SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 1/12 SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 2/12 EE 311 Electronic Devices & Circuits 2 Lecture 17 9.1 MOS Differential Pair (continue…) Kwang W. Oh, Ph.D., Associate Professor SMALL (Sensors & MicroActuators Learning Lab) Department of Electrical Engineering University at Buffalo, The State University of New York 113C Davis Hall, Buffalo, NY 14260-1920 Tel: (716) 645-1205, Fax: (716) 645-3656 kwangoh@buffalo.edu, http://www.SMALL.Buffalo.edu 9.1.6 Cascode Differential Amplifier ● Gain can be increased via cascode configuration – discussed in Section 8.5. 9.38 ≡ || 9.39 9.40 Figure 9.13: (a) Cascode differential amplifier; and (b) its differential half circuit. SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 3/12 Exercise 9.6 SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 4/12 9.13(a) . ● ● 0.1mA 2 0.18μm ● , , , ⇒ , , , ⇒ 2 0.4 2 . . . 0.1 0.2 2 0.1 , , , ● 0.1 0.2 12.5 50 36kΩ 1mA/V . 1 1 ● ● ● 2 , , , ● ● 0.36μm ∥ 36 36 36 36 1.296 Ω 1.296 Ω 1 1296 ∥ 1296 EE 311 Electronic Devices & Circuits 2 Lecture 17 9.2 The BJT Differential Pair 648V/V Kwang W. Oh, Ph.D., Associate Professor SMALL (Sensors & MicroActuators Learning Lab) Department of Electrical Engineering University at Buffalo, The State University of New York 113C Davis Hall, Buffalo, NY 14260-1920 Tel: (716) 645-1025, Fax: (716) 645-3656 kwangoh@buffalo.edu, http://www.SMALL.Buffalo.edu SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 5/12 SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 6/12 The BJT Differential Pair 9.2.1 Basic Operation ● Figure 9.14 shows the basic BJT differential-pair configuration. ● It is similar to the MOSFET circuit – composed of two matched transistors biased by a constant-current source – and is modeled by many similar expressions. ● To see how the BJT differential pair works, consider the first case of the two bases joined together and connected to a common-mode voltage VCM. Figure 9.15: Different modes of operation of the BJT differential pair: (a) the differential pair with a common-mode input voltage VCM; (b) the differential pair with a “large” differential input signal; (c) the differential pair with a large differential input signal of polarity opposite to that in (b); (d) the differential pair with a small differential input signal vi. Note that we have assumed the bias current source I to be ideal. Figure 9.14: The basic BJT differential-pair configuration. SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 7/12 SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 8/12 Exercise 9.7 E9.7 1. 2. 3. 4. 5. With some reasoning It can will turn off, be seen that will carry all the and current . 0.7V → : ON The common emitter will be 0.7V at which means that the EBJ of will be reverse biased by 0.7 0.5 0.2V → : OFF The collector voltage will be 5V 6. 5 1 0.7V . Illustrated in Figure 9.15(a). ● Since Q1 and Q2 are matched, and assuming an ideal bias current I with infinite output resistance, this current will flow equally through both transistors. 9.2.2 Input Common-Mode Range 0.7 0.5 0.2V OFF . ● Refer to the circuit in Figure 9.15(a). ● The allowable range of VCM is determined at the upper end by Q1 and Q2 leaving the active mode and entering saturation. ● Equations (9.41) and (9.42) define the minimum and maximum common-mode input voltages. 4.3mA 0.7V 0.7V ON 0.4V 5V α 5 1 4.3 0.7V 1 Figure E9.7 9.41 ≃ 0.4 0.4 0.3V ← 1 2 9.42 ≅ 0.7 0.3V 0.4 SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 9/12 SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 10/12 Exercise 9.8 9.2.4 Small-Signal Operation ● 0.4 ≅ 1.9V 0.4 0.4 2.5 1 . 5 ● Small-signal: ● ≪ 2 2 Figure 9.18 2.5 ● 0.3 0.7 1.5V ● 9.50 thru 9.58 : 2 0.4V 9.41 ≃ 2 0.4 2 9.55 9.56 2 ● 9.61 1 2 ● 9.63 thru 9.67 : 0.4 2 2 2 2 9.42 ≅ 0.7 ● 0.4 0.3 V ← 9.68 0.3V SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 11/12 Figure 9.19 Figure 9.20: A differential amplifier with emitter resistances. Only signal quantities are shown (in color). SUNY–Buffalo | Electrical Engineering EE 311 Electronic Devices & Circuits 2 Lecture 17 | Chapter 09 | 4/8 | 12/12 With Emitter Resistance Re Example 9.3 ● While the input differential resistance of the MOS pair is infinite, that for but can the bipolar pair is only be increased to by including resistances in the two emitters. The latter action, however, lowers . ● 9.62 1 2 2 ● ● (a) The input differential resistance ● 9.69 ≅ ● Voltagegain frombasetocollector Totalresistanceincollector Totalresistanceinemitter . 0.5mA Each transistor is biased at 1 2 50Ω 2 2 100 1 50 ● (b) The overall differential voltage gain (for simplicity neglect 0.8 50 150 ≅ 40kΩ 40V/V )