opamp roll-off

[audioguru2]

The 741 opamp and most other opamps have a single RC filter that creates a rolloff at 6dB/octave whhich is 20dB/decade.
Depending on its location in the opamp the rolloff capacitor affects the max slew rate.
The datasheet for the 741 opamp shows that its max slew rate occurs when its output is 28V p-p and the frequency is only 9kHz. Then any input waveform begins to appear like a triangle waveform with ramps. The max p-p output above 9kHz for a 741 opamp is reduced in level as shown on a graph in its datasheet because its output cannot slew (ramp) fast enough.

The 741 opamp is a 42 years old antique. A modern "general purpose" opamp costs the same and has much better performance.

 

[indulis]

A 20db/decade roll-off of an opamp approaches the limitation of the device. You extend the bandwidth of an opamp by reducing gain.

KevinIV, where do you get this stuff ... do you just make it up as you go along??? Get a good electronics book, read it, study it and get a good grasp of the fundamentals.

You "claim" you "... extend the bandwidth of an opamp by reducing gain". Now ponder the difference between extending the "usable bandwidth" of your circuit by reducing the gain and the op-amps actual bandwidth. I know I shouldn't ask, but will... do you think you have changed the opamps bandwith from what it was with reduced gain??

 

[indulis]

I was thinking I should have asked that no one give him any hints or the answer

 

[Hero999]

I was thinking I should have asked that no one give him any hints or the answer… oh well.

I've deleted my previous post, he hasn't logged on since so hopefully he hasn't seen it, unless he sneaked on as a guest.

 

[Kevin Weddle]

The max p-p output above 9kHz for a 741 opamp is reduced in level as shown on a graph in its datasheet because its output cannot slew (ramp) fast enough.

I was thinking the loss was due to voltage loss at the base emitter junctions because of capacitance. And that the slew rate specification guarantees low signal distortion.

 

[Kevin Weddle]

I was thinking I should have asked that no one give him any hints or the answer

 

[audioguru2]

I was thinking the loss was due to voltage loss at the base emitter junctions because of capacitance. And that the slew rate specification guarantees low signal distortion.

The base-collector capacitance reduces the gain of a transistor much more than the base-emitter capacitance because base-collector capacitance (Miller capacitance) is negative feedback.

The loss of output at high frequencies is because the output cannot slew fast enough. Instead of following the waveform at the input, the output has ramps. The ramps are slow so the output level at high frequencies is reduced.

The slew rate limit causes every high frequency waveform to become triangle waves. Then the distortion is extremely high.

 

[Kevin Weddle]

Do low frequency opamps have any advantage over high frequency opamps?

 

[audioguru2]

Low frequency opamps like the LM358 dual and LM324 quad use low power supply current which causes poor performance at higher frequencies. Their low current also causes 3% crossover distortion.

I have never used a high frequency opamp. Most ordinary audio opamps perform very well up to 100kHz.

 

[Hero999]

I'll post what I did before but deleted.

Here's how the roll-off limits the high frequency gain and how a reduction in closed loop gain and increased negative feedback, increases the bandwidth.

Going back to the graph posted by audioguru.
http://www.electronics-lab.com/forum/index.php?topic=19813.0

If the gain is set to 1000, the 6dB bandwith will be 1kHz.

If the gain is reduced to 100, the 6dB bandwidth will be increased to 10kHz.

Of course the bandwidth of actual op-amp remains the same, it's the bandwidth of the circuit which increase as more negative feedback is applied.

The obviously doesn't make any difference to the slew rate which is a limitation of the op-amp's ability to change from one voltage to another quickly so the 741 is still limited to <8kHz in most applications.

Do low frequency op-amps have any advantages?

Lower frequency op-amps are more stable than high frequency op-amps and are cheaper too.

 

[gogo2520]

Hey guys
Is KevinIV a BOT? he sure  dose comes up with some systematic question and answers.
                                                gogo

 

[audioguru2]

According to the occasional PM's I get from Kevin, he is a human living in Texas.
Maybe he didn't learn electronics in University like some of us.

 

[Kevin Weddle]

Lower frequency op-amps are more stable than high frequency op-amps and are cheaper too.

Low frequency opamps are more suitable for low frequencies than high frequency opamps? I'm not so sure because opamps have a bandwidth that extends to 0Hz. A high frequency opamp would maybe incorporate newer technology bipolar transistors, and maybe extra transistor amplifier stages to make up for loss.