Apr 30, 2010 #1 K kwanmajun Newbie level 5 Joined Apr 30, 2010 Messages 8 Helped 0 Reputation 0 Reaction score 0 Trophy points 1,281 Location orlando Activity points 1,331 Hi, recently i got a interview question about the poles" we have a open loop gain 100db, with phase margin 55, bandwidth 10MHz, then where are the two poles?" Please give me the details answers, thanks, waiting for your answers. thanks~
Hi, recently i got a interview question about the poles" we have a open loop gain 100db, with phase margin 55, bandwidth 10MHz, then where are the two poles?" Please give me the details answers, thanks, waiting for your answers. thanks~
Apr 30, 2010 #2 WimRFP Advanced Member level 5 Joined Apr 16, 2010 Messages 1,589 Helped 594 Reputation 1,176 Reaction score 538 Trophy points 1,393 Location Netherlands Activity points 15,026 How to find poles? Hello, 100 dB = gain of 100k. So first -3 dB point is around 10M/100k = 100 Hz. I asume that the 10 MHz bandwidth is based on the slope of the first pole. So around 1kHz to 1 MHz you have a 90 degrees phase shift. 55 degrees phase margin (at unity gain I guess) means that the second pole introduces a phase shift of 180-90-55 = 35 degrees at 10 MHz. As the last pole does less then 45 degrees, it is above 10 MHz. I did a fast phasor diagram calculation, the second pole must be around 14 MHz. The gain/phase versus frequency plot in the datasheet should show you the -20dB/dec slope, and the further decreasing slope above 10 MHz.
How to find poles? Hello, 100 dB = gain of 100k. So first -3 dB point is around 10M/100k = 100 Hz. I asume that the 10 MHz bandwidth is based on the slope of the first pole. So around 1kHz to 1 MHz you have a 90 degrees phase shift. 55 degrees phase margin (at unity gain I guess) means that the second pole introduces a phase shift of 180-90-55 = 35 degrees at 10 MHz. As the last pole does less then 45 degrees, it is above 10 MHz. I did a fast phasor diagram calculation, the second pole must be around 14 MHz. The gain/phase versus frequency plot in the datasheet should show you the -20dB/dec slope, and the further decreasing slope above 10 MHz.
Apr 30, 2010 #3 K kwanmajun Newbie level 5 Joined Apr 30, 2010 Messages 8 Helped 0 Reputation 0 Reaction score 0 Trophy points 1,281 Location orlando Activity points 1,331 How to find poles? Got it, thanks man~