About Op Amp circuit signal amplification

[foxjuly]

Hi guys,
   I  built a circuit using LM741 to amplify the signal from a computer fan. The circuit map and the result from the oscilliscope is attached as a pdf file. Yellow line indicate the input signal, while blue line is the output signal. So, my question is that
    1.  why the input signal is affected as I  adjusted R3 from 0 to 10 kOhm?
    2. why the output/input ratio gradually increase and then decrease as R3 is increased from 0 to 10 kOhms?
Thanks a lot!
Best!
Jiang
   

[Audioguru]

The design of a lousy LM741 is 50 years old. Its inputs do not work when they are anywhere near 0V in your circuit or if they are anywhere near the supply voltage. The output cannot go down to 0V and cannot go up to the supply voltage.

When the variable resistor is 0 ohms then the input voltage of the opamp is too close to ground so it does not work. The signal passes through the resistors and the opamp does not invert, it does nothing.
When the variable resistor is 10k ohms then the output of the opamp clips because it cannot go any lower.

[foxjuly]

When variable resistor goes to 10K, the Vin+ is 4.5 volts, so why the output signal is significantly lower than the input?

[foxjuly]

Also, I am so puzzled that when the variable resistor goes from ~200 ohms to 1 kohms, the input signal seems to be distorted and when variable resistor goes from 1 k to 10 k, the input signal just got amplified and reached the maximum to become a square wave. So what is the underlying mechanism?

[danadak]

As stated previously. The 741 can only tolerate a CM range at its inputs of
+/- 12 on a +/- 15 V supply. Depending on manufacturer.

So thats telling you the inputs have to be

Vpossupply - 3 >= Vin >= Vnegsupply + 3   or   6 >= Vin >= 3 in your case

The output has similar constraints for light load.


A calculator for an ideal opamp that CM to both rails on inputs and outputs -


https://masteringelectronicsdesign.com/summing-amplifier-calculator-java/


Biasing for single supply operation -

http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sboa059&fileType=pdf


Also the datasheets characterize the 741 operation at +/- 15V. Your 9V supply might be
too low to operate the 741.


Suggest you look into a RRIO OpAmp capable device.

http://www.ti.com/amplifier-circuit/op-amps/general-purpose/products.html#p480=2;2&p22typ=0.0015;5&p78=In;Out

https://m.eet.com/media/1138867/18049-241228.pdf



Regards, Dana.

[David Hess]

A 741 will work fine on 9 volts as long as you stay within its input and output voltage limits.  Despite the age of its design, it is still a fine operational amplifier.

For general purpose DC applications, this often means including a second operational amplifier to use as a rail splitter to produce a centered ground.

[danadak]

Searching web I see a common reference that 741 min operating voltage is
+/- 4.5 volts, so 9V battery might not yield a very long discharge operating
cycle for design.

And we have the fact that today's datasheet no characterization data on
other than +/- 15 V supply. Like especially CM range allowed at low voltage.

My vote is work with a better "todays" opamp. Preferably RRIO.


Regards, Dana.

[David Hess]

The 741's bias circuit is just 39k of resistance in series with 2 Vbe drops so it would normally only be specified for operation at +/-15 volts however the open loop gain versus supply voltage graphs show operation down to +/-2 volts at which point the open loop gain is 1/6th of its normal value and bandwidth and slew rate will be something like 100kHz and 0.05V/us instead of 1MHz and 0.5V/us.  The normal minimum would be +/-5 volts giving at least a +/-2.5 volt input and output range.

The input range is Vcc-1 to Vee+1.5 volts.  The output range is Vcc-1 to Vee+1.5 volts.  The output range could be improved in one direction with a pull-up or pull-down resistor or current source connected to the output like is commonly done with the 358/324.

The first battery charge and discharge current monitor that I made used a 9 volt battery for power and a pair of 741s with one acting as a rail splitter.  The output voltage design range was +/-2 volts to drive the 0 to 50uA current meter.  It worked but the input precision of the 741 was not quite up to what was required due to high input bias current and drift so I replaced it with a 308.

The contemporary 301A only drops to 1/3rd of its gain and had much less of a bandwidth and slew rate loss because of its superior bias circuit but the 741 became the standard because of its built in compensation.

[danadak]

Thanks David, definitely learned something from your comments, eg. the Aol vs Supply.
Had no idea 741 would do anything at such low voltages. And looking at internal bias
I see max of 2 Vbes + some amount of Vcb to keep devices active region worst case turnoff........


Regards, Dana.

[David Hess]

Thanks David, definitely learned something from your comments, eg. the Aol vs Supply.

Most of this is from the bias current generator inside the 741 producing a proportionally lower current at lower supply voltages.  I used the 301A as an example where the bias current generator operates as a constant current source making its characteristics more independent of supply voltage and that is common with later designs.  Additional slowdown comes from increased junction capacitances at lower voltages but there is not anything to be done about that other than using a different part.

Had no idea 741 would do anything at such low voltages. And looking at internal bias
I see max of 2 Vbes + some amount of Vcb to keep devices active region worst case turnoff........

Exactly, adding up the Vbe drops and Vce drops to stay out of saturation gives a good idea of the input and output voltage limits.  Conveniently for the 741, the low bias currents produced by low supply voltages also lower the saturation voltages.

As danadak pointed out, there are much better parts for low voltage operation but the 741 will certainly work below 9 volts.  In the past, a lot of applications used the 741 or 301A with a -3 to -5 volt negative supply and +12 to +24 or even higher positive supply for easy operation down to ground.  For 9 volt battery applications with a rail splitter to make +/-4.5 volts, an operational amplifier other than the 741 or 301A would often be used for lower current operation to preserve battery life.  An example of a part like this from the time of the 741 is the 776 which might be considered an improved 741 specified to operate down to +/-1.2 volts with a programmable supply current.  Whereas a 741 would draw 500uA at +/-5 volts, a 776 could be configured to draw 1uA or less at any supply voltage.  A modern part like that would be something like an LT1006 which also happens to be a single supply operational amplifier.

[foxjuly]

Hey guys, thanks a lot for your discussion and suggestions. Even though I could not understand most of what you are talking about, I manage to solve the problem using a DM7414 Schmitt Trigger.