# 3 Stage FET-BJT Amplifier Analysis

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#### eceturtle

Joined Jul 18, 2015
1

We're supposed to analyze this circuit and this particular question asks us to find the drain current of the mosfet. I keep getting an answer of Id = 3.54mA and Vds = 6.3675V. Since the gate voltage is zero Vgs = 0-Vs= -Vs. Using the current flowing through the source resistance, I came up with Vs = IsRs - 7.5V. Since the source current is equal to the drain current, Id = 0.5*kn*(Vgs-Vtn)^2 = 0.5*kn*(-Vs-Vtn)^2. I plugged in the eqn for the source current in the expression for Vs which leaves Vs as the only unknown and then obtain Vs = -1.8411V, then I plug in Vs into the eqn for the drain current and solve for Vd, for which I get Vd = 4.5264V. Thus Vds = 6.3675V.
Can someone point out where I'm making an error?

#### WBahn

Joined Mar 31, 2012
26,415
Are you taking into account lambda? Though it is small enough I would not expect it to have a huge effect.

Also, of the answers offered, how many are physically possible?

#### Jony130

Joined Feb 17, 2009
5,350
But can you explain me one thing. How can this N-MOS be ON when his gate is at 0V and Vt has positive value Vt = 1V?

#### WBahn

Joined Mar 31, 2012
26,415
But can you explain me one thing. How can this N-MOS be ON when his gate is at 0V and Vt has positive value Vt = 1V?
The gate is at 0V, but the source is pulled down toward -7.5 V through a 1600 Ω resistor.

#### Jony130

Joined Feb 17, 2009
5,350
Are you trying to say is that M1 is a depletion MOSFET ?

#### WBahn

Joined Mar 31, 2012
26,415
Are you trying to say is that M1 is a depletion MOSFET ?
Nope.

The gate voltage itself is irrelevant except in relation so the source voltage. It is the gate-source voltage that matters. If the gate is at 0 V and the source is at -2 V, then Vgs = 2 V and you have plenty of overdrive available.

#### Jony130

Joined Feb 17, 2009
5,350
OK I see it now. I simply didn't notice that the source is not at ground as usual.

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