MOSFET CS Amplifier (VTC)

Thread Starter

jegues

Joined Sep 13, 2010
732
See figure attached for the question and the figures given.

I'm having a little trouble for part a), I know at point A,

\[v_{GS} = V_{t}\] and the transistor is just turning on, and since \[v_{DS} > 0 \] we will enter saturation.

For point B,

\[v_{GS} - V_{t} = v_{DS}\]

but how do I determine Rd from this? I haven't been told anything about vGS or iD.

Hopefully once I get the ball rolling on part a), parts b), c) & d) should fall into place.

Thanks again!

EDIT:

a) Vgs = Vds + Vt, Vds = 0.6V

\[i_{D} = \frac{1}{2}k_{n}(v_{GS} - V_{t})^{2}\]

\[i_{D} = \frac{V_{DD} - V_{DS}}{R_{D}}\]

\[i_{D} = 1.25mA, \quad R_{D} = 1.6k \Omega\]

b)

\[v_{DS} = 0.5V \quad \rightarrow \quad v_{GS} = 1V\]

\[v_{DS} = 2.5V \quad \rightarrow \quad v_{GS} = 3V\]

I'm stuck on part c) now.
 

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Jony130

Joined Feb 17, 2009
4,931
In the first place tell me why you want to find Rd ??
Rd is given and its equal to Rd = 24kΩ

As for question a

The point A is equal to

\[V_{ds} = V_{t}\]

And point B

\[V_{ds} = V_{gs} - V_{t}\]

And we also know that:

\[V_{ds} = I_{d} * R_{d}\]

And

\[I_{D} = \frac{1}{2}K_{n}(V_{GS} - V_{T})^{2}\]

So you need to solve this equation

\[V_{dd} - (I_{d} * R_{d}) = V_{gs} - V_{t} = V_{dd} - \frac{1}{2}K_{n}(V_{GS} - V_{T})^{2}*Rd = (V_{gs} - V_{t})\]
 
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Thread Starter

jegues

Joined Sep 13, 2010
732
In the first place tell me why you want to find Rd ??
Rd is given and its equal to Rd = 24kΩ

As for question a

The point A is equal to

\[V_{ds} = V_{t}\]

And point B

\[V_{ds} = V_{gs} - V_{t}\]

And we also know that:

\[V_{ds} = I_{d} * R_{d}\]

And

\[I_{D} = \frac{1}{2}K_{n}(V_{GS} - V_{T})^{2}\]

So you need to solve this equation

\[I_{d} * R_{d} = V_{gs} - V_{t} = \frac{1}{2}K_{n}(V_{GS} - V_{T})^{2}*Rd = (V_{gs} - V_{t})\]
Sorry I had posted the wrong question by mistake, the correct one is up there now.
 

Jony130

Joined Feb 17, 2009
4,931
And can you tell me.
How can you get Vgs --> 3V for Vds = 2.5V for Vdd = 2.5V ?
 

Jony130

Joined Feb 17, 2009
4,931
I must have done that incorrectly.

How do I get the value of Vgs for Vds = 2.5V

Is part a) okay?
The part a) looks okay except the last part Vgs=3V for Vds = 2.5V.
To get 2.5V as a Vds the drain currant must be equal to zero.
Then there will be no voltage drop across Rd resistor and Vds = Vdd.
 

Thread Starter

jegues

Joined Sep 13, 2010
732
The part a) looks okay except the last part Vgs=3V for Vds = 2.5V.
To get 2.5V as a Vds the drain currant must be equal to zero.
Then there will be no voltage drop across Rd resistor and Vds = Vdd.
So as a result, Vgs = Vt = 0.5V, correct?
 

Thread Starter

jegues

Joined Sep 13, 2010
732
For part C),

I solved the 2 equations for id for Vds and found that Vds = 0.077V, id = 1.51mA and rd = 50.85 ohms.

Is this correct?
 

Thread Starter

jegues

Joined Sep 13, 2010
732
Here's my work up to Part C), I'm unsure if the Vds->Id->rd I got for part C) is correct or not.
 

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