Example #7
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)
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Example #7
(c)
(d)
(e)
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Example #7
2. All the MOS transistors in the feedback transconductance amplifier (series
(series-series)
series) of the circuit below
are sized to operate at |Vov|=0.2V.
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 outp
output, at the drain of Q1, and at the drain
of Q2.
(b) Find an approximate expression and value for Af≡ for the case ≫ 1.
(c) Use feedback analysis to obtain a more precise value for Af.
(d) Find the value for Rof.
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Example #7
3. The feedback transresistance amplifier below utilizes two identical MOSFets biased by ideal current
sources I=0.5mA. They are both sized to operate at Vov=0.2V and have Vt=0.5V, VA=10
10V, and RF=10kΩ.
(a) If Is has a zero dc component, find the dc voltage at the input, at the drain of Q1, and at the output.
(b) Find gm and ro for Q1 and Q2.
(c) Find the expressions and values of A for the new A circuit in terms of gm1, ro1, gm2, ro2, and RF.
(d) What is β?
(e) Find the gain with feedback, Af
(f) Derive expressions for Rinput and Routput of the new A circuit, and Rif and Rof for the feedback circuit.
(g) Evaluate A, Aβ, Af, Rinput, Routput, Rif, and Rof for the component values given.
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Example #7
4. For the amplifier circuit below assume that Vs has a zer
zero dc component. Find the dc
d voltages at all
nodes and the dc emitter currents of Q1 and Q2. Let the BJTs have β=100.. Use feedback
feedbac analysis to find
Vo/Vs and Rif. Let VBE=0.7V.
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Example #7
5. For the amplifier with characteri
haracteristics shown in the graphs below,, what is the minimum closed-loop
closed
voltage gain that can be obtained for phase margins of 90o and 45o? (Assume
Assume frequency-independent
frequency
feedback)
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Example #7
6. For a multipole amplifier having a first pole at 3 MHz and a dc open-loop
loop gain of 60 dB is to be
compensated for closed-loop
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
e frequency of the second pole is
15MHz and if it remains unchanged while additional capacitance is introduced as mentioned, find the
frequency to which the first pole must be lowered so that the resulting amplifier is stable for closed-loop
closed
gains as low as unity. Byy what factor is the capacitance at the controlling node increased?
7. The op amp shown below has an open
open-loop gain of 105 and a single-pole rolloff
olloff with ω3dB=10rad/s.
(a) Sketch a Bode magnitude plot for the loop gain.
(b) Find the frequency at which |Aβ
β|=1 and find the corresponding phase margin.
(c) Find the closed-loop
loop transfer function, including its zero and poles. Sketch
ketch the magnitude of the
transfer function versus frequency and label the important parameters on your sketch.
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Example #7
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