Operational amplifier integrators for the measurement of the delay times of microwave transistors

A simple low frequency technique for the measurement of subnanosecond delay times with an operational amplifier connected as a summing integrator is described. Previous integrator theory is extended to incorporate second-order effects. The operational amplifier gain-bandwidth product is shown to set a limit to the sensitivity of the integrator. For a gain-bandwidth product of 1 GHz the smallest measurable delay time is estimated to be about 5 ps, which is small enough for measurements on the fastest microwave transistors. Expressions for the delay times of transistors are derived from the hybrid-π model. Examples of measured delay times of microwave transistors are given and the results are compared with theoretical predictions.