How can I use a LM741 as an Inverting pulse amp?

Thread Starter

slavano

Joined May 10, 2016
7
Hello,

I am trying to set up a circuit using a LM741CN op amp to take in an 20mv negative pulse and amplify it to be a 2.5v positive pulse so that I can have it be read by a digital pin on an arduino Due. The pulse frequency is expected to range from 100 000HZ-800 000HZ. How should I set up my circuit??? The most I have been able to do is increase the pulse, without inverting it, to 6.4v using diagrams online.

Thank you in advance
 

crutschow

Joined Mar 14, 2008
24,130
A 741 is too slow to amplify such a high frequency pulse.
You need a faster amp. Where do you buy your electronic parts?
Does the pulse go for 0V to 20mV negative?
 

Thread Starter

slavano

Joined May 10, 2016
7
I would still like to try it because our initial results of for the frequency was inconclusive.

our pulse ranges from -15mV to 35mV from a baseline of 0v.

we bought this part from digikey.
 

Papabravo

Joined Feb 24, 2006
12,774
I would still like to try it because our initial results of for the frequency was inconclusive.

our pules ranges from -15mV to 35mV from a baseline of 0v.

we bought this part from digikey.
What you don't know is that the 741 has problems with frequencies above 10 kHz. It is bandwidth and slew rate limited; Don't waste your time with this ancient relic; do yourself a favor and bin those parts.
 

#12

Joined Nov 30, 2010
18,186
I would still like to try it because our initial results of for the frequency was inconclusive.
You can't turn a mule into a race horse, but you can try if you want to. The problem is, it will never win a race (provide a useful output).
 

hp1729

Joined Nov 23, 2015
2,304
Hello,

I am trying to set up a circuit using a LM741CN op amp to take in an 20mv negative pulse and amplify it to be a 2.5v positive pulse so that I can have it be read by a digital pin on an arduino Due. The pulse frequency is expected to range from 100 000HZ-800 000HZ. How should I set up my circuit??? The most I have been able to do is increase the pulse, without inverting it, to 6.4v using diagrams online.

Thank you in advance
It is a question of available gain at what frequency. You can get that gain at about 10 Hz, but at 800 KHz it drops to a gain of 1.
Yes, slew rate just isn't there.

(edited to add ...)
I can't find an op amp that will get you there in one stage.
 
Last edited:

ErnieM

Joined Apr 24, 2011
8,011
You are looking to create an amp with a gain bandwidth product of 10^8.

Taint gonna get that in no 741.

One would be lucky to get it using two cascaded amps.
 

AnalogKid

Joined Aug 1, 2013
8,259
A gain of 125 (42 dB) and bandwidth of 800 kHz is a gain-bandwidth product of 100 million. That's a lot.

The 741 will not work for this application. The open loop gain is way to low, but you could overcome that with several amplification stages in series. The problem is that to recreate an 800 kHz pulse you need a very fast output stage risetime, and the 741 just plain isn't that fast.

First, determine the minimum slew rate you can tolerate in the output pulse shape. Based on that' select an opamp. A standard opamp gain stage needs at least 20 dB of negative feedback *at the highest frequency of interest* to stabilize the stage gain, frequency response, and output impedance. You might have to have two or three gain stages in series to achieve this. For example, if you have three gain stages in series, each needs to make 14 dB of forward gain at 2.4 MHz (800 kHz plus the third harmonic to improve risetime). With some gain headroom for negative feedback, that means each opamp part must have an open loop gain of at least 34 dB. Still a lot. At 1 MHz, an NE5534 (a much faster and quieter opamp) has an open loop gain of around 26 dB, still not enough. Consider a current-feedback opamp or high-speed comparator.

ak
 
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