2.DC Current Sources and SinksLecture 25 Multistage Amplifiers (II) DC VOLTAGE AND CURRENT SOURCES Outline 1. DC Voltage Sources 2. DC Current Sources and Sinks 6.012 Spring 2009 Lecture 25 1 Reading Assignment:Howe and Sodini, Chapter 9, Sections 9-3-9.4Final: Monday, May 19, 2009, 9am, Johnson Ice Rink1. DC Voltage Sources Characteristics of DC Voltage Sources : • A well controlled output voltage • Output voltage does not depend on current drawn from source ⇒ Low Thevenin Resistance Consider a MOSFET connected in “diode configuration” I-V characteristics: ( ) ( )22 22 TnDSoxnTnGSoxnD VVC L W VVC L W I −=−= µµµµµµµµBeyond the threshold voltage, the MOSFET looks like a “diode” with quadratic I-V characteristics 6.012 Spring 2009 Lecture 25 2How does one synthesize a voltage source with this? Assume a current source is available VGS = VDS takes a value needed to sink current iD = IREF + iOUT = W 2L µµµµnCox vOUT − VTn( )2 Then: iOUT = W 2L µµµµnCox vOUT − VTn( )2 − IREF Solving for vOUT: vOUT = VTn + IREF + iOUT W 2L µµµµn Cox 6.012 Spring 2009 Lecture 25 3Synthesizing Voltage Sources (contd.) vOUT is a function of IREF and W/L of MOSFET: • IREF ↑ ⇒ vOUT ↑ • W/L ↑ ⇒ vOUT ↓ Small Signal Equivalent Circuit Model: Rout = 1 gm || ro ≈ 1 gm Rout is small (good!) 6.012 Spring 2009 Lecture 25 4PMOS voltage source Same operation and characteristics as NMOS voltage source. PMOS needs to be larger to attain the same Rout. 6.012 Spring 2009 Lecture 25 5:3. DC Current Sources and Sinks Characteristics of Current Sources • A well controlled output current • Supplied current does not depend on output voltage ⇒ High Norton Resistance Connect a voltage source to the gate of another MOSFET IOUT ≈ 12 WL 2 µµµµnCox(VREF −VTn)2 IREF ≈ 21 WL 1 µµµµnCox (VREF − VTn)2 Then: IOUT = IREF W L 2 W L 1 IOUT scales with IREF by W/L ratios of two MOSFETs ⇒ Current Mirror Circuit Well “matched” transistors important. 6.012 Spring 2009 Lecture 25 6DC Current Sources and Sinks (contd.) Small Signal Equivalent Circuit Model: Rout 2 = ro2 I-V characteristics of NMOS current source: 6.012 Spring 2009 Lecture 25 7PMOS Current Source PMOS Current Mirror: • NMOS current source sinks current to ground • PMOS current source sources current from positive supply 6.012 Spring 2009 Lecture 25 8 source:3. Multiple Current Sources Since there is no DC gate current in MOSFET, we can tie up multiple current mirrors to single current W Similar idea with NMOS current sinks: IOUTn = IREF L n W L R 6.012 Spring 2009 Lecture 25 9Multiple Current Sources and Sinks Often, in a given circuit, we need current sources and sinks. We can build them all out of a single current source. IOUT1 = IREF W L 1 W L R IOUT2 = IREF W L 2 W L R IOUT4 = IOUT1 W L 4 W L 3 = IREF W L 4 W L 1 W L 3 W L R 6.012 Spring 2009 Lecture 25 10Generating IREF Simple circuit: VDD − VOUTIREF = R IREFVOUT = VTn + W µµµµnCox 2L For large W/L: IREF ≈ VDD − VTn R • Advantages – IREF set by value of resistor • Disadvantages – VDD also affects IREF. – VTn and R are functions of temperature ⇒ IREF(T). In the real world, more sophisticated circuits are used to generate IREF that are VDD and T independent. 6.012 Spring 2009 Lecture 25 11current source.What did we learn today? Summary of Key Concepts • Voltage source easily synthesized from reference current source using MOSFET in diode configuration • Current source easily synthesized from current source using current mirror circuit. • Multiple current sources and sinks with different magnitudes can be synthesized from a single current source. • Voltage and current sources rely on the availability of well “matched” transistors in IC technology. 6.012 Spring 2009 Lecture 25 12MIT OpenCourseWarehttp://ocw.mit.edu 6.012 Microelectronic Devices and Circuits Spring 2009 For information about citing these materials or our Terms of Use, visit:
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