From a measurement of the rise time of the output pulse of an amplifier whose input is a small amplitude square wave, one can estimate the following parameter of the amplifier:

The circuit of Fig. is an example of feedback of the following type

In a series regulated power supply circuit, the voltage gain A_v of the ‘pass’ transistor satisfies the condition:

In the circuit of fig. Determine the resistance R_o seen by the output terminals, ignore the effects of R₁ and R₂.