I have the following problem to solve from this year's exam
The picture and the asc files are attached!
I have chosen random mosfets and BJTs, but for the problem I have the following transistors info
BJT's
hFE = 200
1/hoe = 200 kΩ
hie = 25 mV/Ib
MOSFETs
K= 0.5 mA/V²
Vth = 1 V
The Vo is at R4.
First question is to find the Qpoint for the MOSFETs and for the BJTs. Teacher also says to ignore the base current of Q3 when finding the Qpoint of M1 and M2.
This is the circuit:
I started with the following:
Q1
Vbe = 0.67 V
Vce = Vbe = 0.67 V
Vc = Vcc + Vce = 15 V + 0.67 V = 14.33 V
I_R3 = (Vcc  Vc) / R3 = (15 V  (14.33 V)) / 7.2 kΩ = 4.07 mA
I'm not sure if what I done is correct or not, but I think it is!
Now, I'm also not sure if the following is correct or not:
I_R3 = I_C1 + I_B1 + I_B2
I_B1 = I_B2 = I_C1 / hFE

Vcc + R3*I_R3 + Vce1  Vcc = 0
R3*(I_C1 + I_B1 + I_B2) + Vce1 = 2*Vcc  Vce1
R3*(I_C1 + 2*(I_C1 / hFE) ) = 2*Vcc  Vce1
R3*I_C1*(1 + 2/hFE) = 2*Vcc  Vce1
I_C1 = (2*Vcc  VCe1) / ( R3*(1 + 2/hFE) )
I_C1 = (30 V  0.67 V) / ( 7.2*(1 + 2/200) )
I_C1 = 4.03 mA

I_B1 = I_B2 = 4.03 mA / 200 = 20.15 μA
So, I already have Q1 Qpoint which is:
Q_point Q1  (0.67 V; 4.03 mA)
Now, I'm not sure if I already can evaluate Vce2, but I think I can't.
I tried to go now for the MOSFETs:
I_C2 = I_B2 * hFE = I_C1 = 4.03 mA
I_D1 = I_S1 = I_C2 / 2 = 2.02 mA
I_D1 = K* (Vgs  Vth)²
2.02 mA = 0.5 mA/V² * (Vgs  1)²
Vgs1 = 1 V or Vgs1 = 3 V
Only the 2nd value is of interest because for the MOSFET to be on the active zone, Vgs must be greater than Vth, so
V_gs1 = 3 V
Now, I'm stuck here! I have a couple of questions:
In this kind of circuits, both MOSFETS V_GS must be equal or not? If so, I think I can keep going, if not, I'm really stuck! I can't figure out a way to find V_DS!!!
The picture and the asc files are attached!
I have chosen random mosfets and BJTs, but for the problem I have the following transistors info
BJT's
hFE = 200
1/hoe = 200 kΩ
hie = 25 mV/Ib
MOSFETs
K= 0.5 mA/V²
Vth = 1 V
The Vo is at R4.
First question is to find the Qpoint for the MOSFETs and for the BJTs. Teacher also says to ignore the base current of Q3 when finding the Qpoint of M1 and M2.
This is the circuit:
I started with the following:
Q1
Vbe = 0.67 V
Vce = Vbe = 0.67 V
Vc = Vcc + Vce = 15 V + 0.67 V = 14.33 V
I_R3 = (Vcc  Vc) / R3 = (15 V  (14.33 V)) / 7.2 kΩ = 4.07 mA
I'm not sure if what I done is correct or not, but I think it is!
Now, I'm also not sure if the following is correct or not:
I_R3 = I_C1 + I_B1 + I_B2
I_B1 = I_B2 = I_C1 / hFE

Vcc + R3*I_R3 + Vce1  Vcc = 0
R3*(I_C1 + I_B1 + I_B2) + Vce1 = 2*Vcc  Vce1
R3*(I_C1 + 2*(I_C1 / hFE) ) = 2*Vcc  Vce1
R3*I_C1*(1 + 2/hFE) = 2*Vcc  Vce1
I_C1 = (2*Vcc  VCe1) / ( R3*(1 + 2/hFE) )
I_C1 = (30 V  0.67 V) / ( 7.2*(1 + 2/200) )
I_C1 = 4.03 mA

I_B1 = I_B2 = 4.03 mA / 200 = 20.15 μA
So, I already have Q1 Qpoint which is:
Q_point Q1  (0.67 V; 4.03 mA)
Now, I'm not sure if I already can evaluate Vce2, but I think I can't.
I tried to go now for the MOSFETs:
I_C2 = I_B2 * hFE = I_C1 = 4.03 mA
I_D1 = I_S1 = I_C2 / 2 = 2.02 mA
I_D1 = K* (Vgs  Vth)²
2.02 mA = 0.5 mA/V² * (Vgs  1)²
Vgs1 = 1 V or Vgs1 = 3 V
Only the 2nd value is of interest because for the MOSFET to be on the active zone, Vgs must be greater than Vth, so
V_gs1 = 3 V
Now, I'm stuck here! I have a couple of questions:
In this kind of circuits, both MOSFETS V_GS must be equal or not? If so, I think I can keep going, if not, I'm really stuck! I can't figure out a way to find V_DS!!!
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