Voltage across Q4, Q5 transistors

rchua77 · 29th November 2004, 06:50 PM

Hello,

I have hardwired a pair of Aleph 5 boards and the output offset voltages are well, 53mV and 62mV, voltage across source resistors are 0.6v, voltage across zener Z5 is 9.5v.

But the voltage across transistors Q4 and Q5 are 6.5v. Has anyone encountered such a problem before and would this cause a problem for the amplifier. I have varied the value of the R17 (4k7) resistor but it only reduces the voltage across the transistors by 0.5v.

Thanks for any help rendered

Chua

Nelson Pass · 29th November 2004, 10:03 PM

Which are Q4 and Q5 in your schematic? Are they the NPN
transistors?

rchua77 · 30th November 2004, 07:26 AM

Hi

Yes they are the NPN transistors on the schematic.

Nelson Pass · 30th November 2004, 03:18 PM

This implies a Vgs on the output devices of 5.9 Volts which
seems a bit high. What type of outputs are you using, how
many, and what value Source resistors?

rchua77 · 30th November 2004, 07:15 PM

I'm using source resistors of 1Ohm. Running at from a bench supply of +/-20v. I'm using total 4 MOSFETS (Infineon MOSFETS)
here's the datasheet. http://www.infineon.com/cmc_upload/d...20N60CFDv1.pdf
Incidentally, the Vgs averaged out around 5.20 for these devices.

Just out of curiousity, having used matched differential input pairs (IRF9610), i couldnt quite get the DC offset below 100mV. Temperature of course plays a big role. But would swopping and changing 9610s until i get an offset as low as possible be a good method of reducing offset? Even though the Vgs of the input differential pair wont be matched anymore.

Gaucho · 30th November 2004, 09:36 PM

I think your best bet would be to try to get the Vgs on the output mosfets matched.

Gaucho · 30th November 2004, 10:11 PM

It maybe possible to 'quasi' match the existing MOSFETS in the circuit by placing some resistance from either the gate-to-Source or from the drain-to-Source of the output MOSFET pins. The idea of putting resistance from the drain-to-Source, however doesn't seem very practical in an output stage.

Perhaps Mr. Pass can advise us if this is or isn't reasonable solution?

Nelson Pass · 30th November 2004, 10:45 PM

Quote:

Originally posted by rchua77
I'm using source resistors of 1Ohm.

Typical source resistors in an Aleph are .47 to 1 ohm.
Are you actually using 10 ohms in the amp?

I see not. I looked up the Mosfets, and they do indeed
run about 6 volts Vgs. No reason why it won't work, but
you should increase the resistor off the drain of Q1 (the left
handed input transistor) to compensate and have equal
current sharing on the input diff pair.

rchua77 · 1st December 2004, 08:42 AM

Thank you for the help. I will re-insert the matched pairs, and use a pot trimmer in place of the resistor in the Drain of the input differential pair Q1 and trim it until the dc offset voltage is ideal.

Once again, many thanks!

rchua77 · 1st December 2004, 09:59 AM

New update, I have replaced the matched input differential pair and inserted a pot trimmer in place of the resistor at the Drain of Q1.

I have adjusted the trimmer to give nearly 10mV offset. Give and take due to temperature. But there is a difference in the trimmer values when i measured them, one is 235 Ohms, and the other is 535 Ohms.

Should this be a concern or should I call it a day and be grateful there was no magic smoke

29th November 2004, 06:50 PM	#1
rchua77 diyAudio Member Join Date: Jul 2003 Location: Back in Singapore	Voltage across Q4, Q5 transistors Hello, I have hardwired a pair of Aleph 5 boards and the output offset voltages are well, 53mV and 62mV, voltage across source resistors are 0.6v, voltage across zener Z5 is 9.5v. But the voltage across transistors Q4 and Q5 are 6.5v. Has anyone encountered such a problem before and would this cause a problem for the amplifier. I have varied the value of the R17 (4k7) resistor but it only reduces the voltage across the transistors by 0.5v. Thanks for any help rendered Chua

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29th November 2004, 10:03 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Which are Q4 and Q5 in your schematic? Are they the NPN transistors?

30th November 2004, 07:26 AM	#3
rchua77 diyAudio Member Join Date: Jul 2003 Location: Back in Singapore	Hi Yes they are the NPN transistors on the schematic.

30th November 2004, 03:18 PM	#4
Nelson Pass The one and only Join Date: Mar 2001	This implies a Vgs on the output devices of 5.9 Volts which seems a bit high. What type of outputs are you using, how many, and what value Source resistors?

30th November 2004, 07:15 PM	#5
rchua77 diyAudio Member Join Date: Jul 2003 Location: Back in Singapore	I'm using source resistors of 1Ohm. Running at from a bench supply of +/-20v. I'm using total 4 MOSFETS (Infineon MOSFETS) here's the datasheet. http://www.infineon.com/cmc_upload/d...20N60CFDv1.pdf Incidentally, the Vgs averaged out around 5.20 for these devices. Just out of curiousity, having used matched differential input pairs (IRF9610), i couldnt quite get the DC offset below 100mV. Temperature of course plays a big role. But would swopping and changing 9610s until i get an offset as low as possible be a good method of reducing offset? Even though the Vgs of the input differential pair wont be matched anymore.

30th November 2004, 09:36 PM	#6
Gaucho diyAudio Member Join Date: Mar 2004 Location: Stuck in the 80's	I think your best bet would be to try to get the Vgs on the output mosfets matched.

30th November 2004, 10:11 PM	#7
Gaucho diyAudio Member Join Date: Mar 2004 Location: Stuck in the 80's	It maybe possible to 'quasi' match the existing MOSFETS in the circuit by placing some resistance from either the gate-to-Source or from the drain-to-Source of the output MOSFET pins. The idea of putting resistance from the drain-to-Source, however doesn't seem very practical in an output stage. Perhaps Mr. Pass can advise us if this is or isn't reasonable solution?

1st December 2004, 08:42 AM	#9
rchua77 diyAudio Member Join Date: Jul 2003 Location: Back in Singapore	Thank you for the help. I will re-insert the matched pairs, and use a pot trimmer in place of the resistor in the Drain of the input differential pair Q1 and trim it until the dc offset voltage is ideal. Once again, many thanks!

1st December 2004, 09:59 AM	#10
rchua77 diyAudio Member Join Date: Jul 2003 Location: Back in Singapore	New update, I have replaced the matched input differential pair and inserted a pot trimmer in place of the resistor at the Drain of Q1. I have adjusted the trimmer to give nearly 10mV offset. Give and take due to temperature. But there is a difference in the trimmer values when i measured them, one is 235 Ohms, and the other is 535 Ohms. Should this be a concern or should I call it a day and be grateful there was no magic smoke

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