Voltage-to-current converter offers precisely defined dead-band

August 06, 2013 //By Marian Stofka
Voltage-to-current converter offers precisely defined dead-band
Marian Stofka of the Slovak University of Technology explains how a voltage-to-current converter is able to offer precisely defined dead-band.

In the control of nonlinear and linear systems, sometimes a slight nonlinearity is intentionally inserted into the closed control loop. “Slight” here means that the output of the inserted element starts to vary linearly, but only once the input exceeds a nonzero threshold value. For inputs from zero to the threshold, the output of the element has zero value. The result is that adding of this nonlinear element can quench damped oscillations, which are typical for feedback systems.

The unipolar voltage-to-current converter in Fig. 1 is based on a classical connection of op-amp IC2a and NPN transistor Q1. The regulated current flows through emitter resistor R E, which acts here as a simple passive current-to-voltage converter. In the closed-loop, actual negative feedback is the voltage fed to the inverting input of IC2a. A dead-band in the V-I characteristic is created by current flowing from the source of reference voltage, VREF, through resistors R D, R E to ground. The reference voltage VREF is derived from the band-gap cell IC1, resistive divider R a, R b, and op amp IC2b.

Figure 1  Output current of the converter rises linearly for input voltage rising within the range of 20mV to 200mV, while it has zero value for V IN below 20mV.  

To evaluate this dead-band quantitatively, you can firstly assume V IN equal to zero. The op-amp tries to force V ED to zero as well. This however is not possible, as the B-E junction of Q1 acts now as a reverse-biased diode. The result is that the emitter current of Q1 is zero, and consequently the voltage-drop at resistor R E, is:

              (1)

As the same voltage appears at inverting input of the op amp, its output is saturated at zero-level.

When you start to elevate the level of V IN from zero value, the situation remains steady until VIN value exceeds that of

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