Improve FET-based gain control

One problem with standard FET gain-control circuits is increased noise when you use the FET as a part of a resistive attenuator in series with an op amp. This configuration attenuates the signal before amplification; hence, it requires much higher gain bandwidth and better noise performance from the op amp. When you substitute the FET for the gain-setting resistor in a noninverting op-amp circuit, distortion limits the circuit configuration to applications in which the input voltage is less than a few hundred millivolts. The FET imposes this limitation, because the channel-depletion layer is a function of V_DG and V_GS. The improved circuit in Figure 1 uses the FET as part of the feedback loop.

Figure 1.	The drain-source resistance of the FET controls the gain of the op-amp stage.

The voltage across the FET is limited in this application, and the noise performance is good. An added bonus is improved linearity performance. The transfer function for the improved circuit is as follows [1]:

When R₂ + R₃ = R₁ and R₄ = R_DS (FET drain-source resistance), the transfer function reduces to

The minimum drain-source resistance for the FET on hand, J271, is 76 Ω at V_GS = 0 V. The actual V_DS at the inception of distortion varies with each FET, but keeping V_DS lower than 200 mV usually prevents distortion. In the design in Figure 1, the FET drain-source voltage is limited to approximately 100 mV to prevent distortion. The divider action between R₃ and R_DS creates V_DS from the output voltage, according to the following equation:

You can calculate R₃ as 24.5 kΩ and select 24 kΩ. The parallel value of R₂ and R₃ determine the maximum circuit gain. Selecting R₂ as 3 kΩ yields R₁ equal to 27 kΩ and a maximum gain of –37. The measured gain at V_C = V_GS = 0 V is –36.1, which correlates well with the calculated value. R_A and R_B are feedback resistors that linearize the FET's V_GS versus R_DS transfer function. You can normally obtain adequate linearization with equal-value resistors, but you can also control the slope of the transfer function by setting the resistor ratio. The graph in Figure 2 shows that R_A modifies the transfer function and linear control-voltage range (V_GS). The p-channel FET, J271, requires a positive control voltage, but you can use a negative control voltage with an equivalent n-channel FET, such as the J210. The circuit is versatile and provides low distortion, wide range, good linearity, and low cost. The TLC071 op amp has low input-bias currents and has provisions for input offset-voltage correction.

Figure 2.	The ratio RA/RB in Figure 1 controls the slope of the gain-control transfer function.

Reference

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