EDN Access— 10.12.95 Piecewise linear amplifier eschews diodes

Design Ideas: October 12, 1995

Piecewise linear amplifier eschews diodes

Brian Harrington,
Analog Devices, Wilmington, MA

A common implementation of a piecewise linear amplifier uses diodes in the feedback loop of an op amp. As the diodes become forward-biased, they effectively switch in resistors that alter the closed-loop gain of the amplifier. This implementation has two disadvantages. First, the diodes' shunt capacitance leads to poor high-frequency performance. Second, the breakpoints of the transfer function are subject to the 2-mV/øC drift of the diodes' forward-bias voltages. The circuit of Fig 1a avoids both problems by using a clamping amplifier, IC₂, to set the breakpoints.

If the signal applied to the noninverting input (pin 3) of IC₂ is between the clamp voltages, IC₂ works as a standard op amp. If the signal is greater than the upper clamp voltage, V_H, the amplifier disconnects the input signal at pin 3, and V_H becomes the noninverting input. Likewise, if the signal at pin 3 is below the lower clamp voltage, V_L, then the amplifier also disconnects the signal input, and V_L becomes the noninverting input.

Fig 1b graphically illustrates the operation of the circuit. When V_IN is between V_H and V_L, the circuit is a standard noninverting amp with gain equal to 1+(R₅/R₆). When V_IN is greater than V_H, V_H becomes the noninverting input to the amplifier. The transfer function from V_IN to V_OUT comprises two parts under this condition. From V_IN to V_H, the signal is attenuated by a factor of K_H=1+(R₂/R₁). From V_H to V_OUT, the gain remains G. This leads to an overall gain of G/K_H. The circuit behaves similarly when V_IN is below V_REFL. The gain in that condition is G/K_L, where K_L=1+(R₃/R₄).

Careful layout ensures adherence to the desired nonlinear transfer function over a 5-MHz bandwidth. Three factors limit the temperature stability of the breakpoints: the tracking of the resistors' temperature coefficients, the 10-µV/øC offset-voltage drift of IC₂, and the temperature stability of the reference voltages. You can generate the reference voltages using precision-voltage references or DACs. To maintain accuracy, buffer the references with a high-speed op amp, such as IC₁, to provide a low source impedance throughout the input signal bandwidth. (DI#1765)