AD797, OPA134 or discrete buffer stage?

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Hi everyone,

Im looking for a useable Op-Amp or possibly a simple discrete circuit to use as an input buffer on an amplifier I'm planning. The buffer stage should be unity gain and preferrably low noise and low distortion.

I've currently been looking at the AD797 and OPA134. Judging by the datasheets (and price...) I understand that the AD797 performs better with respect to distortion and noise. However, as the AD797 is quite costly (this is a somewhat unusual application where I'll need as much as 12 Opamps) compared to the 134, I'm wondering if it's worth the extra expense? Using AD797s will cost US$110, while the same number of 134s will get me there for around $30.

Also - I've been looking for a simple buffer stage as an alternative - does anyone know a suitable circuit that does the same job as the opamps in this context?
 
Thanks for replying, but wouldn't it be a bad idea to put an inverting buffer stage in front of a non-inverting amplifier?

Also, any idea on distortion, PSRR and other performance parameters compared to an IC OpAmp? Since it has only passive loads/current sources on the input stage, wouldn't power supply noise get quite easily injected into the signal path?
 
Specs are not known to me,.
This is no opamp design for optimizing any spec. With the named transistors you could build a MM-amplifier. Because of the low amplification distortions are low. I would take a stabilized power supply anyway

It is a design to built with little effort and cost. It can be modified for +1.
 
ventle said:
Thanks for replying, but wouldn't it be a bad idea to put an inverting buffer stage in front of a non-inverting amplifier?

Also, any idea on distortion, PSRR and other performance parameters compared to an IC OpAmp? Since it has only passive loads/current sources on the input stage, wouldn't power supply noise get quite easily injected into the signal path?
If you choose a modern high performance opamp you don't have to worry about much really.

The first question: An inverting amp is a bit sensitive for the signal source and it's output impedance (including a pot) and the input impedance can't be made particulary high. The obvious advantage isn't so obvoius.
 
Thanks for sharing your thoughts everyone.

By FET follower, do you mean a single JFET operated in class A mode, input on gate and output taken from the source (something like Nelson Pass' Bride of Zen preamp)?

peranders, thanks for your input on the LME49710, it seems to be almost on par with the AD797 at just a fourth of the cost. Is this your OpAmp of choice for this kind of stuff?
 
Nordic said:
My favourite of the Fet stages is the Pedja Rogic buffer...


Quite interresting circuit, seems like it can be made for around $1 per buffer. Simulations indicates the PSRR is quite good as well (no visible signal injection even when adding 8V rms 50Hz ripple voltage). Did you end up using a regulated power supply in your preamp, or just stick with the raw, filtered one?
 
That's good news Nordic, as the transformer I'll be using for the power amp has an additional secondary coil with 12-0-12 Volts RMS (approx 15,5-0-15,5 Volts DC after rectification). Regulation means extra power dissipation and avoiding this is always a good thing;)
 
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