What is the important of Gain Bandwidth of op amp

Thread Starter

alifaqil25

Joined Nov 17, 2019
2
Hello, I am required to design an active band pass filter and I needs to design it at high frequency around FM Radio frequency (>80MHz). Do the GBW of op amp needs to be selected accordingly to the required frequency or it does not matter? And what is GBW indicates actually?
 

Ian0

Joined Aug 7, 2020
1,305
It's the gain multiplied by the bandwidth.
If you have 10MHz GBW, then you can get a gain of 10 at 1MHz bandwidth or a gain of 1000 at 10kHz bandwidth.
It's very important in filters. A Sallen-and-Key structure requires a lot more of it than does a MFB.
 

LvW

Joined Jun 13, 2013
1,187
It's very important in filters. A Sallen-and-Key structure requires a lot more of it than does a MFB.
Sorry - that is not the case. The MFB topology requires - in theory - an infinite gain (sometimes this filter is called "Infinite-gain Filter"). There is a technical parameter called "active sensitivity" - that is the sensitivity of a transfer function against gain errors. This sensitivity figure for an MFB-structure is worse (higher) than for the S&K topology.
On the other hand - S&K filters have higher PASSIVE sensitivities (against parts tolerances).
As we can see - as always in electronics - everything is a trade-off between conflicting effects.
 

DickCappels

Joined Aug 21, 2008
6,972
There was a time when active filters were very rare because to get much bandwidth we had to make discreet transistor amplifiers. At that time we made complicated LC filters using reference books like Handbook of Filter Synthesis by Zverev for guidance. This book was once very expensive and out of my reach while raising a family. Now there is a pdf version free.
 

Ian0

Joined Aug 7, 2020
1,305
Sorry - that is not the case. The MFB topology requires - in theory - an infinite gain (sometimes this filter is called "Infinite-gain Filter"). There is a technical parameter called "active sensitivity" - that is the sensitivity of a transfer function against gain errors. This sensitivity figure for an MFB-structure is worse (higher) than for the S&K topology.
On the other hand - S&K filters have higher PASSIVE sensitivities (against parts tolerances).
As we can see - as always in electronics - everything is a trade-off between conflicting effects.
You're right - I was thinking of Biquads and State-Variables not MFBs
 

Papabravo

Joined Feb 24, 2006
15,127
Hello, I am required to design an active band pass filter and I needs to design it at high frequency around FM Radio frequency (>80MHz). Do the GBW of op amp needs to be selected accordingly to the required frequency or it does not matter? And what is GBW indicates actually?
To work in the FM band, 88-108 MHz., I would be looking for an opamp with a GBW of at least 1 GHz. as a starting point. I have experimented with the OPA847, but it is kinda like grabbing a tiger by the tail.
 

ronsimpson

Joined Oct 7, 2019
1,132
An a almost related subject; there is a slew rate specification on op amps that is important at high speed.
Example: 1V/uS is the fastest some amps can move the output. So with a small signal they can run faster than with a large signal.
1610202349957.png
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1610202433690.png
 

crutschow

Joined Mar 14, 2008
26,422
Since the Sallen-Key filter typically uses a unity gain op amp, it is possible to use a high frequency emitter or source follower circuit in place of the op amp.
The filter parameters would need to be adjusted slightly for the gain being slightly less than one, and its offset voltage must be allowed for.

Below is the LTspice comparison of a 10kHz 2-pole Sallen-Key filter with an op amp and with a transistor voltage follower.
Note that the value of the feedback capacitor was slightly changed (empirically) to get the same rolloff characteristics.
Also note that the follower gain is less than 1 in the passband.

Of course the follower would need to use a higher frequency transistor for an 80MHz filter.

\1610206597621.png
 

LvW

Joined Jun 13, 2013
1,187
Since the Sallen-Key filter typically uses a unity gain op amp, it is possible to use a high frequency emitter or source follower circuit in place of the op amp.
The filter parameters would need to be adjusted slightly for the gain being slightly less than one, and its offset voltage must be allowed for.
Yes - I like to add that the original publication from Sallen&Key in 1955 was based on single transistor stages.
 

crutschow

Joined Mar 14, 2008
26,422
Below is the circuit with a compound complementary emitter follower output for output offset close to zero and higher gain.
Note that no change was now needed to be made in the capacitor value to get essentially the same response:

1610237694706.png
 
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