RE: Top questions on op amp power dissipation – Part 2

Sunney Kumar — Wed, 15 Sep 2021 06:37:11 GMT

This is very informative, thank you [mention:7e07bc3660804f54a2a53742ac2a03c8:e9ed411860ed4f2ba0265705b8793d05] and TI for always educating.

RE: Top questions on op amp power dissipation – Part 2

Thomas Kuehl — Thu, 02 Feb 2017 16:45:10 GMT

Hello YSR1991,

I am glad you found the information useful. Thank you for the encouraging feedback.

Best Regards, Thomas

Precision Amplifiers Applications Engineering

RE: Top questions on op amp power dissipation – Part 2

YSR1991 — Thu, 02 Feb 2017 07:48:16 GMT

Thanks for very useful information.

RE: Top questions on op amp power dissipation – Part 2

Thomas Kuehl — Mon, 16 Feb 2015 16:58:44 GMT

Hi Wang,

Let me illustrate why the output transistor dissipation is highest when the output voltage is one-half the supply voltage. Let's use an example of an op-amp in a gain of +1 V/V where ideally the output voltage is equal to the input voltage. The op-amp will be powered by a +5 V supply and have a 100-Ohm load resistor on the output. The input voltage is stepped from 1 V to 4 V, in half-volt increments, and the output transistor power dissipation is calculated. Here are the results:

Vs+ = 5 V, RL = 100 Ω

Vin = Vout V(xstr) Iout Pd (xstr)

1 V 4 V 10 mA 40 mW

1.5 V 3.5 V 15 mA 52.5 mW

2 V 3 V 20 mA 60 mW

2.5 V 2.5 V 25 mA 62.5 mW

3 V 2 V 30 mA 60 mW

3.5 V 1.5 V 35 mA 52.5 mW

4 V 1 V 40 mA 40 mW

We see that the Pd(xstr) is maximum when the amplifier output voltage is half the supply voltage. It is purely a matter of the voltage across the output transistor and the current flowing through it when operating at this point in the curve that results in the power dissipation being at maximum.

Regards, Thomas

RE: Top questions on op amp power dissipation – Part 2

wang siyang — Mon, 16 Feb 2015 07:46:52 GMT

Maximum power dissipation will occur when VS and IQ are at maximum and the output is biased to a dc level half the Vs+ supply voltage

why it happens at half the Vs+ supply voltage?

RE: Top questions on op amp power dissipation – Part 2

Thomas Kuehl — Mon, 25 Aug 2014 18:05:07 GMT

Hello MKarger,

Actually, in this single-supply circuit configuration the upper MOSFET is sourcing all the current to the load. The lower MOSFET isn't involved in providing any current to the load. It is essentially in its quiescent state.

You can find why the power was really less than I originally caculated in my response to David. I've made some corrections to the text.

Thank you for close attention. You guys are keeping me honest!

Regards, Thomas

RE: Top questions on op amp power dissipation – Part 2

Thomas Kuehl — Mon, 25 Aug 2014 17:10:20 GMT

Hello David,

Thank you for spotting the error! I shouldn't have summed the IQ(max) and Io currents and used it in the total power calculation. The Io portion flows through the load and only half the supply voltage is involved in its contribution to the total power dissipation.

I making an edit to the text where I use your corrected equation. The correct information needs to be here for reference purposes.

Thanks again, Thomas

RE: Top questions on op amp power dissipation – Part 2

MKARGER — Mon, 25 Aug 2014 17:08:03 GMT

I believe David is correct. The solution presented assumes that both output FETs are conducting the full Io, when actually each of them conducts 0.5 * Io when the output is mid-scale.

RE: Top questions on op amp power dissipation – Part 2

David Ciaffa — Mon, 25 Aug 2014 01:43:15 GMT

I believe there may be a small error in the equations for Figure 2. As written, the equations calculate the total power dissipation of the entire circuit, rather than the power dissipated by the op-amp. I believe the total power dissipated in the op-amp is given by:

Pd(max) = (Vs+) * Iq(max) + ((Vs+) - Vo) * Io = ~6.5 mW

Correct?

RE: Top questions on op amp power dissipation – Part 2

Thomas Kuehl — Tue, 12 Aug 2014 15:58:00 GMT

Hello Ahmet,

I am glad you find this information informative!

Thank you, Thomas

RE: Top questions on op amp power dissipation – Part 2

Thomas Kuehl — Tue, 12 Aug 2014 15:54:31 GMT

Hello Zarek,

Your feedback is appreciated. I am always interested in learning what our customer's want to know about operational amplifiers and how to best apply them.

Thank you, Thomas

RE: Top questions on op amp power dissipation – Part 2

Ahmet Cetin — Tue, 12 Aug 2014 09:46:51 GMT

Very informative. Thank you.

RE: Top questions on op amp power dissipation – Part 2

Zarek Galicia — Tue, 12 Aug 2014 06:32:24 GMT

This is one of the best information i can acquire for the op amp!! Thanks for everything TI!