This circuit breaks an
input voltage signal down into its components: (1) the absolute value
and (2) the polarity or ‘sign’ (+ or –). It will handle direct input
voltages as well as alternating voltages up to several kHz. With a
supply voltage of ±9 V, the input level should remain below ±6V. The
circuit consists of two sections, each having its own function.
Operational amplifiers IC1a and IC1b form a full-wave rectifier, its
output terminal supplying the absolute value of the input signal, while
operational amplifiers IC1c and IC1d examine the polarity of the input
voltage. For negative input voltages, the output of IC1a goes high.
Consequently D2 is reverse-biased so that IC1a has no effect on the
rest of the circuit. IC1b then acts as an inverter because its
amplification is (–R5/R3) or –1. Thus, the output voltage is positive.
For positive input voltages, D2 conducts and the amplification of IC1a
is -1. The output voltage is then determined by the sum of currents that
flow through R3 and R4. Taking into account the polarities and the
value of all resistors, the overall amplification becomes
–R5/R3 + (–R5/R4) ↔ (–R2/R1) = –1 + 2 = 1
This means that the value of the output voltage at the output
terminal is the same as the input voltage, but the polarity is always
positive. The accuracy of the rectification process is determined by the
accuracy of resistors R1-R4; close-tolerance (1%) types are
recommended. At low input voltages (smaller than 20 mV), the input
offset voltage of the operational amplifiers may introduce significant
errors. If this is the case, use individual operational amplifiers
instead of an array of four (TL061, TLC271, AD548, …), because they have
pins for offset voltage compensation. Alternatively, use an operational
amplifier with a low offset voltage like the OP07. In the polarity
detector IC1c acts as a comparator, with a certain amount of positive
feedback due to R7 and R8.
This feedback causes a hysteresis of 20mV that prevents oscillation
when the input voltage changes slowly. IC1d is an ordinary inverter. For
input voltages above 10mV, the SIGN output terminal will swing to almost the positive supply. When the input voltage is below –10mV, the SIGN
terminal drops low, almost to the negative supply voltage. For input
voltages between these two thresholds, the output voltage is well
defined, too, because it stays at its previous level. This circuit is
the perfect complement to the ‘+/– voltage on bargraph display’ circuit
discussed elsewhere in this blog.
The |Uin| and SIGN outputs of the present circuit may be directly connected to Uin and CONTROL
IN inputs of the bi-directional bar display. The ±6 V sign indicator
signal may be used as the control voltage for the +/– voltage display as
long as the reference voltage remains smaller than 3 V. Although
presented as a pair, both circuits may of course be used individually
for other purposes.