Chapter_10

Willy Sansen 10-05 101 Current-input Operational Amplifiers Willy Sansen KULeuven, ESAT-MICAS Leuven, Belgium willy.sansen@esat.kuleuven.be Willy Sansen 10-05 102 Table of contents • Operational current amplifier • Configurations current amplifiers Willy Sansen 10-05 103 Current or voltage amplifier IIN RF Av AR IIN vOUT = RFIIN vOUT vOUT vOUT = ARIIN Willy Sansen 10-05 104 Operational current amplifier Gain & Speed IB +- 1 : B2 M1 M6 M5 IB IIN M2 M4 M3 AR = B2ROUT = B2 ro5//ro6 CL BW = 2π CLROUT 1 AR BW = 2π CL B2 SR = B2 CL IIN ~ IDS IDS1 : B2 1 : 1 Willy Sansen 10-05 105 Operational current amplifier : noise SR = B2 CL IIN diDS2 = 4kT gm df 2 3 diIN2 ≈ 2 diDS2 N S = IIN diIN2 BW π 2 N S ~ IIN IDS IDS IDS IB +- 1 : 1 B2 M1 M6 M5 IB IIN M2 M4 M3 CL 1 : B2 1 : Willy Sansen 10-05 106 Gain and noise in MOST cascodes RBB > RL iinIBiinRB iinRBiinIB RL gmrDS > 1 gmRL 1+gmRB vout RL vout iin vout iN iN iin iN vout iN vout iN vout iN rDS > RL RB gmRB 1 1+gmRB RL RL RBBRBB gmRBB RL gmRBB 1 gmrDSRB gmrDSRBB RB gmrDS 1 gmRBB 1 rDS IL IL Willy Sansen 10-05 107 OTA in feedbackloop IB +-vIN GBW = 2π Cc gm1 RS RF + - - + RS RF Cc M1 AvV = - 1 + RS RF gm1 jωCc 1 Willy Sansen 10-05 108 OCA in feedbackloop IB +- 1 : B2 IB vIN RS RF + - - + RS RF Cc M1 AvI = - 1 + RS RF jω RS Cc 1 Willy Sansen 10-05 109 Comparison AvV and AvI RS RFAvV = - 1 + RS RF gm1 jωCc 1 RS RF 1 + RS RF jω RS Cc 1 AvI = - AvV f AvI f 1 1 GBW RF /RS RS = ct RF GBW is ct 2π RS Cc 1 RS Cc is ct gm1 = 0.2 mS; Cc =1 pF; GBW = 32 MHz; AvV = 1000; BW = 32 kHz 1000 32 kHz 1000 32 kHz Willy Sansen 10-05 1010 Advantage AvI RS RF 1 + RS RF jω RS Cc 1 AvI = - AvI f 1 RS = ct RF 2π RS Cc 1 RS Cc is ct AvI f 1 RF = ct RS 2π Rf Cc 1 RF Cc is ct More AvI More BW RS small Cc =1 pF; RS = 100 Ω; RF = 100 k Ω; BW = 1.6MHz 1000 32 kHz 1.6MHz 1000 Willy Sansen 10-05 1011 Limitations advantage AvI AvI f 1 RF = ct RS 2π Rf Cc 1 AvI f 1 RF = ct RS 2π Rf Cc 1 RS small RS limited to RS + : Bipolar2gm1 1 Willy Sansen 10-05 1012 Table of contents • Operational current amplifiers • Configurations Current amplifiers Willy Sansen 10-05 1013 Single-stage OCA- 1 (AD846) I1 +- I1 vOUT Cc/2 Cc/2 I2 I2 BW = 80 MHz SR (G=1) = 450 V/µs 6.5 mA (5V) 0.45 µA Willy Sansen 10-05 1014 Single-stage OCA- 2 (AD846) BW(Hz) RF = 100 kΩ A = 10 : RS = 10 kΩ A = 100 : RS = 1 kΩ RF = 10 kΩ A = 10 : RS = 1 kΩ A = 100 : RS = 100 Ω RF = 1 kΩ A = 10 : RS = 100 Ω A = 100 : RS = 10 Ω RS + > 10 Ωgm1 1 Willy Sansen 10-05 1015 Two-stage OCA- 1 (AD8011 bipolar) IB +- 1 : B2 IB + - RS RF CpM1 vOUT 1 : B2 1 iIN Willy Sansen 10-05 1016 Two-stage OCA- 2 (AD8011 bipolar) I2 +- I2 + - RS RF CpM1 vOUT 1 iIN Cc/2 Cc/2 I1 I1 M2 Willy Sansen 10-05 1017 Two-stage OCA :AD8011 I2 +- I2 + - RS RF Cp vOUT 1 iIN Cc/2 Cc/2 I1 I1 Requires complementary bipolar transistor process BW = 340 MHz (G=1) 57 MHz (G=10) SR (G=1) 3500 V/µs 1 mA (5V) 15 µA Willy Sansen 10-05 1018 Two-stage OCA (LM3900) Cp iIN- iIN+ vOUT iIN--iIN+ 1.3 mA GBW = 2.5 MHz SR (G=1) = 0.5 V/µs 1.4 mA (4-36V) 0.03 µA Willy Sansen 10-05 1019 Current feedback opamp Ref.Bales, JSSC Sept. 97, 1470-1474 110 MHz/ 1.5 mA 230 V/µs Av = 2 in 100 Ω fT = 3.8 GHz Willy Sansen 10-05 1020 Table of contents • Operational current amplifiers • Other current amplifiers