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7th October 2010, 07:47 AM  #1 
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Join Date: May 2008

How do we calculate the values of the boot strap resistors ?
Hi
How do we calculate the values of the boot strap resistors ? For any standard pushpull complementary amps, assuming we use a vas transistor with a ft of 120 mhz at 10 ma, do we just need to ajust the boot strap resistors to have 10 ma for the vas transistor ? Let say, as an example, we need as low as few hundred ohm resistors to get that 10 ma vas current, is there a minimum value for the boot strap resistors ? Is there mores to take account in the boot strap resistors values calculations ? Thanx Paul Last edited by tessier; 7th October 2010 at 08:05 AM. 
7th October 2010, 02:43 PM  #2 
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Join Date: May 2008

Hi
Nobody who can reply me ? Pleeeaaaase Thanx Paul 
7th October 2010, 03:11 PM  #3 
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Join Date: Oct 2007

If you want 10ma of current, the you use the resistors that give you 10 mA. You must be dealing with lowish Vcc's if you're getting values of "several hundred ohms". At low rails you may not need or want 10mA unless either the fT or linearity really does fall like a stone at lower current. The optimum split has no easy answer  other than equal values resulting in the minimum size bootstap cap for a desired open loop pole frequency.

8th October 2010, 09:38 AM  #4 
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Join Date: May 2008

Hi
Yes, it's 16 volts rails. Thanx Paul 
8th October 2010, 12:27 PM  #5 
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Join Date: May 2008
Location: Smoky Mountains , Tennessee

Yes , Resistance for low rails like that will be sub 1K (820R X 2)@ 10mA+ VAS current , I took my "blameless" , added a bootstrap (blame ES like) and needed 2 X 5.6K for 75v rails to get my 6.5mA VAS standing current. At +  20V , the same amp needed 2 X 1.5K to remain at that 6.5mA. (below)
Gain needs to be lower at 20v , clipped at .25v input.. 38db gain .. no, no OS
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