Output current of an OpAmp

[engr_joni_ee]

Hi, I am wondering for which applications we use OpAmp with high output current ? Normally, if we have a current pulse as sensor output we need to use transimpedance amplifier to convert the current pulse to voltage signal and then ADC sample it to process digitally with FPGA but how about the output current capability of an OpAmp where we need to consider it and for which applications it matters. I also notice that OpAmp with high output current have low bandwidth.

 

[danadakk]

What are your current, voltage swing, slewrate, BW requirements ?

750 Mhz @ 800 mA @ 2100 V/uS too slow for you ? (this is a unity G buffer)

Here is a pretty fast part, 175 mA drive - https://www.analog.com/en/products/ad8009.html#product-overview

Selection Table for High Output Current Op Amps ≥ 100mA | Parametric Search | Analog Devices

Analog Devices' Selection Table for High Output Current Op Amps ≥ 100mA lets you add, remove, and configure parameters to display; compare parts and choose the best part for your design.

www.analog.com

Needless to say PCB layout, bypass C technology, stability considerations..a challenge at these speeds.

A Practical Guide to High-Speed Printed-Circuit-Board Layout | Analog Devices

This article addresses high-speed layout from a practical perspective, the various found when designing board layouts for high-speed circuitry.

www.analog.com

Regards, Dana.

 

[KlausST]

Hi,

driving a 50 Ohms transmission line.... or a transducer, or a low ohmic sensor...

I build an LRC meter able to drive some 100mA...

Klaus

 

[engr_joni_ee]

AD8009 datasheet shows output load driving current 175 mA and power supply current 14 mA. If I have five of such op amp in a circuit then while searching for linear regulator for the op amp power supply, which current I should consider that linear regulator provide or delever ?

--- Updated Mar 17, 2022 ---

Hi,

driving a 50 Ohms transmission line.... or a transducer, or a low ohmic sensor...

I build an LRC meter able to drive some 100mA...

Klaus

If the current pluse need to be converted into voltage pulse using transimpedance amplifier which will be passed through a low pass filter and then ADC differential driver for ADC then how much the output current of the transimpedance amplifier at the first stage is important ?

 

[danadakk]

AD8009 datasheet shows output load driving current 175 mA and power supply current 14 mA. If I have five of such op amp in a circuit then while searching for linear regulator for the op amp power supply, which current I should consider that linear regulator provide or delever ?

--- Updated Mar 17, 2022 ---

If the current pluse need to be converted into voltage pulse using transimpedance amplifier which will be passed through a low pass filter and then ADC differential driver for ADC then how much the output current of the transimpedance amplifier at the first stage is important ?

Total current, OpAmp + Load.

A bit of advice, that OpAmp is so easily unstable because of layout, strays, so consider
buying their eval board to see layout and bypass C's chosen, etc....

Also caps for high speed work, ceramics, for the same C do not all share the same esr response
characteristics. So test several different manufacturers in a 50 ohm jig, of a proto board with
the OpAmp, to make sure they exhibit wide response ESR characteristics. When used on OpAmp
they will be supplying current for the transients, so critical in avoiding droop and pulse distortion.


Regards, Dana.

 

[KlausST]

Hi,

14mA is the "quiescent current". This means (unless otherwise noted)
* no load current
* stable DC (input and output) conditions

If you run the Opamp with high frequency, bus still no load current, it's supply current may be higher than 14mA.

So, as dana mentioned ... but add some headroom.

***
And I agree with dana regarding layout and eval board.

Klaus

 

[dick_freebird]

As you approach the short circuit protection threshold your main
amplifier attributes are likely to suffer (more or less, depending on
design details).

Some SC limit circuits work only on the back end while others reach
back all the way to the diff-pair tail loads. When they get close to
"lighting up" you will see Vio, AVOL degrade in any case - how abrupt
and how close you can get to the spec Isc is something I'd recommend
you characterize for yourself (do you need to back off by 10%? 20%?).

Simple SC protection schemes are likely to have a really bad tempco
of limit-point, too - use a positive-TC Pbase resistor against a
negative-TC Vbe and you get a compound tempco and a lot of slop.
And of course you can expect die self-heating when you push output
current, meaning you have history effects in the milliseconds range
in such applications along with some thermal drift in accuracy params
(we try, but you can only do so much with thermal layout balancing
if you're not going to autozero continuously).

 

[engr_joni_ee]

In fact I am trying to understand how to search linear regulator power supply for the OpAmp given that from the datasheet we know that the output load driving current and power supply current of AD8009 are 175 mA and 14 mA respectively.

The OpAmp AD8009 is supposed to power up by + 5 V and - 5 V.

Let's suppose I have five AD8009 OpAmp chips in a circuit then linear regulator is appropriate, the one that deliver 100 mA or the one which deliver 1000 mA ?

 

[danadakk]

Simple algebraic summation of all active device currents including the load
current they are driving. If N are driving at a time then its sum of the N's Load
currents + 5 x Iquiescent ......

Regards, Dana.

 

[BradtheRad]

Audio amplifier is a popular role for high current op amps. LM383 can deliver 3.5 Amperes. Runs on 12V positive polarity making it ideal for automotive sound systems.

Another use is power inverters since 60 Hz is a low audio frequency. It may take some searching to find an op amp that runs on bipolar AC.
Or, to drive a load with true bipolar AC, bridge it between two LM383 supplied by positive polarity.