You take the high road.........
Since this a transconductance amplifier, the open loop output impedance is determined by the load impedance i.e. it has a very high open loop output impedance. The closed loop output impedance is this load impedance divided by the amount of feedback (i.e. the open loop gain divided by closed loop) as the open loop gain falls of with increasing frequency, the amount of feedback falls of and the output impedance rise and goes from resistive to inductive. If this happens in the audio range you will freguency response errors. Inter action with cable capacitance can also cause stability issues and the design must be done with the cable capacitance in mind. Conventional amps with follower output stages exhibit this property as well. The difference being that thier open loop output impedance is several orders of magnitude lower causing the inductance to be much lower value. Both types can be unstable with certain capacitive loads. a wise designer puts a build out resistor between the amp output and the capacitive load.
The AD823 is an example of a transconductance amplifier. This is done in a few op amp designs to be able to swing the output voltage very close to the power supply rails. There are several very successful audio power amps that are transconductance amps as well.
http://www.datasheetcatalog.com/datasheet/A/AD823.shtml
Since this a transconductance amplifier, the open loop output impedance is determined by the load impedance i.e. it has a very high open loop output impedance. The closed loop output impedance is this load impedance divided by the amount of feedback (i.e. the open loop gain divided by closed loop) as the open loop gain falls of with increasing frequency, the amount of feedback falls of and the output impedance rise and goes from resistive to inductive. If this happens in the audio range you will freguency response errors. Inter action with cable capacitance can also cause stability issues and the design must be done with the cable capacitance in mind. Conventional amps with follower output stages exhibit this property as well. The difference being that thier open loop output impedance is several orders of magnitude lower causing the inductance to be much lower value. Both types can be unstable with certain capacitive loads. a wise designer puts a build out resistor between the amp output and the capacitive load.
The AD823 is an example of a transconductance amplifier. This is done in a few op amp designs to be able to swing the output voltage very close to the power supply rails. There are several very successful audio power amps that are transconductance amps as well.
http://www.datasheetcatalog.com/datasheet/A/AD823.shtml
Fred,
While isolating the load from the amp is standard practice in the application data books, I have never found it necessary to do so. Of course designing for the masses there is no telling what some one will connect to the output.
However, it is sometimes mandatory to have a resistor isolating your probe when looking at high performance and high impedance parts.
More open loop gain, more slew rate and no noise please.
While isolating the load from the amp is standard practice in the application data books, I have never found it necessary to do so. Of course designing for the masses there is no telling what some one will connect to the output.
However, it is sometimes mandatory to have a resistor isolating your probe when looking at high performance and high impedance parts.
More open loop gain, more slew rate and no noise please.
I have..... and why I do it.
Driving an unterminated transmission line (an interconnect) with an inductive source impedance and terminating it with a 10K to 100K load and anyone's guess for capacitance (RCA connector, input filter, etc.) makes for very unpredictable response and overshoot/stability at RF frequencies and we both know how much RF is floating around. It may be very worth while for preamps to have build out resistors for predictable RF stability. One could make the argument that that might be why different cables sound different. One of our well respected designers on the forum (that leaves me off of the list of suspects) even has an RC trap on the output of one of his preamps. I think it pretty well established that an RC filter on the input of an amp is a good idea and I pick values such that the series R is close to the value of the characteristic impedance of the interconnect. A compensation cap across the feedback resistor is a pretty common path into the other input of the diffamp. many prefer to use output stages without loop feedback and outside the preamps feedback loop to lessen these influences. I learn lot reading app note and many application guys are very smart engineers. Right Mr. Jung?
Resistors....... their not for short circuit protect any more.
Driving an unterminated transmission line (an interconnect) with an inductive source impedance and terminating it with a 10K to 100K load and anyone's guess for capacitance (RCA connector, input filter, etc.) makes for very unpredictable response and overshoot/stability at RF frequencies and we both know how much RF is floating around. It may be very worth while for preamps to have build out resistors for predictable RF stability. One could make the argument that that might be why different cables sound different. One of our well respected designers on the forum (that leaves me off of the list of suspects) even has an RC trap on the output of one of his preamps. I think it pretty well established that an RC filter on the input of an amp is a good idea and I pick values such that the series R is close to the value of the characteristic impedance of the interconnect. A compensation cap across the feedback resistor is a pretty common path into the other input of the diffamp. many prefer to use output stages without loop feedback and outside the preamps feedback loop to lessen these influences. I learn lot reading app note and many application guys are very smart engineers. Right Mr. Jung?
Resistors....... their not for short circuit protect any more.
It is interesting that I made a modified version of this design to make the Grateful Dead Line driver, that has to send the stereo signal from the mix board in the audience to the stage, 100 ft or more. This is probably the nastiest load that anyone here would ever encounter.
Actually transconductance amps are MORE STABLE than op amps. This is because, instead of ringing, they become more compensated by the load capacitance.
An op amp would have a follower of some kind. In this case, the cap load is buffered from the compensation, and this creates a separate second rolloff of the high frequencies. This is what causes ringing.
The synthetic inductance is another issue, and an interesting one. I suspect that you could build a 'pathological' transconductance amp that had low gain-bandwidth and low open loop bandwidth that could be problematic. This particular design is high gain-bandwidth and high open loop bandwidth, so it may not have as much problem with synthetic inductance. However, it is an interesting question.
Actually transconductance amps are MORE STABLE than op amps. This is because, instead of ringing, they become more compensated by the load capacitance.
An op amp would have a follower of some kind. In this case, the cap load is buffered from the compensation, and this creates a separate second rolloff of the high frequencies. This is what causes ringing.
The synthetic inductance is another issue, and an interesting one. I suspect that you could build a 'pathological' transconductance amp that had low gain-bandwidth and low open loop bandwidth that could be problematic. This particular design is high gain-bandwidth and high open loop bandwidth, so it may not have as much problem with synthetic inductance. However, it is an interesting question.
interesting
Interesting. I have seen feedback take from the second gain stage and leave the output stage outside the loop in some class A amps. I have also seen some of the feedback taken from the output and some from the driver or second gain stage i.e. it set the closed gain around the output lower than the closed loop gain of the amplifier before the output stage.......... where did I see that? It was an oldy but goody inverting design with a two pair complementary jfet front end and a class A bipolar output stage. I think it was in some hobbiest magazine. Anyone remember? I can remember the amp design which was very novel at the time......but who was the designer.......... I just can't remember
Interesting. I have seen feedback take from the second gain stage and leave the output stage outside the loop in some class A amps. I have also seen some of the feedback taken from the output and some from the driver or second gain stage i.e. it set the closed gain around the output lower than the closed loop gain of the amplifier before the output stage.......... where did I see that? It was an oldy but goody inverting design with a two pair complementary jfet front end and a class A bipolar output stage. I think it was in some hobbiest magazine. Anyone remember? I can remember the amp design which was very novel at the time......but who was the designer.......... I just can't remember
I beg your pardon?
"Fred is just being " " He is implying that I use op amp configurations for power amp design. So why not for line drivers?"
WHAT???????? I did no such thing. The only thing I was trying get across was that feedback around a follower seems often to be avoid by some audio designers and I seem to see it being done more lately.
You made the case for transconductance amp and convinced me that it might be useful to look into more closely for line level circuits.Nested feedback loops are becoming more common as well. Since no seems to want to jump on my clue, the JC3 is the amp where I believe I first saw it. While I gently and respectfully encourage you not to read between the lines. I try to be a what so see is what I meant type of guy. Subtlety is not a trait I am accused of very often. As for reading between the " " I will take it as a mild rebuke in the interest of public decorum and invite you to respond with confidential Email, if I am too far off the mark. If I know anything at all about you from previous Email it is that you are not shy....... Mr. Pass has recently encourage me to play nicely with others and the last thing I would want to do is disappoint him.
Are we on the same page on this?
Respectfully,
Fred
"Fred is just being " " He is implying that I use op amp configurations for power amp design. So why not for line drivers?"
WHAT???????? I did no such thing. The only thing I was trying get across was that feedback around a follower seems often to be avoid by some audio designers and I seem to see it being done more lately.
You made the case for transconductance amp and convinced me that it might be useful to look into more closely for line level circuits.Nested feedback loops are becoming more common as well. Since no seems to want to jump on my clue, the JC3 is the amp where I believe I first saw it. While I gently and respectfully encourage you not to read between the lines. I try to be a what so see is what I meant type of guy. Subtlety is not a trait I am accused of very often. As for reading between the " " I will take it as a mild rebuke in the interest of public decorum and invite you to respond with confidential Email, if I am too far off the mark. If I know anything at all about you from previous Email it is that you are not shy....... Mr. Pass has recently encourage me to play nicely with others and the last thing I would want to do is disappoint him.
Are we on the same page on this?
Respectfully,
Fred
time time time is my side yes it is.
You claim to know me pretty well for a guy who has had lunch with me three times and at Akbars the last time. Food to sooth the savage breast, Right Jam?
The watch analogy is a bit suprising since I don't wear one. Are you still sore about me getting lost and showing up 20 minutes late for lunch number two?
jewilson said:John,
No, Fred is one of those fellows that has to tell you how to build a watch when you only want the time, it just his nature. He knows your big in discrete art of analog design.
You claim to know me pretty well for a guy who has had lunch with me three times and at Akbars the last time. Food to sooth the savage breast, Right Jam?
The watch analogy is a bit suprising since I don't wear one. Are you still sore about me getting lost and showing up 20 minutes late for lunch number two?
Jam:
In Japan, Toyota sells a car called the "S-Car Go!" Reckon your photo would be a great promo shot for that Toyota.
On second thought, maybe not...
Thanks always for the stream o' grins,
jonathan carr
PS. Oops! The Toyota is called FunCarGo, not SCarGo. It was Nissan that used the SCarGo name for a car model. Think it was about 10 years ago, mebbe a few years older than that.
In Japan, Toyota sells a car called the "S-Car Go!" Reckon your photo would be a great promo shot for that Toyota.
On second thought, maybe not...
Thanks always for the stream o' grins,
jonathan carr
PS. Oops! The Toyota is called FunCarGo, not SCarGo. It was Nissan that used the SCarGo name for a car model. Think it was about 10 years ago, mebbe a few years older than that.
John,
I remember this Grateful Dead stage set up, if it was under this time you mean, the linedriver have a lot of cables and power amp inputs to drive.
--Bo
I remember this Grateful Dead stage set up, if it was under this time you mean, the linedriver have a lot of cables and power amp inputs to drive.
--Bo
An externally hosted image should be here but it was not working when we last tested it.
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