EE 140 ANALOG INTEGRATED CIRCUITS SPRING 2011 C Nguyen PROBLEM SET 5 Issued Tuesday Feb 22 2011 Due Tuesday Mar 1 2011 5 00 p m in the EE 140 homework box in 240 Cory 1 Calculate the output current IOUT of the CMOS Widlar current source as shown in Fig PS5 1 IIN 100 A VDD 3V W L 1 W L 2 25 k 200 A V and R2 4k Assume 0 and ignore the body effect VDD IIN IOUT R1 M1 Fig PS5 1 M2 R2 Fig PS5 2 2 Let s consider a low voltage cascoded current mirror as shown in Fig PS5 2 Ensure that all devices are operating in saturation Ignore all capacitances Assume 0 Ignore the body effect The goal is to design a low voltage cascoded current mirror Two 10 A reference ideal current sources I1 and I2 are available and the desired output current is IOUT 100 A Size the transistors in order to provide the maximum headroom at the IOUT node Find an expression for the widths and the lengths of M1 M2 M4 and M5 relative to the width and the length W L of M3 Use the same length for all transistors Size M1 M2 M3 and M4 so that these devices all have the same Vdsat 3 Determine the output current and output resistance expressions as a function of VOUT and IOUT respectively for the bipolar current mirror shown in Fig PS5 3 Find the output current and output resistance if VOUT 1V 3V and 10V Assume beta is infinite VBE on 0 7 V VA 100 and that all transistors are identical EE 140 ANALOG INTEGRATED CIRCUITS SPRING 2011 C Nguyen VCC 5 V IOUT R 1 k VOUT Fig PS5 3 4 Calculate the bias current of the Widlar current source shown in Fig PS5 4 as a function of R nCox W L 1 and W L 2 Ignore body effect channel length modulation and assume W L 3 W L 4 VDD VDD VDD M3 M4 IIN MC IBIAS W L 2 W L 1 MB M1 M2 W L 1 MA R Fig PS5 4 Fig PS5 5 EE 140 ANALOG INTEGRATED CIRCUITS SPRING 2011 C Nguyen 5 For the circuit of PS5 5 assume that W L C W L Ignoring body effect find W L A and W L B so that VDS A VDS B VDSAT C Draw the schematic of a double cascode current mirror that uses this circuit of PS5 5 to bias both cascode devices in the output branch For this current mirror calculate the output resistance the minimum output voltage for which all three transistors in the output branch operate in saturation and the total voltage across all the devices in the input branch Big hint Look at Fig 4 12 in Gray and Meyer
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