Capacitive-Division Traveling-Wave Amplifier with 340 GHz Gain-Bandwidth Product

Transcription

1 Hughes Presented at the 1995 IEEE MTT-S Symposium UCSB Capacitive-Division Traveling-Wave Amplifier with 340 GHz Gain-Bandwidth Product J. Pusl 1,2, B. Agarwal1, R. Pullela1, L. D. Nguyen 3, M. V. Le 3, M. J. W. Rodwell1, L. Larson 3, J. F. Jensen 3, R. Y. Yu1,4, M. G. Case1,3. 1 University of California, Santa Barbara 2 Now with Hughes Space & Communications Company 3 Hughes Research Laboratories 4 Now with Rockwell International Science Center Support: California MICRO / Hughes Research Laboratories ARPA Thunder and Lightning Program, AFOSR

2 Capacitive-Division Traveling-Wave Amplifier Traveling-wave (distributed) amplifier : standard broadband IC Capacitive division TWA: Ayasli, 1988 broadband power Capacitive division TWA: significantly larger gain-bandwidth product This work: InGaAs/InAlAs HEMT CDTWAs Result: 11 db gain, 96 GHz bandwidth Record 340 GHz gain-bandwidth product

3 Principles of Traveling-Wave Amplifiers Z 0 L /2 C d L C d Output L/2 Synthetic Drain Line Cascode FETs Input L/2 L L/2 Z 0 Synthetic Gate Line Broadband circuit. FET input / output capacitances absorbed into synthetic transmission lines. Gain-bandwidth limited by line losses resulting from FET resistive parasitics.

4 Synthetic Transmission Lines in TWAs line sections L line section inductance gate-source capacitance Characteristic impedance: C line section capacitance Z 0 = LC Cutoff frequency: f cutoff = 1 π LC

5 TWA Bandwidth Limited by Gate-Line Losses R i R i R i R i C gs input impedance of1st FET C gs FET input resistance Ri causes gate line attenuation transistors far from input not driven at high frequency this limits gain-bandwidth (Ayasli, 1982): C gs C gs Nth FET S f 2 21 high f τ 4πR i C gs...if drain line losses are small (cascode TWA)

6 Cascode Cell: Negligible Drain-Line Losses R i g m V g' s R ds C gs V g' s R i C gs V g' s g m V g' s R out R ds ( 1 + g m R ds ) much larger than R ds Cascode cell: very large output resistance drain line losses nearly eliminated model valid for frequencies significantly below fmax

7 Normal cascode TWAs are not optimal designs TWA Gain 15 Gains, db 10 5 Cascode MAG f/f max (frequency normalized to f max ) Gain-bandwidth is well below MAG of cascode cell because 50Ω load much smaller than cascode R out

8 Examine the Gain-Bandwidth Limit: S f 2 21 high f τ 4πR i C gs R i g m V g' s C gs V g' s R out Decreasing Ri increases gain-bandwidth... How can we decrease input resistance Ri?

9 Capacitive Division Reduces Input Resistance transistor sizes doubled V in 2C gs 2C gs added capacitor forms 2:1 voltage divider with 2W 2W 2C gs model V in 2 2C gs R i /2 2g m V g's V in 2C V g' s gs R out /2 same input capacitance, same net transconductance input & output resistances reduced 2:1 2:1 division shown; higher ratios possible

10 Capacitive Division Increases TWA Gain-Bandwidth V in transistor sizes C div increased K:1 KC gs capacitive voltage divider with ratio: C div C div + KC gs = 1 K input & output resistances reduced K:1 gate line losses reduced K:1 K:1 more stages can be used: more gain at any design bandwidth, gain improved K:1 S 21 2 f high f K τ 4πR i C gs

11 How Much Can Gain-Bandwidth Be Improved? Gain, db normal 2:1 division 3:1 division Cascode Cell MAG 0 simulation f/f max (frequency normalized to f max ) Large division ratios: drain losses now significant, limits gain-bandwidth gain-bandwidth approaches MAG limit optimal circuit big FETs, difficult layout

12 Implementation / Design Features Hughes Research Laboratories low noise HEMT InAlAs / InGaAs / InP HEMT 0.15 µm gate length ft = 160 GHz, fmax = 260 GHz Regular, periodic structure: element values from design equations not computer optimized all cells have same FET sizes, same line lengths Conservative design: 2:1 capacitive division ratio designed for positive gain slope vs. frequency design gain-bandwidth well below MAG limit common-gate damping resistors: stabilization

13 Die Photo

14 Measured Results: InP Capacitive-Division TWA S 21, db Wafer A: 10 db gain, 92 GHz bandwidth 5 0 Wafer B: 8 db gain, 98 GHz bandwidth Frequency, GHz Measured by UCSB 200 GHz on-wafer network analyzer Difference due to variation in Cgs & gm

15 Measured Results: InP Capacitive-Division TWA S 21, db 10 5 Wafer C: measured on 50 GHz HP8510 Wafer D: measured on W-Band HP Frequency, GHz 11 db gain, 96 GHz bandwidth 340 GHz gain-bandwidth product

16 DC-50 GHz Return Losses & Reverse Isolation 0-10 S 22 db S S Frequency, GHz S22 resonances due to test configuration (bias probe) Good input and output return losses

17 7-100 GHz Return Losses & Reverse Isolation 5 0 measured by UCSB 200 GHz wafer probe system S db S Frequency, GHz S12 better than 15 db below 100 GHz

18 Capacitive-Division TWA with 340 GHz Gain-Bandwidth Product Traveling-wave amplifier: broadband gain block Ayasli, 1988: capacitive division TWA Capacitive division can sizably improve gain-bandwidth product Results for InGaAs/InAlAs amplifier: 11dB gain, 96 GHz bandwidth record 340 GHz gain-bandwidth product This work: conservative design results below limits of device technology Designs with 150 & 200 GHz target bandwidths currently in fabrication.