Drift of MOSFET source voltage

namui · 5th February 2001, 07:06 AM

Hi folks,
Let a simple amp be constructed this way :
1) Two single-ended class A MOSFET amps driven by a set of balanced input
and driving the load across the source pins of the two MOSFETs
2) Each amp is biased by an active current source connected between the
MOSFET's source pins and the negative supply voltage and the drain pins
are connected directly to the positive supply voltage.
So it's two followers driving the load in balanced fashion.
The potential problem is the drift in the source pin voltages of the two amps
which will appear across the load as drifting offset, caused by the temperature
increase. Unless the two MOSFETs are exactly the same, the Vgs variation will
be slightly different.
Is there any way besides capacitive coupling to the load or bias servo circuit
to keep the offset voltage practically low and constant (says within +/- 50mV) ?
Would source resistors help ?

Thanks,

[Edited by namui on 02-05-2001 at 02:12 AM]

djk · 5th February 2001, 12:00 PM

If the amp is to be a no global feedback design a servo or coupling cap will be required.To eliminate the active current source see this patent. http://www.delphion.com/cgi-bin/view...d/US04229706__ This will cut power disipation in half.A FET follower may replace the BJT follower.If a discrete op-amp is used only one is required.This amp may be constructed with a single 5687 or 6H30Pi dual triode tube connected as a differential pair driving a pair of Magnetec BUZ900D.Bias and DC balance to be set up like a PP tube amp.If the pair of Magnetec outputs are on a common heatsink the Vgs is stable enough that the DC off-set will not drift.One pair of the Magnetec will deliver 100 watts in class A with this design or 200 watts in class AB.The amp described in the patent was the Sumo Model 9.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
MOSFET current source as triode load	Craig405	Tubes / Valves	46	14th April 2010 12:06 PM
Do I have to match voltage Gate-Source of mosfet in SMPS?	Charoenchai	Car Audio	5	17th November 2006 09:28 PM
temp drift in MOSFET Vgs	SemperFi	Solid State	6	29th June 2006 04:50 PM
load on source follower choke or mosfet?	AudioGeek	Solid State	57	2nd September 2004 02:42 AM
simple MOSFET source folower suggest me please	aiwonsi	Solid State	22	26th July 2003 04:56 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

5th February 2001, 07:06 AM	#1
namui diyAudio Member Join Date: Nov 2000	Hi folks, Let a simple amp be constructed this way : 1) Two single-ended class A MOSFET amps driven by a set of balanced input and driving the load across the source pins of the two MOSFETs 2) Each amp is biased by an active current source connected between the MOSFET's source pins and the negative supply voltage and the drain pins are connected directly to the positive supply voltage. So it's two followers driving the load in balanced fashion. The potential problem is the drift in the source pin voltages of the two amps which will appear across the load as drifting offset, caused by the temperature increase. Unless the two MOSFETs are exactly the same, the Vgs variation will be slightly different. Is there any way besides capacitive coupling to the load or bias servo circuit to keep the offset voltage practically low and constant (says within +/- 50mV) ? Would source resistors help ? Thanks, [Edited by namui on 02-05-2001 at 02:12 AM]

5th February 2001, 12:00 PM	#2
djk diyAudio Member Join Date: Feb 2001 Location: USA	If the amp is to be a no global feedback design a servo or coupling cap will be required.To eliminate the active current source see this patent. http://www.delphion.com/cgi-bin/view...d/US04229706__ This will cut power disipation in half.A FET follower may replace the BJT follower.If a discrete op-amp is used only one is required.This amp may be constructed with a single 5687 or 6H30Pi dual triode tube connected as a differential pair driving a pair of Magnetec BUZ900D.Bias and DC balance to be set up like a PP tube amp.If the pair of Magnetec outputs are on a common heatsink the Vgs is stable enough that the DC off-set will not drift.One pair of the Magnetec will deliver 100 watts in class A with this design or 200 watts in class AB.The amp described in the patent was the Sumo Model 9.