Four methods of achieving an input impedance greater than 100 kΩ are shown in the ac schematics of Fig. 11.50.

(a) Neglecting flicker noise and capacitive effects, derive expressions for the equivalent input noise voltage and current generators of these circuits. For circuit (i) this will be on the source side of the 100-kΩ resistor.

(b) Assuming that following stages limit the bandwidth to dc-20 kHz with a sharp cutoff, calculate the magnitude of the total equivalent input noise voltage in each case. Then compare these circuits for use as low-noise amplifiers from low source impedances.

Fig. 11.50:

100 k 10 , . Vs 1 k 10 Ic-0.5 mA , = 100, r,= 100 (ii) Ic=0.5 mA, =100, r, = 100 (1) 10 k 10 Ic- 13 A = 50 , - 100 (i) 3 1 mA (iv)