616 Chapter 8 � Feedback

ii

vo

+

–

M1 5 kΩ

10 kΩ

M2 5 kΩ M3 5 kΩ

Figure 8.48 An ac schematic of a shunt-shunt feedback amplifier.

8.7 The ac schematic of a shunt-shunt feed-back amplifier is shown in Fig. 8.48. All transistorshave ID = 1 mA, W/L = 100, k′ = 60 �A/V2, andλ = 1/(50 V).

(a) Calculate the overall gain vo/ii, the looptransmission, the input impedance, and the outputimpedance at low frequencies. Use the formulas fromtwo-port analysis (Section 8.5).

(b) If the circuit is fed from a source resistanceof 1 k in parallel with ii, what is the new outputresistance of the circuit?

8.8(a) Repeat Problem 8.7(a) with all NMOS tran-sistors in Fig. 8.48 replaced by bipolar npn transistors.All collector currents are 1 mA and β = 200, VA =50 V, and rb = 0.

(b) If the circuit is fed from a source resistanceof 1 k in parallel with ii, what is the new outputresistance of the circuit?

8.13 A feedback amplifier is shown in Fig. 8.49.Device data are as follows: βnpn = 200, βpnp = 100,|VBE(on)| = 0.7 V, rb = 0, and |VA| = ∞. If the dcinput voltage is zero, calculate the overall gain vo/vi,the loop gain, and the input and output impedance atlow frequencies. Compare your answers with a SPICEsimulation.Also use SPICE to plot the complete large-signal transfer characteristic and find the second andthird harmonic distortion in vo for a sinusoidal inputvoltage with peak-peak amplitude of 0.5 V at vi.

Q1 Q2

Q3

vi

+

–

vo

+

–

1.25 kΩ

10 kΩ

+6 V

–6 V

5 kΩ

1 kΩ6 kΩ

Figure 8.49 Feedback amplifier circuit.

Problems 617

Q1

Q2

Q4

Q3

vi

+

–

500 Ω

500 Ω

500 Ω

500 Ω

78 Ω 280 Ω

+3 V

–3 V

+3 V

50 Ω

50 Ω

200 Ω

200 Ω

vo

+

–

Figure 8.50 Balanced series-shuntfeedback amplifier.

8.15 A balanced monolithic series-shunt feedbackamplifier is shown in Fig. 8.50.

(a) If the common-mode input voltage is zero, cal-culate the bias current in each device. Assume that βF

is large.

(b) Calculate the voltage gain, input impedance,output impedance, and loop gain of the circuit at lowfrequencies using the following data:

β = 100 rb = 50 VA = ∞ VBE(on) = 0.7 V

8.16 How does the loop gain T = af of the circuitof Fig. 8.50 change as the following circuit elementschange? Discuss qualitatively.

(a) 50 emitter resistor of the input stage

(b) 500 feedback resistor

(c) 200 load resistor on the output

618 Chapter 8 � Feedback

Q11

vi

+

–

7 kΩ

7 kΩ

400 Ω

Q10

400 Ω

Q9

RL = 2 kΩ

300 Ω

Q8

1.4 kΩ

1.1 kΩ1.1 kΩ

10 kΩ

2.4 kΩ2.4 kΩ

Q7

Q3

Q6

Q5

Q4

Q1 Q2

300 Ω

640 Ω 640 Ωvo

+

–

+6 V

–6 V

Figure 8.51 Circuit diagram of the 733 wideband monolithic amplifier.

8.20 Acommercial wideband monolithic feedback amplifier (the 733) is shown in Fig. 8.51. This consists of a local series-feedback stage feeding a two-stage shunt-shunt feedback amplifier. The current output of the input stage acts as a current drive to the shunt-shunt output stage.

(a) Assuming all device areas are equal, calculatethe collector bias current in each device.

(b) Calculate input impedance, output im-pedance, and overall gain vo/vi for this circuit atlow frequencies with RL = 2 k. Also calculate theloop gain of the output stage.

Data: β = 100, rb = 0, ro = ∞.

(c) Compare your answers with a SPICE simula-tion of the bias currents, input and output impedances,and the voltage gain.

8.21 If the 723 voltage regulator is used to real-ize an output voltage Vo = 10 V with a 1-k load,calculate the output resistance and the loop gain ofthe regulator. If a 500- load is connected to the reg-ulator in place of the 1-k load, calculate the newvalue of Vo. Use SPICE to determine the line regula-tion and load regulation of the circuit. Use I1 = 1 mA,β = 100, VA = 100 V, Is = 10−15 A, and rb = 0.