# Suggestion for an high speed rail-to-rail op amp

Discussion in 'The Projects Forum' started by simo_x, Jan 28, 2015.

1. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
Hi all,

I am looking for an high speed rail-to-rail output op amp for $\pm$ 5 V voltage output swing.

I need to work with a signal of a maximum frequency $\approx$ 10 MHz, maximum closed loop gain would be $\approx$ 5, so if I am not wrong the minimum slew rate required is $\approx$ 315 V/µ, but I would say 400 V/µ to stay large, and the Unity-Gain Bandwidth would be of minimum $\approx$ 50 MHz.

I need to drive at least 50 ohm impedance.

The olny one op amp I found well suitable for my needs is the LM7171 but it's not rail-to-rail output.

Do you have any suggestion ?

Last edited: Jan 28, 2015
2. ### GopherT AAC Fanatic!

Nov 23, 2012
7,111
5,761
I would suggest that you change the circuit. If only one device exists that comes close to meeting your needs and still doesn't meet your needs completely, I would suggest that there must be another way to solve your goal. Think about it, all circuits ever designed have been made without a 400V/uV rail-to-rail op amp.

Again, I think a redesign is needed.

#12 likes this.
3. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,906
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I do not understand how an output that swings ±5V can be driven off a single supply, unless you have a series DC blocking cap in there.

You may find it easier to split the two function into a voltage amp followed by a current amp. The (now sadly obsolete) LH0002 current amp makes a great driver up to about 30MHz; google it and borrow the schematic if need be, it's 4 transistors and 4 resistors.

4. ### crutschow Expert

Mar 14, 2008
16,186
4,324
Current feedback op amps aren't limited by the gain-bandwidth product so their bandwidth is not significantly affected by the closed-loop gain value.
They are also inherently less susceptible to stray circuit capacitance at the summing junction.
As noted in the data sheet, they do require the input and feedback resistors to be within a certain (relatively low) range for proper frequency response.

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5. ### DickCappels Moderator

Aug 21, 2008
3,812
1,034
I wonder whether this is a serious request. The specified performance is certainly beyond that of jellybean op amps.

I would love to suggest an amplifier, but I doubt one really exist. Rail-to-rail amplifiers don't really swing rail-to-rail, usually limited by the saturation voltage of the output devices; in some applications when driving a high impedance load, the output can swing almost to the rails but at ±100 ma, it is unlikely that you will find an opamp that swings all the way from rail-to-rail, especially with that high slew rate.

Have you considered using a pair of transistors on the output to boost the output current and range? It might even help improve the slew rate problem (but 350 V/us...?).

6. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
Dear Dick, yes, this is a serious request. I am sorry If I made you doubt about it.

When I wrote $\pm$ 5 V swing I kept in mind the drop out of the Op Amp. So, I was referring to a few mV.. It's true that I did not specified it, I am sorry.

BTW check this 'jellybean' op amp: http://www.ti.com/lit/ds/symlink/ths4221.pdf

Bye.