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ECE142_2008FALL_EXAM1__[0] - ECE142A Fall 2008 Mid—Term...

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Unformatted text preview: / ECE142A Fall 2008 Mid—Term Examination 10am - Noon Thursday October 30" 2008 AD. 1. [30] A two-transistor amplifier, with a linear load is reproduced in Figure 1. VCC a, Draw the complete small signal equivalent. circuit for the amplifier; include components arising from base-width modulation. Label your important nodes with any intermediate voltages that you may require during your derivation of an expression f or the gain. 1). From the circuit in part (a), write down expressions for the nodal voltages or currents and thus derive a symbolic expression for the voltage gain: explain concisely the reasons [or any approximations you make. 2. [16] Sketch the current—voltage characteristics for the transistors shown in Figure 2. Assume that the magnitude of the respective threshold voltages is about 1V. Use the attached sheet for your sketches. + + + + + + + + (a) (b ) (C) (d) (e) (f) (g) 00 Figure 2 ECEI42A. Fau2008 8. [24] Current sinks and sources serve dual roles in analog circuits. Explain what these are: include a schematic illustrative of each role. Vdd Vdd Apart from the current, current sinks and sources are characterized by two other parameters. “1 M2 M1 “2 With the aid of Figures 3 a 4, explain what these Iref are and write down values for each, for the two M3 M4 c1rcu1ts, H1 symbohc form. Iout ._ Iref The current sink in Figure 5 (M5 & M6) is used to retain the advantage offered by a cascode —T— mm arrangement whilst extending the operating range Figure 3 Figure 4 of the circuit. To ensure that M5 and M6 operate in the saturation region, by what factor must the aspect ratio of M1 be altered in relation to the other transistors? {Hintz Think about the different ways in which the voltage on the node connecting the gates of M1 and M2 can be expressed, remembering that me = V05 — VT} {Assume that all transistors have the same threshold voltage; that. all transistors have the same aspect ratio, with the exception of M1; that Ian e Itch a It”. Ignore any factors introduced by the body cITectJ Figure 5 4. [80] a) Using the principle of the Widlar current source, Vittoz and Fellrath developed the circuit shown in Figure 6 in which the resistor R is used to set the current. If the currents sourced by the cascode current mirror to the drains of M1 and M2 are identical, showthat: 1 W W =— 0r — =4”— gm, Rf [L1 (L) The p-channel transistors are all matched; ignore any consequences of the body‘eflcect and assume that the relationship between the drain current and the gate-source voltage is adequately modeled by: ID =K(VGS_VT)2 Figure 6 ECE142A Fall 2008 b) Show that if the current sources IQ and [BE in Figure 7 are approximately equal, then the currents in both arms of the differential pair are also equal. {Assume that the input bias currents for Q1 and Q2 are equal, and that the magnitude of these currents is significantly less than the quiescent current} Figure 7 {Don ’t forget to vote neXt Tuesday. From many, many perspectives, this may be one oftbe most important elections of all time. If you need to take time to vote and cannot make it to class next Tuesday, that’s OK - but you must send me an email explammg why;l TIC 10/ 30/08 ...
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