Gbwp

[zesla]

Hi there,

for an inverting configuration of LF357 with the gain of 5.6 or 15db and the unity gain freq(GBWP right?) of 10MHz the bandwidth would be 1.8MHz.

Now my question is what it tells realy? Is it telling that the gain will be constant at 5.6 from 0Hz to 1,8MHz?

Thanks

 

[Ubergeek63]

no that is telling you that the product of gain and frequency will be ... unless it is a current feedback amp.

 

[zesla]

Sorry, I did not get your meaning at all.

 

[audioguru]

At 1.8MHz, the output level will be -3dB (times 0.707) the lower frequency level.
But the output will be low above 400kHz due to slew rate limiting.
The LF357 is obsolete and is not made anymore.

 

[zesla]

At 1.8MHz, the output level will be -3dB (times 0.707) the lower frequency level.
But the output will be low above 400kHz due to slew rate limiting.
The LF357 is obsolete and is not made anymore.

Thanks audiguru,
I thought that the freq responce would be flat till 1.8MHz. so the gain will strat at 0 with 5.6 (15dB) and falls to 4 (12dB) at 1.8MHz, right? but the bellow curve does not indicate so?

What is the relationship between the GBWP and SR? Any fornula or link please?

Thanks

 

[audioguru]

so the gain will start at 0Hz with 5.6 (15dB) and falls to 4 (12dB) at 1.8MHz, right? but the below curve does not indicate so?

You are correct. But the curve is made with straight lines instead of curves.

What is the relationship between the GBWP and SR? Any formula or link please?

There is no formula since the slew rate is determined separately from the GBWP.
A lousy old 741 opamp has a GBWP of 1MHz but its slew rate causes trouble above only 9kHz.
A TL071 opamp has a GBWP of 3MHz but its output slews perfectly up to 100kHz.

 

[zesla]

You are correct. But the curve is made with straight lines instead of curves.

Is the gain assumed to be -3dB near or on the 18MHz curve but almost flat before it?
I mean before reaching to 1.8MHz do we have a rather constant 5.6 gain?

There is no formula since the slew rate is determined separately from the GBWP.
A lousy old 741 opamp has a GBWP of 1MHz but its slew rate causes trouble above only 9kHz.
A TL071 opamp has a GBWP of 3MHz but its output slews perfectly up to 100kHz.

So how could you find those 9KHz and 100KHZ for those OP-AMPs?

So How do I treat with an op-amp's GBWP and SR?


THanks.

 

[audioguru]

The response is a small amount less at 1/5th the cutoff -3dB frequency.
The 1.8MHz cutoff is at -3dB and the response is down a small amount at 360kHz.

The datasheet for every opamp lists is slew-rate limited response by showing a chart of the "Maximum Peak Output Voltage vs Frequency".

I don't use a 741 opamp for audio because it is too noisy (hiss) and its poor slew rate messes-up the highest octave of audio.

 

[crutschow]

To determine the highest frequency and amplitude you can use for a given op amp slew-rate, you calculate the maximum slew rate of a sine-wave as 2*Pi*Vp*f where Vp is the peak sine-wave amplitude and f is its frequency.

 

[zesla]

audioguru,
How could you find out that an OP-AMP like 741 has trouble above only 9kHz? experience or any formula?

Thanks

 

[audioguru]

audioguru,
How could you find out that an OP-AMP like 741 has trouble above only 9kHz? experience or any formula?

You simply look at the graph on its datasheet. It has trouble above 9kHz. Many opamps (like the TL07x) work perfectly up to 100kHz.
You can calculate the max frequency by using the slew rate spec. The max frequency is the frequency at full output swing that turns a sine-wave into a triangle wave due to a low slew rate. As the frequency gets higher then the output swing gets lower because the output cannot swing fast enough.