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Hi,I'm sure this is a very basic question. I've been looking up threads on biasing opamps into class A, especially in Thorsten's El Cheapo phono stage. I found a schematic that shows me how to do this - is this correct? The emitter connects to the opamp output, the base connects to V-, and the collector connects to V- through a resistor. I might have the emitter/collector wrong, because (a) I'm really rusty with all this, and (b) the datasheet doesn't show any arrows on the FET.
Anyway, on this diagram, V- is -18V, the opamp is an LT1115, the FET is a 2N4304, and the resistor says "~ 250 ohms, select for 2mA".
Questions:
* Is that the way to hook this up?
* How does one calculate the required resistor value? 18V / 2mA = 9K, which isn't anywhere close to 250, so the calculation obviously isn't that straightforward.If it makes any difference, I'm using an OPA2134, and the supply voltage is 12V. I found a post in the archives where someone used a 2N5485 for this, so I'll probably do the same thing.
Thanks in advance for all help.
Saurav
Follow Ups:
using Ibias ~ .56 mA from Risch's 2nd reference thread ( see post below ) this whole discussion is probably irrelevant to OPA2134OPAx134 data sheet gives open loop output impedance = 10 Ohm
from Zout = 1/2 * ( 26 mV / Ibias ) we can say output stage Ibias > 1.3 mA, unless they've used additional local feedback around the output stage transistors
also why FET constant current source? Bipolar source with vref diode or Wilson current sink can set Ibias to < 5% tolerance with no trimming required
> from Zout = 1/2 * ( 26 mV / Ibias ) we can say output stage Ibias > 1.3 mA, unless they've used additional local feedback around the output stage transistorsWhere did the 26mV come from? And are you saying that I need more bias current, or that it's already biased to 1.3mA and therefore doesn't need any more, or something else?
> also why FET constant current source? Bipolar source with vref diode or Wilson current sink can set Ibias to < 5% tolerance with no trimming required
No idea one way or the other. I'm just following a recommendation I found on this forum. What are the disadvantages to using a FET, other than the need to measure and trim the resistor value?
I'm using a formula for output resistance of a complementary emitter follower to guess at bias current used in OPA134 - 26 mV is a constant from bipolar transistor theoryIf they (BurrBrown/TI) are not using local (nested) feedback around the output stage this should give a lower limit on output stage bias current, so yes I'm saying the OPA134 op amp could already have ~ 1.3 mA bias current
without voltage swing and load impedance to calculate load current drive requirement (including loading from feedback network and possible cable capacitance) we can't say how much bias current is required for class A operation in your application
it's possible that buffering the op amp output (inside the feedback loop) is a better use of the extra transistor and resistor with the advantage of nearly eliminating thermal feedback errors and reducing the load on the op amp output to below it's output bias current level resulting in class A operation -- of course this isn't a simple addition to a finished product, and I couldn't recomend it to someone without an oscilloscope to check for stability
FET current sources have a certain minimalist elegance but bipolar designs can work down to lower volatges without trimming or parts selection, tweakers can go for elegance, product designers really hate trimming in production
For simple resistors that are connected from the negative rail to the output of the opamp, see:http://www.AudioAsylum.com/audio/tweaks/messages/33222.html
and related postsand
http://www.AudioAsylum.com/audio/tweaks/messages/47624.html
Note that there was a typo in this post, it should read 27 kOhm, rather than 2.7 k Ohm for the resistor value.
Hand selected and adjusted FET's are OK, but many people adjust them for way too much current, many op-amps do not respond that well to heavy Class A biasing.
It is my personal opinion that 3 mA is too much bias, and beyond optimum for many op-amps, all but the most capable output units.
1 to 2 mA is just fine in all but special cases, and then the whole idea needs to be carefully evaulated.
Jon Risch
> 1 to 2 mA is just fine in all but special cases, and then the whole idea needs to be carefully evaulated.Could you elaborate on that please? Do you mean careful evaluation is needed for anything above 1 - 2 mA, or re-evaluate the idea of whether you even need to use a FET if 1mA is all the current you need? I'm not sure what exactly you meant.
Thanks for the links.
Saurav
In terms of bias current levels, not use of a resistor vs. a FET.
Jon Risch
Maybe it is precisely that simple - I re-checked the other post, and he used 4K with 12V to get 3mA, which fits the V = IR formula. Maybe I'm just making this more complicated than it needs to be :)Saurav
See? I was right the first time :) Tnanks guys... I can't access that link from work, so I'll read it when I get home.If I want to go the measuring route - what's a ballpark to start in? I don't want to fry the opamp because I picked a value that's totally off the scale. I think I want around 3mA, from what I've read. It seems that the optimum value is also opamp dependent, so does it make sense to play with say a 10K pot, and then put in fixed resistors once I've found the point I like?
Also, if I try this - what should I be listening for? What changes in the sound when I do this class A biasing, and what would change by increasing or decreasing the current? Also, how could I tell from the datasheet if I need to bias to V- or V+? I read a thread where someone said that some opamps should be biased the other way.
One last question - can I do this for the opamps in my CD player's output stage too?
So many questions :)
Thanks,
Saurav
First, PICK A FET. The best FET in this case should have a somewhat lower Idss, maybe 10 ma or less. However, almost any FET will work. It is OK to use a pot, but 10K is too high. Try to use a 2K or lower pot, so that you can get better resolution.
This is the easiest way: Get a 9V battery and some clip leads. Use the 9V battery as your test voltage source. wire up a test assembly with the fet, pot, and add a 100 ohm resistor between the DRAIN of the FET and the (+) terminalof the 9V battery. Of course, connect the pot with the wiper of the pot connected to the input of the pot to make it a variable resistor. Put the pot between the source and the gate of the FET. Make the GATE of the FET, that is now connected to the pot, the most (-)V part of the circuit, by connecting it to the NEGATIVE TERMINAL of the 9V battery. Put the multimeter ACROSS the 100 ohm resistor and adjust the pot until you get .3V across the 100 ohm resistor as this is 3Ma.
> First, PICK A FET.OK, the 2n5485, since someone else used it too. Idss: min 4mA, max 10mA, @ 15V Vds.
Thanks for the detailed explanation, that is a lot clearer now. The 100 ohms going to +9V simulates the load of the opamp that I'm trying to bias, and the rest of the circuit matches the schematic I have, so I can follow that.
The 100 ohm resistor is there to measure across with a cheap voltmeter. The end value will probably be between 350-600 ohms for the current adjust resistor.
Even I can access it from work. It's weird some of the sites I can't access though, many electronics-related.The info at the link pretty much echoes what John said. Bottom line is, you really have to measure to do a proper job. My info assumes your multimeter can measure current, John's method may be more accurate with most meters (with an accurate 100 ohms) since they typically measure voltage more accurately than current.
LOL!! 'Web Hosting' is a filtered category. I have no idea why. One day, all of eBay showed up as 'Adult Entertainment'.I have a relatively cheap Radio Shack DMM, so I have no idea how accurate it is at measuring current. I just ordered a bunch of the FETs from Mouser, I'll play around when they get here.
* Is there any reason why I cannot/should not do this to the opamps in my CD player's output stage?
* What is behind the 3mA magic number?
Saurav
< < LOL!! 'Web Hosting' is a filtered category. I have no idea why. One day, all of eBay showed up as 'Adult Entertainment'.They probably have a dumb filter for Baywatch...
< <* Is there any reason why I cannot/should not do this to the opamps in my CD player's output stage?
Nah, worth a try for sure.
< < * What is behind the 3mA magic number?
I don't think it's so much magic as "safe"...it should easily get the op amps into class A biasing without overheating them. Even 2mA will typically do it with most op amps for most circumstances (cable & pre loads) it seems. Be conservative at first, and try higher currents if things are going well.
In case it wasn't mentioned, the sonic benefit is a certain smoothness and also clarity in low level detail that you will come to associate with class A SS.
cfraser is correct. You could have 2-4ma without too much difference in performance. Factory adjusted fet current sources used to be popular, but now we usually make our own. For the record, I punched in 2N5585 into Google and got the Fairchild spec sheet on the part. Everything is on the spec sheet. This device, however, will 'squeak by' in this application, but this part is not the best part for this application. This is because it is an RF part that has only 25V breakdown. In general, it is better to use a J113, or similar 40V part, because it is happier working with higher voltage swings.
Oh well, I already ordered it. I see, since one end is at -12V and the other end (opamp output) can go a few volts positive, that gets closer to the 25V limit. This should be more comfortable in my phono stage than my CD player, I guess.Some day, I'll know enough to think about everything that needs to be thought about :)
< < Some day, I'll know enough to think about everything that needs to be thought about :)...to GOD. OTOH, when you get to that point, you probably won't need a moniker, you can just THINK your post, and we'll all know it, assuming we survive...
NO, it is more complicated than can be easily explained. Each fet has 2 important characteristics that set the current. First is Idss, the second is the Gm. Each fet type has a range and is unique in its Idss-Gm model, and usually we predict a resistor value to get a specific current by a graphical technique used with the Id-Vgs graph that is included an expanded data sheet available for many fets. Trust me, it is easier to measure it, and even cut and try to get the optimum resistor. For example, you can get some gold pins from an old IC socket and put them in place of the fixed resistor. Then you can guess the resistor value and try it first. By using Ohm's law and measuring the voltage drop across the resistor, you can determine if the resistor is too large or too small. Then you try a different resistor that will get you closer to 2ma or so that you ultimately want. You keep trying until you get there.
Check out the link to show you how to do it. It's under something like "op amp class A biasing" for the ART DI/O page. You have to measure the current using the FET's you putting in the circuit, can't calculate the resistor value unless you know the exact characteristics of the specific FET's you're using.
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