1.H6P2: Phase Inverter

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2 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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1.

H6P2: Phase Inverter

The circuit shown below has two MOSFETs. The input signal is presented to the gate of
Q
1
.
Q
2

is configured as the two
-
terminal device that you explored in exercise S10E2.
Here,
Q
2
, with its gate connected to its drain, is used as a replacement for the load
resistor in a common
-
source amplifier.


Assume that the MOSFETs are identical, with parameters
K

and
V
T
. Note that it is hard
to get close to identical transistors if they are
discrete components, but one can get pretty
good matched pairs on an integrated circuit chip.

We want to set the input bias
V
IN

high enough to keep
Q
1

out of cutoff. If we want to be
able to swing the input voltage up and down by
Dv
, what is the minimum v
alue for the
input bias voltage? Write an algebraic expression involving
Dv

and
V
T

for this
minimum value of
V
IN
.

What is the minimum value that the power supply voltage
V
DD

must be to ensure that
both transistors are operating in the saturated region? Write an algebraic expression
involving
V
IN

and
V
T

for this minimum value of
V
DD

in the space provided below:

Notice that
K

did not appear in this answer... Hmmmmmm, very int
eresting...

Now, consider the small
-
signal model of this circuit. What is the transconductance
g
m
1

of transistor
Q
1

at the operating point? Write an algebraic expression for
g
m
1

in terms of
the parameters
K

and
V
T
, and the bias voltage
V
IN
.

What is the
transconductance
g
m
2

of transistor
Q
2

at the operating point? Write an
algebraic expression for
g
m
2

in terms of the parameters
K

and
V
T
, and the bias voltage
V
IN
.

Hmmmmmm...

What is the Thevenin equivalent open
-
circuit voltage of the small
-
signal circui
t, as seen
from the output port? Write an algebraic expression for this
v
out

in terms of the
parameters
K

and
V
T
, the bias voltage
V
IN
, and the incremental input voltage
v
in
.

What is the Thevenin equivalent resistance of the small
-
signal circuit, as seen from the
output port? Write an algebraic expression for this resistance in terms of the parameters
K

and
V
T
, and the bias voltage
V
IN
.


Notice that, for the approximations we

use in this class, the behavior of this circuit is
quite independent of the detailed properties of the MOSFETs. This is very useful.