FETs as Variable Resistors

Fig. 7 is a schematic diagram of the electronic attenuator. Two pairs of field-effect transistors are the variable resistors. The bias voltage (E in Figs. 5 and 6) is quite low 20 mV so the FET's operate in the linear regions of their V-I characteristics, and therefore look like resistors. The resistance of each branch varies from about 30 Q to 1 MQ. The direct current flowing through the FET's (I in Figs. 5 and 6) is an accurate measure of the rms value of the ac input signal, provided that...