Example #7

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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

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(c)

(d)

(e)

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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.

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series) of the circuit below

, and at the drain

Example #7

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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.

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identical MOSFets biased by ideal current

10V, and RF=10kΩ.

, and at the output.

, ro2, and RF.

for the feedback circuit.

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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

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Find the dc voltages at all

. Use feedback analysis to find

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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

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, what is the minimum closed-loop

Assume frequency-independent

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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.

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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

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