Analog Integrated Circuits Fundamental Building Blocks Fundamental - PowerPoint PPT Presentation

Analog Integrated Circuits Fundamental Building Blocks Fundamental Building Blocks Differential amplifiers Faculty of Electronics Telecommunications and Information Technology Information Technology Gabor Csipkes Bases of Electronics Department

Outline  differential and common mode signals – voltage balancing and virtual ground  a differential amplifier with resistive load  input and output voltage range  input and output voltage range  the half circuit concept  small signal and low frequency model  small signal and high frequency model  frequency response  the differential amplifier with current source load  the differential amplifier with current mirror load Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 2

Differential and common mode signals  differential voltage ( v in ) → floating voltage between two ground referenced nodes  common mode voltage ( V CM ) → the component common to both V inp and V inm (average?)  v    v V V   V V in  in inp inm  inp CM  2       V V V V v v inp inp inm inm   V     V V in CM  2  inm CM  2 In most cases V CM is the DC component of V in → virtual ground . Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 3

Differential amplifiers  two common source amplifiers balanced around gound or virtual ground  input signal → differential and common mode components  the output can be either differential (symmetrical/balanced) or referenced to ground → special case: differential amplifier with current mirror load special case: differential amplifier with current mirror load  operation often described through the equivalent half circuit (exceptions!!) → the circuit must be symmetrical The load can be any type discussed for the elementary common source amplifiers equivalent half circuits Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 4

Virtual ground What is virtual ground? → playground balance/scale example This point is not moving relative to ground in spite of the non-zero displacement but its position is not zero → virtual ground position is not zero → virtual ground positive displacement negative relative position to displacement ground not zero  the displacements are measured relative to the virtual ground instead of the proper ground  in terms of variations the virtual ground is no different from proper ground  in a circuit any potential not changing with the signal is virtual ground Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 5

Differential amplifier with resistive load  common source amplifier with resistive load as half circuit  bipolar version can also be used if the technology allows bipolar transistors  differential input and differential output  input common mode ( V CMin ) range and output voltage swing are both important Virtual Virtual ground ground V GS V o min        NMOS: V V V V NMOS: V V V , V CMin DSat 1,2 Th 1,2 o min out S 1,2 DSat 1,2 DD         PMOS: V V V V V   PMOS: V 0, V V CMin DD DSat 1,2 Th 1,2 o min out S 1,2 DSat 1,2 Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 6

Differential amplifier with resistive load  the small signal low frequency model – the large signal behavior given by the DC transfer function: only qualitative description here V out V V inp inm V V om op  g V V   m 1 in out 0    2 2 r || R  DS 1 D  g V V    m 2 in out 0       2 2 2 2 r r || || R R   DS 2 D DC transfer function g V     m 1,2   out A 2 r || R 0 DS 1,2 D    V 2  in R G out m Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 7

Differential amplifier with resistive load  the small signal high frequency model → replace transistors with their small signal equivalents and consider capacitances  calculate the frequency dependent voltage gain A ( s ) as the ratio of V out to V in  the differential input source resistance is neglected  the differential input source resistance is neglected    C C C  1 2 GD 1,2           C C C C 2 2 C C C C 2 2 C C     3 4 L DB 1,2 L The analysis of one half circuit is sufficient → KCL at the output:    sC V V g V V 1 in out   m 1 in out   1 2 2 2 r || R ||   DS 1 D sC   3 Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 8

Differential amplifier with resistive load  the small signal high frequency model     C s   1,2 A 1 A 1 s g     0  0     zp  m 1  A s ( )       s 1 s C C r || R   1 1 1,2 3,4 DS 1 D   p one pole and one right half plane zero caused by the Miller effect  1 1   f      p    2 R C C 2 r || R C  out 1,2 3,4 DS 1 D L     g g  f m 1  zp  2 C  1,2  g     GBW A f m 1 0 p   2 C  L Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 9

Differential amplifier with current source load  common source amplifier with current source load as half circuit  bipolar version can also be used if the technology allows bipolar transistors  differential input and differential output, similar to the resistive load configuration  input common mode ( V CMin ) range and output voltage swing are both important Virtual Virtual ground ground V V GS GS V o min         NMOS: V V V V NMOS: V V V , V V CMin DSat 1,2 Th 1,2 o min out S 1,2 DSat 1,2 DD DSat 3,4         PMOS: V V V V V   PMOS: V V , V V CMin DD DSat 1,2 Th 1,2 o min out DSat 3,4 S 1,2 DSat 1,2 Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 10

Differential amplifier with current source load  the small signal low frequency model V out V V inp inm V V op om  g V V   m 1 in out 0    2 2 r || r  DS 1 DS 3  g V V    m 2 in out 0       2 2 2 2 r r || || r r   DS 2 DS 4 DC transfer function g V     m 1,2   out A 2 r || r 0 DS 1,2 DS 3,4    V 2  in R G out m Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 11

Differential amplifier with current source load  the small signal high frequency model → replace transistors with their small signal equivalents and consider capacitances  calculate the frequency dependent voltage gain A ( s ) as the ratio of V out to V in  the differential input source resistance is neglected  the differential input source resistance is neglected    C C C  1 2 GD 1,2               C C C C 2 2 C C C C C C C C 2 2 C C     3 4 L DB 1,2 DB 3,4 GD 3,4 L The analysis of one half circuit is sufficient → KCL at the output:    sC V V g V V 1 in out   m 1 in out   1 2 2 2 r || r ||   DS 1 DS 3 sC   3 Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 12

Differential amplifier with current source load  the small signal high frequency model     C s   A 1 1,2 A 1 s g     0  0     zp  m 1  A s ( )      s 1 s C C r || r   1 1 1,2 3,4 DS 1,2 DS 3,4   p one pole and one right half plane zero caused by the Miller effect  1 1   f      p    2 R C C 2 r || r C  out 1,2 3,4 DS 1,2 DS 3,4 L     g g  f m 1  zp  2 C  1,2  g     GBW A f m 1 0 p   2 C  L Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 13

Differential amplifier with current mirror load  no equivalent half circuit due to the current mirror load  differential input and single ended output  input common mode ( V CMin ) range and output voltage swing are both important Virtual Virtual ground ground V GS V o min         NMOS: V V V V NMOS: V V V , V V CMin DSat 1,2 Th 1,2 o min out S 1,2 DSat 1,2 DD DSat 3,4         PMOS: V V V V V   PMOS: V V , V V CMin DD DSat 1,2 Th 1,2 o min out DSat 3,4 S 1,2 DSat 1,2 Analog Integrated Circuits – Fundamental building blocks – Differential amplifiers 14