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How to amplify 3-3.5V to 5V PWM signal?

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variaseng

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Hi! I am trying to amplify a 3-3.5V PWM signal to a 5V PWM signal, frequency may range from 500Hz to 5kHz. I created a schematic in simetrix using a non-inverting op-amp(LM139), the figure is shown below. Although I managed to create a PWM signal with 5V amplitude, the frequency somehow became 10kHz (from 1 kHz) and there is a triangular wave form in the Voff part of the PWM. Can you please tell me the problem here, or if there are any other ways to amplify a PWM signal more efficiently? Thanks very much! :)

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hi
A LM139 is an open collector output, it needs a pull resistor to +V

BTW: its a comparator IC not a true OPA

EDIT:
A circuit for a dual supply amp, the bandwidth is VERY limited.
 

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Thank you very much for your reply and for pointing out that LM139 is not a true op-amp! :)

I replaced LM139 with LM324 this time and put it in the same schematic of non-inverting amplifier. Since I need 5V PWM signal from 3-3.5V, I need a 1.47 gain for my amplifier. I provided 1k for R2 value and computed ~2.2k for R1 value. This should theoretically bring out the 5V amplitude but amplitude only reaches until 3.5V at most. I tried other R2 values(2k-10k) but same results. Schematic and waveform output are shown below.

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Again thank you very much for your help! :)
 
hi,

A LM324 with a +5V supply can only swing high to ~ +Vs -1.5V.

To get close to +5V swing out, you need a a 5V rail to rail OPA or use a higher +Vs supply for the LM324
 
The datasheet for the LM324 shows that it is far too slow for PWM. It is also too slow for linear audio. Its square-wave is actually a triangle-wave.
It also shows that its input current is too high for the extremely high value resistors you are using.

Your resistor value ratio is wrong. For a gain of 1.47 then R1 can be 47k and R2 can be 100k.
 
AMPLIFYING A DIGITAL SIGNAL
A Digital signal is only detected as a HIGH or LOW. However if the digital signal does not have sufficient amplitude, it may not be detected AT ALL.
This circuit detects a low amplitude signal and produces a high-amplitude signal.


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You want to use the circuit Colin provided. If you run a simulation on it you should get the attached image. The upper trace is your pulses in and the lower pulses out. Using two transistors maintains the phase relationship.

Ron
 

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Hi! Thank you very much for your replies!

I tried using colin55's suggestion and created a schematic similar to it and the output waveform turned out to be what I needed (schematic and waveform below). Instead of using BC557 (PNP), I used BC857 and instead of BC547, I used BC847 due to parts availability in the local store. I was thinking that this could be the best way since if I use amplifiers, I need to obtain the correct part that is fast enough to process PWM signals. I just want to ask if I use transistors for this, do I still need to consider if these are fast enough to process PWM signals?

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Again, thank you very much for all your suggestions and help!
 
Alright, I will keep this in mind. Thank you very much for all your suggestions and help! I will apply this in the breadboard and update if I encounter hardware problems. Again, thanks! :)
 
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Your PWM modulation (audio?) is 50hz to 5kHz, not the switching carrier. The switching carrier is at least 50kHz or more.
Your resistor values are very high so the capacitance of the transistors and the wiring will produce lowpass filters and mess up ultrasonic pulses.
Similarily, a breadboard cannot be used for high frequencies because of its high capacitance between wires and contact strips.

I used a carrier of 2kHz for the PWM switching carrier on an electric RC airplane motor speed control and it worked well but was very loud.
 
Thank you very much for this, audioguru. Also, I found out that I need to amplify 12 PWM signals so using the transistor way became somewhat inconvenient, since I need to provide 12 NPN and 12 PNP transistors. I created another schematic using a comparator, this time using TLC3704 (Quad, Micropower, Push-Pull Outputs, LinCMOS(TM) Voltage Comparator) and the results are as shown below. Although I have acquired the needed waveforms, I would like to know if there are unseen problems if I do it this way.

Also, this comparator is a bit hard to acquire so I was thinking of replacing it with AD790(Fast precision comparator 45nS soic8 ) or TL3116(Ultra-fast comparator) which are readily available. But I am clueless on how to simulate this in SIMetrix Intro since it is not on the default model library so I have no idea if this will output the same waveforms. 

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Thank you all very much for your help and patience! :)


  
 
 


 
A SIM program uses "typical" spec's. Then in your circuit only typical and better ICs will work and ICs that have spec's less than typical might not work.
I always design a circuit so that minimum spec ICs will work then ALL ICs will work perfectly.

We don't know what your circuit will do. Is it a motor speed control or a heater control? Or is it for a class-D switching amplifier where its switching frequency is very high? For a switching power supply?
 
I am trying to produce 5V PWM signals to feed to motor drivers. These PWM signals originally come from a PIC microcontroller, which typically produces 5V PWM signals, which is what I need. But I need to feed them through a 74LS245 transceiver to the drivers because I want to be able to disable PWM feeding whenever I enable the "E" pin of the transceiver. But the transceiver reduces the 5V PWM amplitude to at most 3.4V, which is not enough. So I need to amplify these PWM signals from the transceiver. (BTW, I'm using the transceiver here as a buffer)

Thanks very much! :)
 
What load will the 5V signals have? The output level from the amplifier will drop when it is loaded.
 
You may want to look at some various darlington array ICs. Along the lines of the ULN2003 or ULQ2003 family. There are no shortage of darlington array drivers with beefy current output. The ULN2003 for example has an on voltage of IC = 200 mA 2.4 volts and 300 mA is 3.0 volts.

Ron
 
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I am trying to produce 5V PWM signals to feed to motor drivers. These PWM signals originally come from a PIC microcontroller, which typically produces 5V PWM signals, which is what I need. But I need to feed them through a 74LS245 transceiver to the drivers because I want to be able to disable PWM feeding whenever I enable the "E" pin of the transceiver. But the transceiver reduces the 5V PWM amplitude to at most 3.4V, which is not enough. So I need to amplify these PWM signals from the transceiver. (BTW, I'm using the transceiver here as a buffer)

Thanks very much! :)
This is a bit confusing. The 74LS family of chips operate at 5V but why do you need it ?
Have the PIC sense/read whatever you are now using to enable the 74LS245. Have the PIC start and stop the PWM as needed.
 
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