I am building a class D Amplifier, I have to build my circuit but, overall in the circuit, My hex inverter is not working. when I am providing a digital PWM signal from the LM393 comparator to the HEX inverter (MC74HC04AN), it should invert the PWM signal, but when I am performing,0 volts is coming in the second pin (output of hex inverter).so when I powered the circuit, Mosfet driver Low input side input voltage is showing is zero.in the High input side,5 Volt is coming. but the low input side is not coming. I have changed my hex inverter but nothing has happened.

Operational Data:-

Is there any difference between MC74HC04A and SN74HC04?

Basic Testing of Hex inverter

I am posting another two pictures which are based on basic testing of Hex inverter. in this experiment, a)- first, I provided square wave of 2.5 volts with 200 khz frequency, duty cycle 50% to the input of Hex inverter, and output was zero volts. b)- after 1st method was not worked properly, I just thought hex inverter will work as digital input 0 to 5 volt. that's why I provided the same data as above written with 2.5 volts dc offset extra to see the results. when I performed I got these results. please see the below pictures(figure 6, figure 7).

# Class D Amplifier overall practical Data

  1. TLC 555 timer generates triangle wave Approx and above 200khz with 2.14 volt.
  2. LM3393 comparator using for comparing two signals, Audio signal( Sin wave - 1 Hz to 20 khz with voltage-2 volt(max)) and Triangular wave(2.14).(input voltage common-mode range reference taking consideration not more than 3.5 volt)
  3. Hex inverter((MC74HC04AN) is used for inverting the PWM signal.
  4. Ir2113 MOSFET driver is used for high-speed switching.
  5. IrlZ44 n MOSFETs are using.

I am posting my practical data which is shown in real-time operations (oscilliscope).

  1. Schematic Diagram of class D amplifier (Figure 1)
  2. Triangle wave (TLC555 Timer). (I am posting two pictures for clarity)(Figure 2, figure 3)
  3. LM393 comparator output (i am posting two pictures for clarity). (figure 4, 5)

Afterward, hex inverter is unable to work. the reason I don't know. please give me a valuable comment.

Class D amplifier schematic figure 1 -Class D amplifier schematic

Triangle Wave Figure 2- Triangle Wave

Triangle Wave Figure 3- Triangle wave

PWM Generated by Lm393 Comparator Figure 4- PWM Generated by Lm393 Comparator

PWM Generated by Lm393 Comparator

PWM Generated by Lm393 Comparator

Basics Testing of Hex inverter Basics Testing of Hex inverter using Function Generator

Output of Hex inverter when basics Test was performing The output of Hex inverter when basics Test was performing

  • 1
    \$\begingroup\$ I fixed your wall of text for you. Use the editor buttons to create properly formatted lists. Use 2 x <Enter> for paragraph breaks. Please move your image explanations as captions below each image and number them. That way we can refer to Figure 1, etc. \$\endgroup\$
    – Transistor
    Jul 11, 2020 at 13:15
  • \$\begingroup\$ Thank you, I will improve it. \$\endgroup\$ Jul 11, 2020 at 13:17

1 Answer 1


Although there are some problems seeing images in the question at the moment, the fundamental problem you describe seems to be just with the behaviour of the 74HC04.

Therefore you should temporarily troubleshoot just that part of the circuit. Don't complicate your troubleshooting (and this question) with other parts like the 555 timer and MOSFET driver. Disconnect those parts and just troubleshoot the inverter.

There is already one big problem in your description so far. You said:

In the schematic shown, in the hex inverter rest of pin is not connected with the ground, but in reality, I have grounded every pin except 1,2,14.

Don't do that! You must not connect unused outputs of the inverters to Gnd. That risks causing excessive current in the inverters and possible damage. Only unused inputs should be fixed at a valid logic level. Unused outputs should be left unconnected.

You might have damaged the 74HC04 devices where you made those incorrect connections. In any case, as I said, you need to focus on checking the behaviour of the inverters (or at least the one inverter you are using in that package) separately from the other parts of your design.

To answer your newly added question:

Is there any difference between MC74HC04A and SN74HC04?

No significant difference in behaviour, provided you stay within normal operational parameters. They are just made by different manufacturers.

  • \$\begingroup\$ Sir, the first time when I performed this circuit, I have not connected the rest of the pin with ground , but nothing is happened , then I tried and connected the ground with the unused pins .but still nothing happened. \$\endgroup\$ Jul 11, 2020 at 13:38
  • \$\begingroup\$ I have performed Hex 74Hc04 inverter alone with the led , when I am providing a digital signal from Arduino it's was working.but when it comes to this circuit. I don't know what's happening. \$\endgroup\$ Jul 11, 2020 at 13:41
  • \$\begingroup\$ @ArijeetRoy - "nothing is happened" That's not useful, unfortunately. Much better description needed. "when I am providing a digital signal from Arduino it's was working.but when it comes to this circuit. I don't know what's happening." Again, saying "I don't know what's happening" isn't helpful. However, from this new info that (at the time of that test) the inverter you tested behaved correctly with a digital input from the Arduino, you need to carry on troubleshooting from there e.g. use just the comparator to drive the HC04 i/p instead of the Arduino, and view its (unconnected) o/p. \$\endgroup\$
    – SamGibson
    Jul 11, 2020 at 14:01
  • \$\begingroup\$ yes, I have done that also. i have connected the LM393 comparator with Hex inverter, and I was checking the output of hex inverter, but results are noisy or unpredictable. \$\endgroup\$ Jul 11, 2020 at 14:17
  • 1
    \$\begingroup\$ As I said before, I believe you need local troubleshooting help. It has taken 1 hour of comments here, just to clarify that in some places you refer to multimeter measurements, and in other places refer to scope measurements. That is just one point, and there are some other points which also need clarification. I have to stop here, as I have other things to do. Perhaps someone else will carry on with the next stage of your troubleshooting for you. But, as I said, getting local help would be much more efficient, as they are more likely to quickly see invalid tests or measurements etc. \$\endgroup\$
    – SamGibson
    Jul 11, 2020 at 15:37

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