N-Channel mosfet amplifier with servo bias

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi,

I'm going to built a new amp whith what i have in my drawers so i did the attached schematic but i have many questions before drawing a pcb because i'm not sure of what i did.

I want to keep bias current constant so i measure it by calculating the difference of the total current trough the output transistors and the current in the speaker. A slow integrator of difference adjusts the current generators Q1 and Q3 .
I want to use only one quad amp-op so i drawed a single phase rectifier for the speaker current and substract it twice to the total current.
When A4 output is 0V, i choose R6 and R11 to have only half of bias current, so the current grows slowly when starting the amplifier.
I'd like to have your comments about amplifier section also (does it work ?)
Can you simulate it with spice ?
Any comment is welcome
Thanks
 

Attachments

  • nmosservobias.pdf
    27.5 KB · Views: 1,431
Hi Perenders
I began to draw a sample and hold that sample voltage on R23 R24 when volage is about 0V at the output but it should be able to sample/hold in 10ns , more, V and I are not in phase because a speaker is not a resistance !! Then i thought this way "could" work........i'm very interrested in the application you speak about.
Maybe measuring the energies dissipated by R22,23,24 could be an other way ?
rgds:rolleyes:
 
I had an idea of an auto bias circuit:

Between the gates, place a microcontroller with a temperature sensor. The output of the MCU is PWM or a DAC. Vgs vs temperature is measured and stored in the MCU. Variyng temp => Varying Vgs

The thing was to let this MCU be floating and be driven directly from ther gate drive voltage.

This idea never left my brain....and got into the computer or lab.
 
Siliconix circuit

I found it, but the article is copyrighted so I can't post the schematic without permission (I emailed Siliconix). Anyway, the article is by Bill Roehr and it may have originally been an AES preprint. It's dated April 1982 and titled "The Autobias Amplifier." Shown are 25 and 50 watt circuits. These are all single-supply, bipolar input, n-channel MOSFET output amps with a big output cap. Brrrr!
 
Peranders : sure we can do nice things whith a microcontroller, i'd preffer to keep it for a direct numeric input amp with class D output....(hope to find a project one day in the digital forum) my desire is not to "compensate" (i could do a curve with ampop, zeners,res, and a temperature sensor, depending of the Vgs caracteristic) but to control with closed loop, just for fun and experimentation.
In my bipolar amp, i see clearly with the scope, the bias current passing through the emitter resistances. This current never gets lower than the minimum value, so i had another idea and will post it soon.

SY : I thing i found the application you are speaking about :
http://wes.feec.vutbr.cz/~gratz/Download/Roehr.ZIP
Single supply and all AC coupled......I don't understand all, Vgs decreases non linear with Id....I have to take time to analyse.

Tks for your posts
 
Hi
I did a new servobias schematic (see attached)

The idea is to compare the current crossing R22 and R23 to a minimum value witch is the image of the bias current. If it goes under this value, a slow integrator will increase the bias current by changing the voltages at Q1 and Q3 emitters.
And if it goes greater ? no problem, a negative current is continuously injected in the input of the integratorso the output of A3+D1 must be 100mw, corresponding to an error of 0.5mA of the bias current.

With this method, there is no need to measure an AC current at the output, all what is usefull is that the continuous part of the current does not go under the choosen value.
(sorry for syntax errors)
:xeye:
rgds
 

Attachments

  • nmosservobias2.pdf
    31.4 KB · Views: 749
You had better use a very fast...FET or FET hybrid op amp for A1. The difference in what is needed to drive this quasi complementary pair output is going to be different for each device. Some very high frequency harmonics are going to be needed here to cancel those created by the bottom IRFP460, acting as a voltage amplifier, or common source. The top one is a source follower or current amplifier. If complementary operation is what you are looking for in this circuit, you should use complementary N-ch & P-ch. However, if you want my opinion, you should rebias this circuit as a class A amplifier and use the top IRFP460 as the output transistor as a source follower(as is now), and use the bottom IRFP460 as a current source, biasing Ids bias through the top one. Direct couple to speaker as is currently shown, I bet you will get better overall results without so much herky-jerky.:rolleyes:

May its not as power efficient as class AB complementary or quasi-complementary, but quality would be better...of course you wouldn't need a servo bias circuit.
 
Bias ideas and comments

Some years back Linear Technology was kind enough to send me some samples of one of their new 8 pin function blocks, the LT1166. This IC is designed for power output stage automatic bias. Although it is not directly suitable in the circuit outlined in this tread it could possibly be adapted to provide a good portion of the required functionality. They had a nice 16 page brochure that may be available on their web site as a application note that gives a great deal of detail on the LT1166.

In general any type of automatic bias adjustment acn add substantially more complexity to a power amplifier circuit with potential negative benefits. With a well designed bias network that monitors heat sink temperature and without doing any kind of sampling the output stage current the output stage current can be kept constant within 1.5% over a temperature range of 110 to 190 degrees Fahrenheit. Such a method can do better than this over the desired operating temperature range of 135 to 155 Fahrenheit, (145 or so being preferred for maximum stability). The bias circuit I used in my 1R1 amplifier that Per-Anders is placing on his web site has the ability to do this. The drawback of using Bias control networks such the one I used in that amplifier is that it is sensitive to construction of the heat sinks and thus it is optimized for a specific heat sink design, and also affected by the placement of the devices on the heat sink.

My feeling is that I’m generally against using any type of auto bias circuit that adjust bias by measuring the voltage drop across resistors placed in series with the output devices. I have a gut feeling that such current sensing method look better on paper than they work in actual practice when applied to audio power amplifiers. Such methods been tried by quite a number of manufacturers and appear to all become abandoned after a period of time. This of course does not mean that we don’t experiment with auto bias and other method to stabilize bias settings.

If a isolated method of controlling bias is required then a very useful circuit of for making a dual opto isolator very linear with an op amp was published in Electronic Design quite a number of years back. It provides a good voltage in versus a current output curve. It used one section of the dual opto for driving a isolated load and the other half of the opto for providing feed back to the op amp. The two led light sources being in series. As I recall the LED current was also part of the feedback loop to the op amp. One of the inputs to the op amp excepted the input drive voltage. I had used it in the past for some experiments and it gave excellent results. Thus such a circuit should be kept in mind when the need for controlling current or bias and isolation is required.

Johannes
 
Hi cunningham
I will try many ampop at the input, i have some ad846 and i dont know what to do with them, they have a 450V/µs slew rate but i don't think i'll need that.
I thing the 2 mosfets are working the same way : Vgs are both created by a current passing through a resistance connected to the source. The top res is not connected to the output. I will connect the top res under the 0.22R of the source and add one in the source of the bottom one, then current gain of the (output devices+R18,R19) will be approx 230 . That is not more than a darlington in bipolar configuration.
Bias (without compensation) is only made by the voltages at Q1 and Q3 bases so the output swing of the ampop must stay low to keep the same bias in the top mosfet then the bottom one is driven and viceversa (it's chance that id=f(Vgs) is flat for the bias zone and vertical after). Thath's why it needs a lot of open loop gain after the ampop. I want to keep an AB class and 2 N-Channel because i have a full gerkin pot of them !!
Bias current will be about 100mA.
What do you think of the fact that, the bias current decreases in the one device then the current increases in the oposite one ?
Rgds
 
Hi Alaskanaudio
Tks for the information about LT1166, it seems to be a very easy way to make a bias control and currents limits. What i found about it is that it has been made for a complementary output. I did not find any on websites of french electronic parts sellers.
I put a irfp460 on a heatsink, tuned Vgs to get Id=100ma (Vrail 36V and a lamp in the drain) and had 3.5V at Vgs. I have increased current to 3A to warm it to 80°C and returned to 100mA, Vgs was then 200mA. (at 20°C it needs 3.6V to have 200mA). I think 0.1V is a "big" values and easy to compensate.
I'm right with you, compensating can't generate unstability as servo coulld do, but, what i want to try, is a very slow corrective closed loop because temperature does not move fast with a big inertia heatsink.
I have not understand yet how works the controller with dual opto isolator you are describing but i'm interrested in it. Could you post a link vhere i could found a schematic ?
Tks for your reply
Rgds
 
Opto isolation bias control

f4bok,

In regards to using dual opto isolators for controlling bias in conjunction with a opamp. I have searched all my old computer files and have not been able to find the circuit diagram. I will continue to look for it. But I may have to draw it into a simulator from memory. My memory however does not recall all details.

Johannes
 
The OPTO isolation circuit

I have tried to remember the isolation circuit I mentioned before in this tread. I believe that the link to the circuit I have drawn below is very close to being correct. It offers a voltage or current input and can convert that into an isolated current output. The circuit shown used a dual OPTO but a three or four section OPTO can be used if more isolated outputs are required. The circuit is not a complete bias system; it only covers the isolation section. Make component or circuit changes as required.

The PDF file is here. Some older versions of acrobat reader will not be able to read the file. http://www.audioamps.com/Misc/OPTO Bias.pdf

This circuit idea was originally published in Electronic Design Magazine in the early 1990’s. The circuit listed here was drawn strictly from memory so it may not be exactly alike. Component values listed were tested in Multisim and may need to be optimized for best input to output current tracking. The tracking range is roughly 0 to 20ma. A larger range is possible. OPTO devices with MOSFET outputs may also be used.

Accuracy is enhanced by the use of multiple OPTO devices in one package which also provides good thermal tracking between the feedback and isolated output section.

Johannes
 
Hi Alaskanaudio
Tks for schematic. Its a good way to have linear output , the 2 optos, in the dual opto must have exactly the same current transfert ratio. I made a prototype of the amplifier, changed many values, it works all right now without the auto bias control, i'll do it in october and will post a new schematic. I have a little non-linearity of voltage output but i'm not sure beacause the output load is made of 2 // 15 Ohm big wired res. slew rate is 30V/µs with an opa27, 33pf in direct feed back of the ampop, 5.6pf in the output feedback, and 5.6pf between BandC of Q 2and Q6 (tip42c today). I did not measure distorsion yet.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.