Find an opamp for an special range

[goldsmith]

Dear All
Hi
Thank you for your time .
I'm looking for an opamp , with GBWP , around 40MHZ . ( +-15 volts ) .
I'll be grateful if you guide me to find it .
Best Regards
Goldsmith

 

[:Dark_Force]

What are the other requirements? Low power, low noise, low offset, high output current...? There are too many Amps that match your specs for +/-15V and < 40MHz to make a recommendation!

 

[shahbaz.ele]

check out OP177
it may solve your problem

 

[FvM]

Surely not OP177.

I guess, you are looking for an unitiy gain compensated OP. It's reasonable to specify at least a slew rate, possibly input current.

Major vendors, e.g. Analog, TI, Linear have web selection tools to find suitable devices. Most likely, availability in your country will be major problem in the end.

 

[mister_rf]

;-) e.g.

LM4562NA/NOPB
**broken link removed**
AD817ANZ
**broken link removed**
AD826ANZ
**broken link removed**
AD827JNZ
**broken link removed**
AD811ANZ
**broken link removed**
AD829JNZ
**broken link removed**
AD847JNZ
**broken link removed**
LM6171BIN
OPA637BP
LT1364CN8
LT1362CN
LT1360CN8
LT1227CN8
LT1127CN
LT1115CN8
LT1037CN8
LT1028CN8
LME49720NA/NOPB
http://www.ti.com/lit/ds/symlink/lme49720.pdf

LME49710NA/NOPB
LM6181IN/NOPB
LM6172IN/NOPB
LM4562NA/NOPB
**broken link removed**

 

[goldsmith]

What are the other requirements? Low power, low noise, low offset, high output current...? There are too many Amps that match your specs for +/-15V and < 40MHz to make a recommendation!

Dear ark_Force
Hi
Thank you for your reply , the offset , as low as possible ( with reasonable price ) . and low power . and lowest noise .

check out OP177
it may solve your problem

Dear shahbaz.ele
Hi Thank you for your effort to helping me .

Surely not OP177.

I guess, you are looking for an unitiy gain compensated OP. It's reasonable to specify at least a slew rate, possibly input current.

Major vendors, e.g. Analog, TI, Linear have web selection tools to find suitable devices. Most likely, availability in your country will be major problem in the end.

Dear FvM
Hi
Thank you so much for your reply . what do you mean by unity gain op amp here ?
Best Regards

Dear mister_rf
Hi my friend
Thank you very much for your huge suggestions ! those seems pretty nice ! you are very helpful friend ! :grin:

Best Regards
And thank you all for your suggestions and helps .
Goldsmith

 

[FvM]

"Unitity gain compensated" means, it can be stably operated down to unitiy gain, e.g. as a buffer or inverting integrator. Not all OPs can.

 

[goldsmith]

Dear FvM
Hi
Thank you very much for your reply .
It means that it's gain can't exceed from one ? what can be the benefit of op amps such as this ? i want an op amp with high value of gain . i told GBWP around 40MHZ it means that it's gain at 40MHZ is 0 dB and at 20MHZ is 6dB . am i right ?
Thank you
Goldsmith

 

[mister_rf]

Unlike the ideal op amp, a practical op amp has a finite gain. The open-loop dc gain (usually referred to as AVOL) is the gain of the amplifier without the feedback loop being closed, hence the name “open-loop.” For a precision op amp this gain can be vary high, on the order of 160 dB (100 million) or more. This gain is flat from dc to what is referred to as the dominant pole corner frequency. From there the gain falls off at 6 dB/octave (20 dB/decade). An octave is a doubling in frequency and a decade is ×10 in frequency). If the op amp has a single pole, the open-loop gain will continue to fall at this rate... The second pole will double the rate at which the open-loop gain falls to 12 dB/octave (40 dB/decade). If the open-loop gain has dropped below 0 dB (unity gain) before it reaches the frequency of the second pole, the op amp will be unconditionally stable at any gain. This will be typically referred to as unity gain stable on the data sheet. If the second pole is reached while the closed-loop gain is greater than 1 (0 db), then the amplifier may not be stable. Some op amps are designed to be stable only at higher closed-loop gains, and these are referred to as decompensated op amps.

https://www.analog.com/static/imported-files/tutorials/MT-033.pdf


:-D If you have time there's more to read here:

https://seit.unsw.adfa.edu.au/staff/sites/hrp/research/helpfulnotes/whyUnityMostDifficult.html
https://www.analog.com/static/imported-files/tutorials/MT-033.pdf

https://www.analog.com/library/analogDialogue/archives/31-2/appleng.html
**broken link removed**
**broken link removed**


And based on that supplementary criteria from the above examples you need to remove dedicated audio application amplifiers, and keep only:
AD817ANZ
AD826ANZ
AD827JNZ
AD811ANZ
AD829JNZ
AD847JNZ
LM6171BIN
LT1364CN8
LT1362CN
LT1360CN8
LT1227CN8

Anyway, to choose a suitable circuit you need to provide additional information about the application in use. ;-)

 

[hshah8970]

Dear All
Hi
Thank you for your time .
I'm looking for an opamp , with GBWP , around 40MHZ . ( +-15 volts ) .
I'll be grateful if you guide me to find it .
Best Regards
Goldsmith

Hi there Goldsmith! A while ago I was facing a similar situation and needed to find an opamp with my specifications. I found Digikey's online opamp searching interface to be the most helpful. The best part is that they ship opamps to almost anywhere. Here is the link for the opamp searching tool:

https://www.digikey.com/product-sea...s-instrumentation-op-amps-buffer-amps/2556125

 

[FvM]

It means that it's gain can't exceed from one ? what can be the benefit of op amps such as this ?

I fear, I didn't make the point clear enough. It's about minimal, not maximal gain.

See e.g. LM7171 versus LM6171. LM7171 is compensated for gains >= 2 (feedback factor <= 0.5), it can't work at a gain of 1. LM6171 can work at all gains or more exactly for feedback factors up to 1.

 

[goldsmith]

Dear mister_rf
Hi
Thank you very much , your links are pretty helpful for me .
Best Regards
Goldsmith

Dear hshah
Hi
Thank you for your good link . it is really helpful.
Regards
Goldsmith

Dear FvM
Hi again
Thank you very much for your help and reply . may i ask another question ? why we can't get gains around 1 for that opamp ? if we try to use it with that gain , what will happen ? and what is the cause of this ambient of gain ?
Thank you very much
Best Regards
Goldsmith

 

[hshah8970]

I also have a question regarding unity gain stability from FvM. How do I check the unity gain stability from the bode plots of the opamps? What phase margin value should be at unity gain? Thanks.

 

[mister_rf]

@hshah8970
I have posted here:

https://www.edaboard.com/threads/255149/#post1093088
More details e.g.:
http://www.analogzone.com/acqt0418.pdf

 

[godfreyl]

why we can't get gains around 1 for that opamp ? if we try to use it with that gain , what will happen ?

It will oscillate at high frequency. The frequency compensation is enough to keep the opamp stable at higher gains, but not at low gains.

 

[goldsmith]

Dear godfreyl
Hi
Thank you for your reply .
But he told , that the gain can't be around one . of course i know if gain be high , the oscillation is possible ( unwanted oscillations due to the improper place of poles )
So i couldn't find relation between your statement and my question . i'll be grateful if you kindly , clarify your meaning .
Respectfully
Goldsmith

 

[FvM]

godfreyl has been answering your question, I think.

To understand the problem, you can sketch the loop gain bode diagram and determine the phase margin.

You can refer to the characterictic of OP37, an OP decompensated for a minimal stable gain of 5.


If you connect it e.g. as +1 amplifier (feedback factor of 1), you get a negative phase margin - the circuit will surely oscillate. With a feedback factor of 0.2, the phase margin is 71°, as indicated in the diagram.

 

[goldsmith]

Thanks FvM . Thank you very much . but is it possible that you tell me , please that How with those specifications it can oscillate ? it is a bit unclear for me , now .
Thank you so much
Goldsmith

 

[FvM]

According to the diagram, phase margin of 0 (= oscillation phase condition) is achieved at 17 MHz, with a open loop gain of about 2. So it will oscillate for feedback factors > 0.5.

 

[goldsmith]

How you told that ? ( at 17 MHZ ? ) can you clarify it , please ?

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is that 0.5*2=1 ?

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or greater than unity ?