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Example #7
Fall 2010 1
1. The figure below shows a series-shunt amplifier with a feedback factor β=1. The amplifier is design
so that vo=0 for vs=0. Use Kn’= 2Kp’=120µA/V2, |Vt|=0.7V, and |VA|=24V.
(a) Ignore the early effect and find the dc currents and overdrive voltage, Vov, at which each of Q1 and
Q5 is operating when Q1 gate is grounded and Q2 is at ground instead of having feedback connected its
gate to the output, Vo.
(b) Find gm and ro of each of the five transistors.
(c) Find the expressions and values of A and Routput of the new A circuit. Assume that the bias current
sources are ideal.
(d) Find the gain with feedback, Af, and the output resistance Rof.
(e) How would you modify the circuit to realize a closed loop gain of 5 V/V? What is the value of output
resistance obtained?
(a) (b)
Example #7
Fall 2010 2
(c)
(d)
(e)
Fall 2010
2. All the MOS transistors in the feedback transconductance amplifier (series
are sized to operate at |Vov|=0.2V.
(a) If Vs has a zero dc component, find the dc voltage at the outp
of Q2.
(b) Find an approximate expression and value for
(c) Use feedback analysis to obtain a more precise value for
(d) Find the value for Rof.
Example #7
All the MOS transistors in the feedback transconductance amplifier (series-series) of the circuit below
v|=0.2V. For all transistors |Vt|=0.4V, and |VA|=20V
(a) If Vs has a zero dc component, find the dc voltage at the output, at the drain of Q1, and at the drain
(b) Find an approximate expression and value for Af≡
for the case ≫ 1.
Use feedback analysis to obtain a more precise value for Af.
3
series) of the circuit below
, and at the drain
Example #7
Fall 2010 4
Fall 2010
3. The feedback transresistance amplifier below utilizes two
sources I=0.5mA. They are both sized
(a) If Is has a zero dc component, find the dc voltage at the input, at the drain of Q
(b) Find gm and ro for Q1 and Q2.
(c) Find the expressions and values of
(d) What is β?
(e) Find the gain with feedback, Af
(f) Derive expressions for Rinput and R
(g) Evaluate A, Aβ, Af, Rinput, Routput
Example #7
3. The feedback transresistance amplifier below utilizes two identical MOSFets biased by ideal current
They are both sized to operate at Vov=0.2V and have Vt=0.5V, VA=10
component, find the dc voltage at the input, at the drain of Q1, and at the output.
Find the expressions and values of A for the new A circuit in terms of gm1, ro1, gm2, r
and Routput of the new A circuit, and Rif and Rof for the feedback circuit.
output, Rif, and Rof for the component values given.
5
identical MOSFets biased by ideal current
10V, and RF=10kΩ.
, and at the output.
, ro2, and RF.
for the feedback circuit.
Fall 2010
4. For the amplifier circuit below assume that Vs has a zer
nodes and the dc emitter currents of Q
Vo/Vs and Rif. Let VBE=0.7V.
Example #7
For the amplifier circuit below assume that Vs has a zero dc component. Find the d
nodes and the dc emitter currents of Q1 and Q2. Let the BJTs have β=100. Use feedbac
6
Find the dc voltages at all
. Use feedback analysis to find
Fall 2010
5. For the amplifier with characteri
voltage gain that can be obtained for phase margins of 90
feedback)
Example #7
haracteristics shown in the graphs below, what is the minimum closed
voltage gain that can be obtained for phase margins of 90o and 45
o? (Assume frequency
7
, what is the minimum closed-loop
Assume frequency-independent
Fall 2010
6. For a multipole amplifier having a first pole at 3
compensated for closed-loop gains as low as unity by
which the pole is formed to reduce the frequency of the first pole.
15MHz and if it remains unchanged while additional capacitance is introduced as mentioned,
frequency to which the first pole must be lowered so
gains as low as unity. By what factor is the capacitance at the controlling node increased?
7. The op amp shown below has an open
(a) Sketch a Bode magnitude plot for the loop gain.
(b) Find the frequency at which |Aβ
(c) Find the closed-loop transfer function, including its zero and poles.
transfer function versus frequency and label the important parameters on your sketch.
Example #7
multipole amplifier having a first pole at 3 MHz and a dc open-loop gain of 60 dB is to be
loop gains as low as unity by using additional capacitance at the circuit node at
which the pole is formed to reduce the frequency of the first pole. If the frequency of the second pole is
15MHz and if it remains unchanged while additional capacitance is introduced as mentioned,
frequency to which the first pole must be lowered so that the resulting amplifier is stable for closed
y what factor is the capacitance at the controlling node increased?
The op amp shown below has an open-loop gain of 10
5 and a single-pole rolloff with
plot for the loop gain.
β|=1 and find the corresponding phase margin.
loop transfer function, including its zero and poles. Sketch the magnitude of the
transfer function versus frequency and label the important parameters on your sketch.
8
loop gain of 60 dB is to be
using additional capacitance at the circuit node at
e frequency of the second pole is
15MHz and if it remains unchanged while additional capacitance is introduced as mentioned, find the
that the resulting amplifier is stable for closed-loop
y what factor is the capacitance at the controlling node increased?
olloff with ω3dB=10rad/s.
ketch the magnitude of the
transfer function versus frequency and label the important parameters on your sketch.
Example #7
Fall 2010 9
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