OPA541: Limit bandwith

Hello Claverie,

The OPA541 has a typical slew rate of 6 V/us. That is as fast as the output voltage can change. If your peak voltage is 28 V it will take about 4.7 us for the voltage to transition from 0 V to the 28 V peak, and another 4.7 us to transition from 28 V to 0 V, or about 9.4 us total. This is nearly the entire 10 us half cycle. This result indicates op amp slew rate limiting is occurring and the output waveform should be a triangle wave.

If you don't need nearly as much output current as the OPA541 provides we might be able to come up with a different power op amp that has higher slew rate.

Regards, Thomas
Precision Amplifiers Applications Engineering

Claverie,

In order to have a square wave at the output, the op amp would have to have an infinite slew rate, which is not possible.  OPA541 slew rate is between  6-10V/us and that is the reason why your square wave input becomes a trianglular wavaform at the output - the output cannot move faster than its slew rate (see below) and for that reason your square wave input becomes a triangular output.

Hi and first Thank you very much for your quick answers

Effectively slew rate causes problem for my application

You told about other amp ...with lower current if I 've well understood (sorry I'm french..)

So what model do you propose that I can cheg available current and chek if it can satisfy my request

Thanks

Regards

Hello Claverie,

In order to amplify a waveform that approaches a true square wave the slew rate of the power op amp will have to much, much faster than 6 or 10 V/us. When I review the 2.5 Ampere output current and the high slewing rate requirements TI doesn't have a power op amp that can cover that combination of needs.

When we don't have a power op amp product that meets the application requirements we try and assist by referring the person who has the requirement to another power op amp producer. Apex Microtechnology, whom TI has no business affiliation whatsoever, produces a wide range of power operational amplifiers. Their line of High-Speed amplifiers offer slewing rates in the hundreds and thousands of V/us , and multi-ampere output current levels. It is likely they have a device that can fulfill your applications requirements. You should be able to find their info on line.

Regards, Thomas
Precision Amplifiers Applications Engineering

Many thanks again
Do you think I can use A TI amplifie with slew rater that i Can follow by a stage of 2 MOS FET to amplify current and voltage
thanks a lot again

Hello Claverie,

Since you are only interested in generating a square wave you really don't have to use a linear circuit approach that involves an op amp. You could probably use a comparator, or digital logic element having sufficient output drive capability to drive the gates of a P-Ch, N-Ch power MOSFET pair. The circuit could be designed so that the MOSFET output slams against one supply rail or the other, and slews very quickly in between.

Regards, Thomas
Precision Amplifiers Applications Engineering

Hi 

Fibally I lower my request in terms of current (now about 1 A)

I've tested the upon schematic 

I've not found a SPICE model for the OPA541AP I've ordered and assemble .. I've only found on net the old one of Burbrown model in TO3 package so the pins are different 

Do you have one for my model which is TO220 model

2 generators 3.3 V to emulate my future logic .. 

Gain about 8 and on this schematic about 10 

The design works but

1. The amplifier heats a lot EVEN whent the 2 generators are OFF (grounded)

Is it normal ? normaly I think it s an Class AB amplifier so It should not so heating ? 

2. The output voltage is about 28V WITHOUT any load or high load (10 mAmp) 

With 1 A it's about 26 V even 24 V... 

IF I read datasheet it should be VS- 4V min .. so why ???,

3. The output resistor should limit about 1.5 A .. but on test it's limit to 1 A ???? 

4 - Is it normal after limitation that the output voltage decrease and wave forms are totaly distored? 

Have you some informations to improve my application ? 

Thanks in advance

Regards

 

Hi 

Regarding schematic I give upper 

Can you help me understanding

If I leave the 2 input non connected he amplifier is -32v and stable

BUT

If I put 2 oscilloscope propes one on each input (+ -) the amplifier is on oscillation between -32 and +32 

Could you explain ?

probes should present hig impedande et somme pf of capacity .. so ?

Have you idee to avoied this problem ? 

and stabilises the amplifier

regards 

Hello Claverie,

If you connect a scope probe to the OPA541 inverting input, the summing junction, that adds the probe capacitance from that node to ground. In doing so, time delay is added to the feedback path from the OPA541 output to the input. Time delay, which equates to phase delay can degrade the circuit's phase margin to the point that the feedback becomes positive and oscillation results.

You can learn much about op amp stability by viewing the TI Precision Labs Op amp series. There is a section specially about op amp stability and how to maintain it. You can find the series here:

Regards, Thomas

Precision Amplifiers Applications Engineering

thanks a lot for documentation

More and normaly last question

When I powerd under +/- 32 V without any input connected or 2 in
put connected to ground the output is stable and the ouput is Minus 32 V
I put a vey high load resistor (about 100 kohms)
so out current is nearly 0

BUT my amplifier seems to heat a lot
Even I ve no heatsink .. is it normal that without any load the temperature is about 50 % C !!
I'm unquiet for an equipement with must work 10 hours per day ..;

Regards

Hello Claverie,

OPA541 pin 2 is the inverting input. It sounds like you are operating the op amp open loop, in which case the output is will slam against one output rail or the other. You indicate it is sitting at -32 V which sounds reasonable if it isn't sinking current from a load.

If the output is at -32 V the only real current flowing though the OPA541 is the operating current, which is typically 20 mA, or 25 mA maximum. The 100 k load only represents a few hundred microamps of current so that isn't significant. Taking the 20 mA of operating current and multiplying it times the 64 V across the V+ to V- pins, results in a quiescent power of 1.3 W.

The package case temperature can be approximated from:

T_C° = T_A° + Pd ∙ θ_JC

Using the RθJC(top), Junction-to-case (top) thermal resistance of 17.4°C/W, and a TA of 25°C:

T_C° = 25°C + (1.3 W)(17.4°C/W) = 47.6°C

You mention the OPA541 isn't mounted on a heat sink in this case, and it appears the device can get quite warm. I am not sure about whether RθJC(top), or RθJB Junction-to-board thermal resistance, would be the correct number to use in that case. But it does appear the device will self heat to the temperature you have observed. 

The OPA541 will go into thermal shutdown if it gets too hot. The amplifier is intended to be used with a heat sink following the recommendations in the datasheet.

Regards, Thomas

Precision Amplifiers Applications Engineering

hi and thanks

I'll will get an heatsink 

another question ..

Is there any danger for the amplifier if I input the 2 inputs to 3.3V WITHOUT power supply of the amplifer ?

could it be damaged?

Thanks

Hi Alain,

the input voltages must never exceed the supply voltages. This is what the datasheet states in section 6.1.

You could eventually (and at your own risk!) clamp the input voltages to the rails by the help of small signal silicon currents like the BAV99 or BAV199, for instance, or even small current Schottky diodes like the BAT54, for instance. An additional current limiting resistor directly at the input should be mounted to furtherly limit the input current.

Or you could take another power OPAmp which can more easily be protected at the inputs.

Kai

hi
My problem is that the input signals will be present even when the power supply of the AMp will be off
I will find a solution to isolate inputs
Thanks
Regards

Hi Alain,

where are the input signal coming from? Are they current limited somehow?

Kai

Hi
From an FPGA (limit 12 mA max)

Alain

We have not heard back from you so we will close this thread. Please reply if further discussion is needed.

Thanks
Dennis