Voltage-to-current converter offers precisely defined dead-band : Page 2 of 3

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.
V ED, given by Equation 1. At that instant the output voltage of op-amp IC2a becomes positive, causing a current to flow through the emitter of Q1. The higher the value of V IN is, the higher is the emitter current of Q1. Due to a strong negative feedback, the dependence I(V IN) is linear for V IN within the V ED to V REF range. To evaluate the output current at full-scale input voltage, V IN = V REF, you can realize that the R D resistor has the same voltage, VREF, at both its ends, so no current flows through it. The equivalent emitter resistor at the full-scale output current is therefore R E itself, and the value of emitter full-scale current is V REF/ R E.

The output current flowing across the collector of Q1 and positive supply terminal, is very close to emitter current:

  where β is the current gain of Q1. The V-I characteristic of the converter is drawn in Fig. 2.

Figure 2

For the 2N3904 transistor, β ≈ 230; thus the collector current is about 0.44% lower than its emitter current. To lower this error, you could use a Darlington pair, or a cascade of two bipolar transistors, in place of Q1. The input voltage, V IN, can be derived either from potentiometer P1 or an external source.

If you choose, for example V DB = 0.1×V REF, V DB = V ED you get from Equation 1 that R D = 9R E. A theoretical dependence of output current on input voltage is depicted in Fig. 2.

Experimentally, V REF = 0.19645V and full-scale emitter voltage VED = 0.19660V have been measured. The V DB has been determined by detecting an abrupt change at the output of IC2a of either zero-to-positive, or positive-to-zero voltage transitions, and by measuring the VIN value

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