# Produce high frequency square wave

Joined Sep 28, 2021
266

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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#### AlbertHall

Joined Jun 4, 2014
12,385
Reduce C1, so R1 is 10k+.
Use a modern chip, not the bad, old, slow '741.

#### Irving

Joined Jan 30, 2016
3,983
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.

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#### MrChips

Joined Oct 2, 2009
30,948
Why are you using an op-amp to produce a square wave?
There are other ways to do this.

#### ericgibbs

Joined Jan 29, 2010
18,997
hi Mrs,
the Slew Rate of 0.5V/uSec is one limiting factor.
E

Added a plot to show the effect of Slew.

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#### ronsimpson

Joined Oct 7, 2019
3,144
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.

Joined Sep 28, 2021
266
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?

Joined Sep 28, 2021
266
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?

#### MrChips

Joined Oct 2, 2009
30,948
LMC555 timer IC can output square wave up to 3MHz.

#### Irving

Joined Jan 30, 2016
3,983
Hi, what op-amp should i use at high frequency then?
What frequency do you want?

Joined Sep 28, 2021
266
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.

#### crutschow

Joined Mar 14, 2008
34,689
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..

#### Irving

Joined Jan 30, 2016
3,983
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.

Joined Sep 28, 2021
266
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
4,006
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?

Joined Sep 28, 2021
266
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

Joined Sep 28, 2021
266
Hmmm...I'm curious.
At first you wanted 100kHz, now 1Mhz or greater.
What are you building that requires Mhz clock speeds?
100kHz sorry. Planning to build an oscillator for inverter

#### Irving

Joined Jan 30, 2016
3,983
Why? Use a proper inverter chip with an internal oscillator and save yourself some pain... Or generate digitally with a microcontroller...

#### eetech00

Joined Jun 8, 2013
4,006
100kHz sorry. Planning to build an oscillator for inverter
Oh...ok

What duty cycle? Adjustable duty? Supply voltage? Is it a master clock?