. The multipole local AC feedback loop (Miller loop) is analyzed, and a method of compensation is described which uses pole splitting and feedforward techniques within the Miller loop. As a result, a general-purpose monolithic p-channel JFET quad operational amplifier has been fabricated with standard low-cost technology; the amplifier achieves a 10-MHz bandwidth and a 45-V/μs slew rate, while consuming only 2.1 mA/amplifier.