The circuit in this example converts a negative pulse into a positive pulse. Although this task seems simple, the magnitude of the negative pulse is -5V~-2V. According to different application requirements, positive pulses also require different pulse widths, while negative pulses are trapezoidal. Pulses must pass through a long distance transmission line to reach a control device. There are multiple circuits that can solve this problem, depending on the pulse amplitude and shape.
Figure 1 shows a circuit that requires only a 5V supply. It has a high trigger threshold that maximizes noise rejection. This circuit requires a large input current that is comparable to the collector current. In addition, it requires a CMOS or TTL (transistor-transistor logic) inverter for triggering at threshold voltage. If the input pulse is trapezoidal, the output pulse width is not equal to the width of the input pulse. The threshold V T- can be calculated as follows: VT-=-[(V+ -V IH)&TImes; R1/R2+0.62], where V T- is the lower voltage threshold, V+ is the supply voltage, and VIH is the 74HC132 High level input voltage. Figure 2 shows the input and output waveforms.
Figure 1. This circuit uses a single power supply and has good noise rejection, but cannot reliably convert trapezoidal pulses.
Figure 2, the output pulse width is similar to the main negative part of the input pulse
Figure 3 is a pulse shaper that converts a 3μs negative pulse into a positive pulse. The width of the output pulse is very close to the width of the input pulse. This circuit does not require a large input current and does not require an inverter. Its voltage threshold is lower than the circuit in Figure 1: V T- ≥ -0.3V, but the circuit in Figure 3 requires two supply voltages: ±5V. Figure 4 shows the waveform of the circuit of Figure 3.
Figure 3, this circuit can be used to convert the pulse polarity, but it requires dual power supplies.
Figure 4, the pulse width is a minimum of 3μs.
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