Because digital systems often have only a single-polarity power supply, a common problem becomes translating an opposite-polarity pulse train into a positive-or negative-pulse output. Here are some simple circuits that can do the job easily and reliably.
For positive-supply systems, the circuit of Figure 1a transforms a negative pulse train into positive-going output pulses. The dual-output comparator provides both inphase and out-of-phase outputs. (If the system requires only one output phase, you can substitute an alternative single-output comparator.) Voltage at the comparator's inverting input swings between 1.8 and 3.0 V. Also, making R1 = R2 sets the noninverting input voltage to 2.5 V. As configured here, the complementary comparator outputs produce positive pulse trains.
For negative-supply systems, the circuit of Figure 1b (which is very similar to that of Figure 1a) transforms a positive pulse train into negative-going output pulses. Voltage at the comparator's inverting input ranges between 1.8 and 3.0 V, and setting R1 = R2 makes the noninverting input voltage equal to 2.5 V. In this instance, the complementary comparator outputs generate negative pulse trains.
In the circuits of Figure 2 and Figure 3, the comparator serves as a buffer, thereby providing an interface configuration for systems whose input signal and system supply voltage are of opposite polarity. The Figure 2 circuit enables a system with positive supply voltage to accept negative pulses. Similarly, the Figure 3 circuit lets a system with negative supply voltage handle positive input signals. Both circuits use npn transistors to levelshift the comparator's outputs by:
[VBE(R5 + R4)]/R5 ≈ 4.5 V
For single-phase outputs, you can choose a suitable single-output comparator.
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