Download 1 Homework Assignment 09 Question 1 Electronics engineers are

55:041 Electronic Circuits. The University of Iowa. Fall 2012.
Homework Assignment 09
Question 1 Electronics engineers are often interested in the input- and output resistance of
amplifiers. Provide two reasons for this and briefly explain. (4 points)
Answer
One reason is that the input resistance of an amplifier can load the source at its input. For
example, if the output resistance of an amplifier is the same as the internal resistance of a sensor,
then only 50% of the signal appears at the amplifier input, and we lose 50% of the signal.
Similarly, the output resistance will determine how the load will affect the output voltage.
The second reason is that the input-and output resistances can also determine the overall
frequency response. For example, if an amplifier drives a long cable, the time constant
associated with the cable capacitance and output resistance of the amplifier will limit the highfrequency response. At the input, the time constant associated with the input resistance and a
coupling capacitor will limit the low frequency response.
Question 2 Consider the amplifier below. In the circuit,
represents a photodetector. Ignore
the MOFET’s capacitances. (a) Classify the type of frequency response (high-pass or low-pass)
the circuit has. (2 points) (b) Determine the 3-dB frequency. (5 points)
Answer
Part (a) The amplifier has a high-pass response.
Part (b) The circuit time constant is
. The 3-dB frequency is the
(
)
⁄
1
(
)(
)
55:041 Electronic Circuits. The University of Iowa. Fall 2012.
Question 3 Explain why the magnitude of the voltage gains of both amplifiers below increase
when the drain/collector bias current is increased. (4 points)
Answer
For the MOSFET amplifier,
(√
) and for the BJT amplifier
. Clearly, the magnitude of the gain increases for both amplifiers when
when the drain/collector bias current is increased
Question 4
(
)
Use BJT impedance scaling and give a reasonable estimate for the output resistance
(10 points)
Answer
⁄(
)
, so that
arrangement of BJTs as a PNP transistor with
impedance scaling
(
⁄
( )(
)
(
)
2
)(
)
. Model the composite
Using BJT
55:041 Electronic Circuits. The University of Iowa. Fall 2012.
Question 5 For the amplifier below,
Estimate the 3-dB bandwidth. Assume that
, and a dc analysis reveals that
. (15 points)
.
Answer
⁄
⁄
For the transistor
resistance looking into the emitter as
Using BJT impedance scaling, we estimate the
(
The time constant associate with
)
is
(
Since
is so much smaller than
The 3-dB bandwidth is
⁄
and
)
we can write
.
3