# It is interesting to note that the input pole

• 858

This preview shows pages 593–596. Sign up to view the full content.

It is interesting to note that the “input” pole magnitude is on the order of the of the transis- tor: is equal to or roughly equal to while the resistance seen to ground is less than . For this reason, the input pole of the CB/CG stage rarely creates a speed bottleneck. Example 11.19 Compute the poles of the circuit shown in Fig. 11.36(a). Assume . in V R S V b V DD V out M M 1 2 in V R S V b V DD V out M M 1 2 (a) (b) C + C SB1 GS1 X Y C DB1 + C + C GD1 GS2 + C DB2 C SB2 Figure 11.36 Solution Noting that and play no role in the circuit, we add the device capacitances as de- picted in Fig. 11.36(b). The input pole is thus given by (11.98) Since the small-signal resistance at the output node is equal to , we have (11.99) Exercise Repeat the above example if operates as a current source, i.e., its gate is connected to a constant voltage. Example 11.20 The CS stage of Example 11.18 is reconfigured to a common-gate amplifier (with tied to the source of the transistor). Plot the frequency response of the circuit. One exception is encountered in radio-frequency circuits (e.g., cellphones), where the input capacitance becomes undesirable.

This preview has intentionally blurred sections. Sign up to view the full version.

BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 569 (1) Sec. 11.6 Frequency Response of Followers 569 Solution With the values given in Example 11.18 and noting that , we obtain from Eqs. (11.96) and (11.97), (11.100) (11.101) With no Miller effect, the input pole has dramatically risen in magnitude. The output pole, how- ever, limits the bandwidth. Also, the low-frequencygain is now equal to , more than a factor of two lower than that of the CS stage. Figure 11.37 plots the result. The low- frequency gain is equal to 15 dB and the -dB bandwidth is around 450 MHz. 10 6 10 7 10 8 10 9 10 10 20 15 10 5 0 5 10 15 20 Frequency (Hz) Magnitude of Frequency Response (dB) Figure 11.37 Exercise Repeat the above example if the CG amplifier drives a load capacitance of 150 fF. 11.6 Frequency Response of Followers The low-frequency response of followers is similar to that studied in Example 11.11 and that of CE/CS stages. We thus study the high-frequency behavior here. In Chapters 5 and 7, we noted that emitter and source followers provide a high input impedance and a relatively low output impedance while suffering from a sub-unity (positive) In reality, the junction capacitances and sustain different reverse bias voltages and are therefore not quite equal.
BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 570 (1) 570 Chap. 11 Frequency Response voltage gain. Emitter followers, and occasionally source followers, are utilized as buffers and their frequency characteristics are of interest. Figure 11.38 illustrates the stages with relevant capacitances. The emitter follower is loaded with to create both a more general case and greater similarity between the bipolar and MOS counterparts. We observe that each circuit contains two grounded capacitors and one floating ca- pacitor. While the latter may be decomposed using Miller’s approximation, the resulting analysis is beyond the scope of this book. We therefore perform a direct analysis by writing the circuit’s

This preview has intentionally blurred sections. Sign up to view the full version.

This is the end of the preview. Sign up to access the rest of the document.
• One '14

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern