Study Questions
Question-1
Consider the series–shunt feedback amplifier of Fig. 11.8(b) that is analyzed in Example 11.2.
(a)If R1 = 1 kΩ, what value should R2 have to obtain a closed-loop gain whose ideal value is 5 V/V?
(b) If gm1 = gm2 = 4 mA/V, RD1 = RD2 = 10 kΩ, and the MOSFET’s ro is very large, use the expression for
Aβ derived in Example 11.2 to find the value of Aβ and hence determine the closed-loop gain Af.
Fig. 11.8(b)
Question-2
Consider the noninverting op-amp circuit of Example 11.4 for the case R1 = ∞ and R2 = 0.
(a) What is the value of β, and what is the ideal value of the closed-loop gain?
(b) Adapt the expressions found in Example 11.4 to obtain expressions for A and Aβ for this case.
(c) For μ = 104, Rid = 100 kΩ, Rs = 10 kΩ, ro = 1 kΩ , and RL = 2 kΩ, find A, Aβ, Af , Rin, and Rout.
Example 11.4 Non-inverting op-amp
Question-3
Figure P11.48(a) shows a feedback transresistance amplifier formed by an op amp and a feedback
resistance RF. Assume that the op amp is modeled by an input resistance Rid, an open-circuit voltage gain
μ, and an output resistance ro.
(a) Show that the feedback factor β, determined as shown in Fig. P11.48(b), is given by β = −1/RF . Hence
find the ideal value of the closed-loop gain Af ≡ Vo/Is. Find RF that results in Af of approximately 1 kΩ.
(b) By setting Is = 0 and breaking the loop at the input terminals of the op-amp, show that the loop gain is
given by
.
(c) For μ = 1000, Rid = 100 kΩ, ro = 1 kΩ, and RF having the value found in (a), what is the actual value of
Af realized?
(a)
(b)
Question-4
Figure P11.73 shows a shunt–shunt feedback amplifier. The MOSFETs have Vtn = 0.6 V, VA = 20 V, and
μnCox = 200 μA/V2. The power supply VDD = 3.3 V, and RL = 2 kΩ.The coupling capacitor CC can be
assumed to be very large.
(a) Perform a dc design to meet the following specifications: ID1 = 100 μA, ID2 = 1 mA, IR2,R1 = 10 μA,
VOV1 =VOV2 = 0.2 V. Neglect the Early effect. Specify the values required for I1, R1, R2, (W/L)1, and
(W/L)2.
(b) Find an expression for β and hence an expression for the ideal value of Vo/Vs.
(c) Find the value of Rs that results in Vo/Vs being ideally −6 V/V.
(d) Find the A circuit and use it to determine the values of A, Ri, and Ro.
(e) Find the value obtained for Vo/Vs.
(f) Find Rin and Rout.
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