I have a question.
With source follower Mosfet, I read somewhere that if you parallel 2 Mosfet the total capacitance is:
Cinput = Cgs1 + Cgd1 + Cgd2
So you have to consider only one Cgs
Is it true? Where this calculation come from?
Thanks
With source follower Mosfet, I read somewhere that if you parallel 2 Mosfet the total capacitance is:
Cinput = Cgs1 + Cgd1 + Cgd2
So you have to consider only one Cgs
Is it true? Where this calculation come from?
Thanks
In a source follower the Cgs is considerably reduced since the source tracks the gate voltage (with a CC load this is pretty accurate tracking and the Cgs isn't seen).
@Giorgio_Milan
Ciss = Cgs + Cgd
From a theoretical point both have to be considered if you want to find out total Ciss, the formula you have typed isn't correct, either whom wrote it must have mistyped it, or perhaps you may have misread it.
Ciss = Cgs + Cgd
From a theoretical point both have to be considered if you want to find out total Ciss, the formula you have typed isn't correct, either whom wrote it must have mistyped it, or perhaps you may have misread it.
Just consider that if both devices are the same and in parallel, the formula must be symmetric in #1 and #2.
Even though the follower topology bootstraps the Cgs, effectively reducing the value, it is still there for both devices.
The bootstrapped value of Cgs is reduced by a factor of (1 - gain factor). Recall that the gain factor is less than unity.
Even though the follower topology bootstraps the Cgs, effectively reducing the value, it is still there for both devices.
The bootstrapped value of Cgs is reduced by a factor of (1 - gain factor). Recall that the gain factor is less than unity.
Noted, thanks all.
Lets do some calculations.
If I use two couple of 2SJ162+2sk1058, with an Rsource added of 0.22, to drive a 8ohm load:
2SJ162:
Cgs = 860pF
Cgd = 40pF
Cds = 360pF
Using this formula:
Cin = Cgd + {Cgs * (1 - Gain)}
gain mos source follower: 8.22/(8.22+1) = 0.89154 -> 1-0.89 = 0.10846
Cin = 860pF*0.10846+40pF = 133.2756pF
2SK1058:
Cgs = 590pF
Cgd = 10pF
Cds = 340pF
Using this formula:
Cin = Cgd + {Cgs * (1 - Gain)}
gain mos source follower: 8.22/(8.22+1) = 0.89154 -> 1-0.89154 = 0.10846
Cin = 590pF*0.10846+10pF = 73.99pF
With two MOS in parallel:
2 mos SJ162: 133.2756pF*2 = 266.5512pF
2 mos 2SK1058: 73.99pF*2 = 147.98pF
Ctot = 174.6pF+84.9pF = 414.5312pF
This is the Cap to consider to calculate the VAS current to drive the MOS
Please feedbacks if I do any mistake
Lets do some calculations.
If I use two couple of 2SJ162+2sk1058, with an Rsource added of 0.22, to drive a 8ohm load:
2SJ162:
Cgs = 860pF
Cgd = 40pF
Cds = 360pF
Using this formula:
Cin = Cgd + {Cgs * (1 - Gain)}
gain mos source follower: 8.22/(8.22+1) = 0.89154 -> 1-0.89 = 0.10846
Cin = 860pF*0.10846+40pF = 133.2756pF
2SK1058:
Cgs = 590pF
Cgd = 10pF
Cds = 340pF
Using this formula:
Cin = Cgd + {Cgs * (1 - Gain)}
gain mos source follower: 8.22/(8.22+1) = 0.89154 -> 1-0.89154 = 0.10846
Cin = 590pF*0.10846+10pF = 73.99pF
With two MOS in parallel:
2 mos SJ162: 133.2756pF*2 = 266.5512pF
2 mos 2SK1058: 73.99pF*2 = 147.98pF
Ctot = 174.6pF+84.9pF = 414.5312pF
This is the Cap to consider to calculate the VAS current to drive the MOS
Please feedbacks if I do any mistake
Are these capacitances used to calculate the values of the MOS Gate resistors to be added?
In case, to have the same Ft on both N MOS and P MOS:
1/(2*Pi*Rgate_N_MOS*74pF) = 1/(2*Pi*Rgate_P_MOS*133pF)
In case, to have the same Ft on both N MOS and P MOS:
1/(2*Pi*Rgate_N_MOS*74pF) = 1/(2*Pi*Rgate_P_MOS*133pF)
for 2sk1058 i use 330R, and for 2SJ162 i use 220R - using lower values may give rise to oscillations
use same values for 1 pair, 2 pair etc
use same values for 1 pair, 2 pair etc