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 the resistances of both branches of the electronic attenuator are equal (these resistances are labeled ra in Figs. 5 and 6). To this end, the FET's in each pair are matched. However, it is impossible to match a pair of FET's so that they track each ac Output

To Log Amplifier No. 2

To Log Amplifier No. 1

Balance

Fig. 7. Using field-effect transistors as variable resistors, the electronic attenuator can vary its resistance over more than an 80 dB range, from about 30 fi to 1 Mil, in response to a control voltage.

Before Balancing w And Offsetting

Before Balancing w And Offsetting

Control Voltage Vc (Input to FET's)

After Balancing And Offsetting

Control Voltage Vc (Input to FET's)

After Balancing And Offsetting

Control Voltage Vc (Input to FET's)

id> After Linearizing With Log ra Feedback id> After Linearizing With Log ra Feedback

Control Voltage Vp (Input to Driver Amplifier)

Fig. 8. Each pair of FET's in the electronic attenuator is matched over part of the resistance range. Feedback is used to linearize the resulting attenuator curve.

other exactly over such a large resistance range. The question is, how well must they track to keep the output error from being too large?

A worst-case analysis has been made, and the results show that a 10% mismatch in the resistances of the two branches of the attenuator causes only about 0.23% error in the output current! This is important, for it means that physically realizable tracking mismatches do not cause serious measurement errors.

The reason for using two pairs of FET's instead of one pair is, again, the difficulty of matching a single pair of them over the entire resistance range. Each pair is matched over part of the resistance range, as shown in Fig. 8. The curves of Fig. 8 are plots of the logarithm of the resistance of one arm of the electronic attenuator (log ra) as a function of the control voltage Vc coming out of the driver amplifier.

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