How do we calculate the values of the boot strap resistors ?
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 7th October 2010, 07:47 AM #1 tessier   diyAudio Member   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 push-pull 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 tessier   diyAudio Member   Join Date: May 2008 Hi Nobody who can reply me ? Pleeeaaaase Thanx Paul
 7th October 2010, 03:11 PM #3 wg_ski   diyAudio Member   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 tessier   diyAudio Member   Join Date: May 2008 Hi Yes, it's 16 volts rails. Thanx Paul
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Quote:
 Originally Posted by tessier Hi Yes, it's 16 volts rails. Thanx Paul
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

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