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Old 3rd June 2002, 04:33 PM   #1
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Default Temperature compensation for output stages

Hi!

I wondering about the temperature compensation circuits for power amps. Does anyone know if there are some smart way (besides using a temp chip and a microcontroller) of creating suitable mV/degree and suitable DC-level sort of speaking?

With BJT it's quite easy, simple use a diode or a transistor as the temperature sensing element. But when we use mosfets we require 8-10 mV/degree C in temperature coefficient. One way to creating this is to use 4 or 5 PN-junctions.

Better ideas out there? The microcontroller thing is not too bad these days, don't you think? A small PIC12 or AVR would do the job and they are also cheap.
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Old 3rd June 2002, 05:16 PM   #2
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peranders, if you are talking about temperature compensating the bias for a complementary MOSFET output stage, you can do this using a "Vgs multiplier," which is just a Vbe multiplier with a MOSFET instead of a BJT as the temperature sensing element.
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Old 4th June 2002, 01:09 PM   #3
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I'm aware of the Vbe multiplier technic but this is not an advance way to control temp coeff and DC-level at the same time.

When we are using mosfets the temperature coeff variates7-10 mV/degree C is my experience. My idea was some smart circuit to adjust the two parameters rather independantly. One idea is to use a low voltage rail-to-rail opamp together with a temperature sensor of some kind.
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Old 4th June 2002, 02:06 PM   #4
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Lightbulb mosfet bias

Us simple folks use a source resistor. But I guess that is not enough intellectual stimulation for everyone. How about Nelson Pass's scheme for the Aleph current source with a mod to remove the AC component from the base of the bipolar transistor. (Hint: two resistors and a capacitor.)

H.H.

http://rube.iscool.net/
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Old 4th June 2002, 02:57 PM   #5
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HarryHaller, please be nice and DO NOT upload pictures which are "only fun". I'm a boring guy and I want to discuss the topic only.
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Old 4th June 2002, 08:23 PM   #6
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I seem to remember (oh, oh, already suspect) a Siliconix FET or MOSFET design book that had the scheme of two Vbe multipliers in series: one to control bias voltage and another to control temperature coefficient.

I'll try to look it up tonight and post later. What I'm saying now might not be making much sense.
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Old 5th June 2002, 09:43 AM   #7
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http://www.analog.com/library/applic...udio/AN211.pdf

The Alexander amp has a circuit with a LM431 to fix the DC-level and a transistor to fix the temperature coeff. The disadvantage of this is that the temperature coeff is somewhat limited. You can't choose 10mV/ degree and have 5 volts across.

To use a microcontroller is nowadays no problem. We have 8-pin devices (PIC, AVR) and good possibilities to read temperature.
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Old 5th June 2002, 09:51 AM   #8
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Default I'm a boring guy and I want to discuss the topic only.

"How about Nelson Pass's scheme for the Aleph current source with a mod to remove the AC component from the base of the bipolar transistor. (Hint: two resistors and a capacitor.)"

I believe I did discuss the topic. Perhaps if you explain why you are trying to control the output stage bias current so precisely it would help.

H.H.
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Old 5th June 2002, 11:32 AM   #9
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Do a websearch on the terms "offset" and "slope". A couple of opamps and a silicon diode should do the trick. The diode forward voltage reduces about 2.2mV per deg C I think.

The offset sets the bias at startup when the devices are at basically room temp. The slope refers to the gain of the circuit which translates into how many mV reduction in bias per deg C increase.

Actually, here's a page with a schematic that illustrates the principle. 5k pot is offset, 10k pot is slope (gain).
http://www.davidbray.org/onewire/barometer.html
Set the offset with a cold amp, then set the slope with a hot amp.

GP.
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Old 5th June 2002, 02:38 PM   #10
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Default Re: I'm a boring guy and I want to discuss the topic only.

Quote:
Originally posted by HarryHaller
Perhaps if you explain why you are trying to control the output stage bias current so precisely it would help.

H.H.
Picture this:

Voltage across the gates of the output stage = 5 V (at room temp)

Temperature coeff = 9-11 mV/ degree C

With a simple transistor solution I will get around 6-8 volts and if the voltage is trimmed to 5-5.5 volts the temperature coeff will be too low.

My problem concerns only MOSFET's, no problems with BJT's.
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