Design and implementation of circuit to convert negative pulse into positive pulse

The circuit in this example converts a negative pulse into a positive pulse. Although this task may seem simple, the amplitude of the negative pulse is -5V~-2V. The positive pulse also requires different pulse widths according to different application requirements, while the negative pulse is trapezoidal. The pulse must first pass through a long transmission line before reaching a control device. There are several circuits that can solve this problem, depending on the pulse amplitude and shape.

Figure 1 shows a circuit that requires only a 5V power supply . It has a high trigger threshold to maximize noise immunity. 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 the threshold voltage. If the input pulse is trapezoidal, the output pulse width is not equal to the input pulse width. The threshold V T- can be calculated as follows: VT-=-[(V+ -VIH)×R1/R2+0.62], where V T- is the lower voltage threshold, V+ is the power supply voltage, and VIH is the high-level input voltage of the 74HC132. 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 approximately equal to the main negative portion 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 power supply voltages: ±5V. Figure 4 shows the waveform of the circuit in Figure 3.

Figure 3. This circuit can be used to invert the pulse polarity, but it requires dual power supplies.

Figure 4, the minimum pulse width is 3μs.

The circuit in Figure 5 goes a step further. It uses an inexpensive LM211 or LM311 IC comparator to produce a positive output pulse that is exactly equal to the width of the input pulse, but the level is adjustable. Resistors R3 and R4 set the comparator's threshold voltage, but it depends on the voltage of the negative supply. The threshold voltage can be calculated using the equation VT- = [V-/(R2+R4)] × R4, where V- is the negative supply voltage. Figure 6 shows the waveform of the circuit.

Figure 5. A comparator in the circuit produces a precise pulse width.

Figure 6, the width of the positive output pulse is essentially equal to the width of the negative input pulse.

If the pulse width is 2μs or longer, the inexpensive LM211 comparator can be used. Otherwise, a high-speed comparator should be used. This eliminates the need for an additional output resistor, R1. The reason why the LM211 requires this resistor is that the IC is an open collector circuit. This circuit requires two supply voltages.

The circuit in Figure 7 converts negative polarity pulses into positive pulses, and the output does not depend on the amplitude of the input pulse. This version uses a single power supply and a 555 timer. It produces a positive output pulse of the required width. Resistors R1 and R3 establish a saturation threshold. This threshold can be calculated as VT-=V+/3×(1-2R3/R1), where V+ is the supply voltage of the 555 timer. Resistor R2 and capacitor C1 set the pulse width. The equation t=1.1R2C2 calculates the duration of the output high state. For the circuit to work properly, the saturation pulse must be shorter than the required pulse width, and the pulse period must be greater than t. Resistor R3 must be at least 1.5kΩ. Resistor R4 is optional.

Figure 7 This pulse converter uses a 555 timer

Compared with the circuits in Figures 1, 3, and 5, the circuit in Figure 7 operates on low resistance loads, and the output can source or sink currents up to 200mA, or can operate under large capacitive loads. The circuit does not require additional inverters or drivers . Resistor R5 is used to protect the IC from short circuits at the output.

Figure 8 shows the waveform of the circuit.

Figure 8 The 555 timer in Figure 7 generates a positive pulse of the required pulse width