| AndrewT |
Hi,
is the OPA228 a good alternative to the LF411 for use as a DC servo in a power amp?
or would the unity gain stable version OPA227 be a better choice?
Are there other cheap alternatives? |
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| jcx |
How about applying the 1st order "tests" of op amp substitution:
Fet or Bjt input?
Unity Gain Sable?
Similar GBW?
I believe you can quickly see that your proposed replacemnt fails all ways - specific circuits may work with different type op amps but in general you don't want to change these classifications when substituing |
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| sreten |
Hi,
Sorry but am I missing something ?
Selecting an op-amp for a DC-servo ?
Just about anything will do AFAIK.
:)/sreten. |
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| SY |
| You want unity gain stability, low offset voltage, and low bias current. The LF411 is just about perfect. |
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| Samuel Jayaraj |
| You also have the following choices: OP07, OP77 and OP90. |
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| AndrewT |
Hi,
so these are the important specs| quote: | | unity gain stability, low offset voltage, and low bias current |
for a DC servo?
In which case the OPA227 is better than OPA228. Except that it's 70% more expensive (£1.76 vs £1.07 each).
I note that 411 has a highish GBW product @15MHz for a unity gain. Is this relatively unimportant? |
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| ACD |
| Just choose an opamp with low offset voltage and one that is stable at low gain, and it will work just fine ;) |
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| peranders |
Not only at low gain, more like unity gain. That is important. Offset voltage is not important because no opamp will have more than 5-10 mV and the limit is < 50 mV (according to me).
One other criteria is also low distortion up to 1 kHz but this is fulfilled with just about any decent type.
My recommendation is, input bias current < 25 nA, SR < 10-15 V/us (= not too fast)
One more thing to think of is max output power at min frequency, this should not saturate the servo. Let's say the absolute max power is 100 W (8 ohms) and the choosen min frequency is 20 Hz, this will cause the servo to heavy action but it must work within it's limits otherwise you'll get some distortion due to a servo which clips. |
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| AndrewT |
Hi,| quote: | | choosen min frequency is 20 Hz, this will cause the servo to heavy action but it must work within it's limits otherwise you'll get some distortion due to a servo which clips |
if the servo rolls off at about 1Hz then how onerous is the above requirement?
What if 10Hz at full power were generated by the source (film sound effect)? |
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| AndrewT |
Hi Peranders,
I have downloaded your sim.
How do I input the critical values to check the effect of low signal frequency on opamp output voltage? And then how do I interpret the results from the sim? |
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| peranders |
Notice that the servo has lower supply voltage than the main amp.
Set the input voltage so you'll get desired output voltage (power). Then make a frequency sweep and observe the servo output. If the servo goes clipping you must make it slower.
As mentioned you must deside max output voltage(power) at the lowest interesting frequency. 10 Hz at full power isn't particular important. 20-25 Hz is probably a better choice. |
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| AndrewT |
Hi Peranders,| quote: | | Set the input voltage so you'll get desired output voltage |
What part of the sim gets changed to allow this?| quote: | | make a frequency sweep |
how?| quote: | | observe the servo output |
N007? and check that output is less than opamp supply less 2volts to avoid clipping? |
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| ash_dac |
Hi,
AD711 is a replacement...says it in the datasheet so it must be true! |
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| AndrewT |
Hi,
if AD says so then you're right.
But AD prices! Thanks anyway. |
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| AndrewT |
Hi,
who makes op77 and op90.
Ti says opa77 is obsolete
and op07, op77, op90, opa90 are not found.
opa07 is a bit slow. |
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| jcx |
| quote: | Originally posted by AndrewT
I note that 411 has a highish GBW product @15MHz for a unity gain. Is this relatively unimportant? |
You may have dropped a decimal place somewhere - LF411 is "min GBW 3 MHz", typ ~ 4 MHz - other than offset V this is a early fet input "jelly bean" part
nA bias current could be bad if your servo uses MegaOhm input R to get long time constant with NP0 or small film cap
there are too many cheap FET and Cmos input op amps to count - looking for Vos trimmed to < 1mV will narrow your choices but then they may qualify as "precision" op amps and command a price premium |
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| AndrewT |
Hi Jcx,
I follow your reasoning.
That's why I started looking for an alternative, but without knowing the really important specs.
I can get 228 for the same price as 411.
The data sheet says a little bit of feedback capacitance corrects the stability for gain of +2 or -2, but omits data for +1 or -1.
The DC servo uses the opamp as integrator, leading me to suspect that the bigger feedback capacitance needed will sort the unity gain requirement.
Yes, there appears to be different data for 411, confusing. |
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| jcx |
the feedback integrator cap is a short circuit near the op amp's unity gain frequency - therefore it imposes a requirement for a unity gain stable op amp
too fast a subtitute means other supply bypass and output loading issues can become problems, probably a 2x faster device wouldn't be too risky - opa132/4 if you can get low Vos, on the lower side of GBW it is likely the inegrator doesn't really care so even <1 MHz low power devices could work
but what's the probelm with buying a lf411 - still being made and as a replacement you shouldn't even have worry about Rhos supply silliness |
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| peranders |
Andrew, you can manage with almost any opamp because you can use resistor values of 47-100 k + 1 uF to get sufficiently low speeds. This will have no effect of 25 nA input bias current.
Change my simulation files to the setup you have. Since it's almost DC it will be a very accurate simualtion. Have you noticed that if you decrease the adjustment range of the servo, it becomes also slower. The only real important thing is a 100% stable opamp at the gain of 1.
My values are perfect for me and AD8620 and OPA2134 will work splendid for me. |
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| AndrewT |
Hi P,
I was hoping for a very short tutorial. |
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| peranders |
| I'll guess you have installed the LTSpice and it's working. Right now I'm sitting in front of a Mac a couple of days ahead and I don't remember exactly how you should do but look after simulation and "RUN". |
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| Tom2 |
Also something to consider is if you swap
opamps in a working circuit and the offset adjustment is being used,
the LF411 offset adjustment pins connect to the negative supply and
opamps like the OPA227 offset adjustment connects to the positive supply.
Tom |
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