Hi All, I have been tinkering with op-amp voltage followers, adding DC offset and the like, experimenting to eventually try making a function generator, just for fun and to learn.
The datasheets all have a "gain bandwidth" but my tests always show distortions way below the advertised bandwidth.
Here is how I test:
* Used my scope's function generator to output a sine wave.
* Configured the op-amp as a voltage follower (buffer): direct connection between the output and the inverted input, function generator signal connected to the non-inverting input.
* I used +12V/-12V for the op-amp supply
* compared on my scope the wave at the output with the original wave from the generator.
* Increased the frequency until I saw distortions of the output signal.
* I also changed the amplitude of the original signal to see if it has any impact. It does, a lower amplitude signal, like less than 1V peak-to-peak, fares better at high frequencies, which makes sense.
I tried first with an LM741CN, and I started seeing distortions at about 50kHz
I then used an LF356N which has a "wide gain bandwidth" of supposedly 2MHz, but I could not get above 100KHz-150KHz before getting distortions.
Here are my questions:
-Why are the distortions starting well below the advertised "gain bandwidth" on the datasheet? Most likely I am not interpreting this data correctly. Is there any other parameter on the datasheet that are more meaningful ?
-Is there any affordable (couple bucks) op-amp which would give reasonably good results at up to 1MHz ? I mean to say that a 1MHz sine wave signal at the input would be output without any noticeable distortion.
Thank you very much for your insights!
Mike
Hi there,
Op amps have two important specifications when looking for reasonable bandwidth. That is because the "power bandwidth" is dependent on the output level as well as the gain bandwidth factor. This means that you can not say that a particular op amp does not work at some frequency like 50kHz because that's not enough information. We also need to know the output level. Assuming a sine wave for testing, that means we need to know the peak of the output. For example, a 1MHz op amp may work fine at 20kHz with only 1 volt peak output sine wave, but at 10 volts peak output sine wave it may become very very distorted. The reason is that the gain bandwidth factor is really like a small signal specification, and internally the op amp output stage has a limited response time usually referred to as the "slew rate".
The slew rate is the ramp time for the output to reach a certain level given a certain input condition that would cause the output to go higher than it was before the input was applied. For example, the LM358 is often specified as 0.5 microseconds per volt. Notice the units of volts are in that. That means that if the input is such that it should force the output to go up by one volt, it would take 0.5 microseconds to reach that point. Now you know a sine wave is curved, so if the slew rate isnt fast enough, the output can not follow the sine wave. That causes a very ugly looking output even with a really clean sine wave input. Obviously you dont want that.
The trick is to calculate the maximum output peak for a sine wave given a certain frequency and the gain bandwidth and the slew rate. You probably already know that we use GBW to estimate like so:
fmax=GBW/Gain
That's easy enough, but we also need to look at the slew rate.
Since the slew rate is taken to be a ramp, we can compare a ramp to the maximum rate of change of the rise of the test sine wave. It just so happens that the maximum rate of change is located at zero volts, so it's when the sine wave crosses zero.
To calculate that we can start with a general sine wave:
v=A*sin(w*t)
where A is the peak and w=2*pi*frequency and t is time and v is the voltage of the sine at any time t.
To find the slope which is the rate of change of voltage, we take the derivative of that and get:
dv/dt=A*w*cos(w*t)
Now since the max slope is when the sine wave goes through zero, we set t=0 and we get:
dv/dt=A*w
Writing this out, we have:
dv/dt=A*2*pi*f
and so on the right is the slew rate of a sine wave at the zero crossing.
Now we can solve for f:
f=(dv/dt)/(A*2*pi)
and with dv/dt being the slew rate we can replace that with the simple "sr" and get:
f=sr/(A*2*pi)
and this is with sr in volts per second.
Converting 0.5 volts per microsecond to volts per second, we multiply by 1e6 and get:
sr=0.5*1e6
and inserting that into the formula for f we get:
f=0.5e6/(A*2*pi)
and since A=3 as above we get:
f=0.5e6/(3*2*pi)=0.5e6/(6*pi)
which of course comes out to:
f=26525.82384864922 Hertz.
So we see that the maximum frequency of operation for that op amp is about 26kHz when the output is 3 volts peak. Of course if we decrease the max output peak we get a higher frequency, and if we increase the max output we get a lower frequency.
That's how the slew rate fits into the calculation for the max frequency of an op amp and you can always solve for the slew rate 'sr' given the frequency you intend to use, then look for an op amp that has that slew rate or better.