CCS/OPA140: Can OPA140 drive 50ohm load as unity gain buffer (low noise, low distortion, please...) ?

Hi Alberto,

Your plan to use a OPA140, JFET input op amp is a good choice because the current noise is typically about 0.8 fA/√Hz. The source impedance would have to be extremely high before any appreciable voltage noise would be contributed by converted current noise. 

The question, however, is how much output swing will you need across the 50 Ohm load?  The OPA140 is capable of about 15 to 20 mA output current . That limits how much it can swing before clipping and high distortion occurs. If the peak output current is assumed to be 15 mA, then the output swing would be limited to 750 mVpk.

Another JFET input op amp to consider that has slightly lower Input voltage noise density and higher output current drive capability is the OPA827. Here's where you can view the datasheet:

Indeed multiple op amps can be configured in essentially parallel configurations to take advantage of the root-sum-squared (RSS) nature of uncorrelated noise sources. Conceptually this should be easy, but because the individual op amps have unique and different electrical characteristics the circuits can become involved to keep all of them happy. I would try achieving the needed performance with a single amplifier first, and resort to something more complex only if necessary.

Regards, Thomas

Precision Amplifiers Applications Engineering

Dear Thomas,

thank you for yor prompt reply.
OPA827 has been reported in this forum to suffer from oscillations when driving even short lenght of coax cable. OPA140 seems to be a bit
more quiter due to the lower gain.bandwidth. However, I definitely will look into it as a viable alternative.
Just to have an escape strategy ready in case of trouble, can you suggest me how to put in parallel 2 or more OPA140 and/or OPA827 in order to improve output driving while put statistic at work in order to lower noise ?
I have seen both 'symmetric' approaches (i.e. identical circuit just replicated in parallel) as well as 'asymmetric' ones (see for instance
the section "Parallel Composite Amplifier Achieves Low Distortion into Heavy Loads" in the LT app. note DN241" Fast Op Amps Operate Rail-to-Rail on 2.7V" ( URL: cds.linear.com/.../dn241f.pdf ).
Any advice/suggestion about how to 'parallel' OPA140 alone or 'in tandem with' OPA827 would be very appreciated.
Thank you again for your time.
Regards,
Alberto

Hello Alberto,

Actually, I would be surprised to find that the OPA827 become sunsatble with small capacitive loads. Here are my reasons why:

• Datasheet Figure 24, Open-Loop Gain and Phase vs Frequency, graphs show a very predictable, 1st-order, -20 dB/dec Aol rolloff and well behaved phase. There appears to be about 70 degrees of phase margin at the unity gain crossover frequency.
• Datasheet Figure 27, Open-Loop Output Impedance vs Frequency, indicates that the open-loop output impedance, Zo, is resistive and flat across the useable bandwidth of the amplifier. No complex Zo issues that can come into play when driving a reasonable capacitive load.
• Datasheet Figure 28, Small-Signal Overshoot vs Capacitive Load, shows that even with 500 pF+ and a worst-case gain of +1 V/V, the overshoot is 55 to 60 %. That indicates that the phase magrin is around 20 %, not high, but still indicates that the circuit is stable with that high capacitive loading.

Even if there was an instability issue with a capacitive load, applying correct compensation should bring the circuit back into a stable condition.

The LT application information about parallel op amps is good and mirrors much of the information that is available on the subject. I did find an E2E discussion that Bonnie Baker addressed regarding the noise benefit of parallel op amps. You can find that E2E discussion here:

e2e.ti.com/.../544356

Regards, Thomas

Precision Amplifiers Applications Engineering

Dear Thomas,

thank you for your reply. The thread from Bonnie Baker is very interesting indeed. I will try myself to simulate different parallel architectures.

Concerning the OPA827 oscillations, please have a look at

It seems that what does trigger the oscillation is the inductance emulated by the trasmission line effect. It is quite subtle, I have to admit.

I will start my first trial with the OPA140, then I will push my fortune ahead with the OPA827.

I have enough to stay busy for a while.

The discussion has been very fruitful. Thank you for your time and suggestions.

Best,

Alberto