Produce high frequency square wave

Thread Starter

MrsssSu

Joined Sep 28, 2021
181
1639056867204.png
Hi readers:),

When decreasing R1 value, output frequency will increase. However, I get bad waveform when high frequency. How do I get a good square wave (100kHz) using this LM 741 chip that I bought?

Simulation is attached below for your reference and modification to the answer :)
 

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Irving

Joined Jan 30, 2016
2,526
Define 'high frequency'

The 741 is an old design and has a very poor 'slew rate' - the speed it can change its output - of about 0.5v/uS so it isn't capable of anything very high frequency.

Also as noted R1 is much too small, its loading the output badly. Use a larger value.
 
Last edited:

ronsimpson

Joined Oct 7, 2019
1,996
What output voltage do you want? Example 0 & 5V?
Frequency=100khz
What are you doing with the square wave?

The LM741:
The output does not pull to the supplies. For a + and - 10V supply will not get you a +/-10V signal.
The output moves slow. 0.5V in 1uS. or 5V in 10uS.
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
181
Define 'high frequency'

The 741 is an old design and has a very poor 'slew rate' - the speed it can change its output - of about 0.5v/uS so it isn't capable of anything very high frequency.

Also as noted R1 is much too small, its loading the output badly. Use a larger value.
Hi, what op-amp should i use at high frequency then?
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
181
What output voltage do you want? Example 0 & 5V?
Frequency=100khz
What are you doing with the square wave?

The LM741:
The output does not pull to the supplies. For a + and - 10V supply will not get you a +/-10V signal.
The output moves slow. 0.5V in 1uS. or 5V in 10uS.
Hi, what op-amp should i use at high frequency then?
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
181
What frequency do you want?
ah as high as modern op-amp can get (Higher than 1MHz. I found one LT1357 which has 25MHz bandwidth. Are there any other much better than this? I seriously thought LM741 could do the job since it is used in high frequency radio. Too bad, I will have to buy a new one.
:)
 

Irving

Joined Jan 30, 2016
2,526
ah as high as modern op-amp can get (Higher than 1MHz. I found one LT1357 which has 25MHz bandwidth. Are there any other much better than this? I seriously thought LM741 could do the job since it is used in high frequency radio. Too bad, I will have to buy a new one.
:)
It might have been used in a HF radio once, for something slow like automatic gain control but you never see them these days in any new design.

While a comparator may be a better option than an op-amp for this purpose, the reality is that an RC-network oscillator like this is a poor choice for most purposes and certainly isn't generally appropriate for anything in the MHz range. What are you planning to drive with it? And do you really need a 10v output swing?

The LT1357 will give you about 1.5MHz max but its not a pretty waveform, and even a LT1724 (200MHz) will only go to 1.7MHz because an RC-network has other limitations. As suggested above a 555 based oscillator will touch 3MHz, but above that you need a more sophisticated approach.
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
181
Use a comparator (e.g. LM339 or LM393) not an op amp.
Op amps are a poor choice to generate high frequency square-waves, especially the old and slow 741..
Hi, thanks again. So, basically for high frequency operation, a comparator must be used. Silly me to think that Op-amp can be used in high frequency operation, however it can be used in low frequency. Other than LM339 or LM393 you suggested, are there any other than is better at high frequency than those because I would like to buy a good comparator?
 

eetech00

Joined Jun 8, 2013
3,087
ah as high as modern op-amp can get (Higher than 1MHz. I found one LT1357 which has 25MHz bandwidth. Are there any other much better than this? I seriously thought LM741 could do the job since it is used in high frequency radio. Too bad, I will have to buy a new one.
:)
Hmmm...I'm curious.
At first you wanted 100kHz, now 1Mhz or greater.
What are you building that requires Mhz clock speeds?
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
181
It might have been used in a HF radio once, for something slow like automatic gain control but you never see them these days in any new design.

While a comparator may be a better option than an op-amp for this purpose, the reality is that an RC-network oscillator like this is a poor choice for most purposes and certainly isn't generally appropriate for anything in the MHz range. What are you planning to drive with it? And do you really need a 10v output swing?

The LT1357 will give you about 1.5MHz max but its not a pretty waveform, and even a LT1724 (200MHz) will only go to 1.7MHz because an RC-network has other limitations. As suggested above a 555 based oscillator will touch 3MHz, but above that you need a more sophisticated approach.
Planning for an oscillator circuit for an inverter (100kHz) I suppose :)
 

Irving

Joined Jan 30, 2016
2,526
Why? Use a proper inverter chip with an internal oscillator and save yourself some pain... Or generate digitally with a microcontroller...
 
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