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How do we calculate the values of the boot strap resistors ?
How do we calculate the values of the boot strap resistors ?
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Old 7th October 2010, 07:47 AM   #1
tessier is offline tessier  Canada
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Default 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.
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Old 7th October 2010, 02:43 PM   #2
tessier is offline tessier  Canada
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Hi

Nobody who can reply me ?

Pleeeaaaase

Thanx

Paul
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Old 7th October 2010, 03:11 PM   #3
wg_ski is online now wg_ski  United States
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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.
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Old 8th October 2010, 09:38 AM   #4
tessier is offline tessier  Canada
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Hi

Yes, it's 16 volts rails.

Thanx

Paul
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Old 8th October 2010, 12:27 PM   #5
ostripper is offline ostripper  United States
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Quote:
Originally Posted by tessier View Post
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

OS
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