open-loop gain vs load condition

panson_hk · 16th July 2010, 05:32 AM

Does open-loop gain of an amplifier change with load condition? Is it worth to investigate? Does it provide us valuable information for improving an amplifier?

stratus46 · 16th July 2010, 05:36 AM

Quote:

Originally Posted by panson_hk

Does open-loop gain of an amplifier change with load condition? Is it worth to investigate? Does it provide us valuable information for improving an amplifier?

If it does it needs to be repaired.

G²

panson_hk · 16th July 2010, 05:44 AM

Output impedance is non zero (voltage amp model). Heavier loads need higher input level to maintain the same output level. This is effectively a lower open-loop gain. Right?

Ipanema · 16th July 2010, 06:06 AM

If the output is not buffered SF/EF, then the OL gain will change with varying output load.

lineup · 06:31 AM

An input voltage will produce a certain amount of current at output.
Most amplifiers (open loop) produce a current that is more or less proportional to the input voltage.
Putting output current across a load with X Ohm will make a voltage on the load.
Current x Ohm = Voltage

When measure Open Loop Voltage Gain, OLG, I think the usual is to use Nominal load.
What the amplifier is intended to drive.
For example 8 Ohm or 4 Ohm.

Another number to measure would be Open Loop Current Gain / Input voltage.
This would be less dependant of the load
but still somewhat different at different loading.

Samuel Groner · 07:37 AM

Quote:

If it does it it needs to be repaired.

If you design one which doesn't do it I'd hurry up and get it patented.

Quote:

Output impedance is non zero (voltage amp model). Heavier loads need higher input level to maintain the same output level. This is effectively a lower open-loop gain. Right?

Spot on.

Quote:

Does it provide us valuable information for improving an amplifier?

Yes and no. If the change in o/l gain is static (i.e. independent of output voltage and current) there's nothing interesting going on--a reduction in loop gain then simply corresponds to a (usually pretty subtle) change in closed-loop gain. Much more interesting are dynamic o/l gain changes, which are just another way of saying that there is some distortion going on.

I suggest you also look at NSC AN-1485.pdf (big file!). Note that the thermal output-input coupling of IC opamps does not apply to discrete power amps to any significant degree but still the measurements are worth looking at.

Samuel

panson_hk · 16th July 2010, 08:06 AM

Samuel, Thank you for the app note link.

16th July 2010, 05:32 AM	#1
panson_hk :) diyAudio Member Join Date: Aug 2005 Blog Entries: 10	open-loop gain vs load condition Does open-loop gain of an amplifier change with load condition? Is it worth to investigate? Does it provide us valuable information for improving an amplifier?

16th July 2010, 06:29 AM	#5
lineup diyAudio Member Join Date: Dec 2005 Location: the north	An input voltage will produce a certain amount of current at output. Most amplifiers (open loop) produce a current that is more or less proportional to the input voltage. Putting output current across a load with X Ohm will make a voltage on the load. Current x Ohm = Voltage When measure Open Loop Voltage Gain, OLG, I think the usual is to use Nominal load. What the amplifier is intended to drive. For example 8 Ohm or 4 Ohm. Another number to measure would be Open Loop Current Gain / Input voltage. This would be less dependant of the load but still somewhat different at different loading. __________________ lineup Last edited by lineup; 16th July 2010 at 06:31 AM.

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
Open loop gain, phase margin etc	ceharden	Solid State	67	22nd November 2011 02:16 PM
Throwing away open loop gain with an opamp near or at unity gain	okapi	Pass Labs	3	25th December 2009 01:23 PM
Open Loop Gain Measurement? Fotios Need Help!	fotios	Solid State	21	22nd April 2008 12:02 AM
open loop gain,how many is enough?	Leolabs	Solid State	28	17th May 2006 09:00 PM
Open loop gain (High or low) ?	JensRasmussen	Solid State	19	16th September 2002 09:26 PM

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

16th July 2010, 05:44 AM	#3
panson_hk :) diyAudio Member Join Date: Aug 2005 Blog Entries: 10	Output impedance is non zero (voltage amp model). Heavier loads need higher input level to maintain the same output level. This is effectively a lower open-loop gain. Right?

16th July 2010, 06:06 AM	#4
Ipanema diyAudio Member Join Date: Jun 2004 Location: KL	If the output is not buffered SF/EF, then the OL gain will change with varying output load.

16th July 2010, 08:06 AM	#7
panson_hk :) diyAudio Member Join Date: Aug 2005 Blog Entries: 10	Samuel, Thank you for the app note link.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)