A 5k 2GHz GaAs Transimpedance Amplifier Using A Low-noise Active Load

At a frequency of 300MHz with gd = 42mS and L = 15nH,rai is half. At a frequency of 2GHz, 1;: is 7.8~10-~~A~/ Hz which is less than M2 and a 200-ohm resistor. Figure 3 shows a schematic of the transimpedance amplifier including the inductive load. This circuit has two gain stages; the first uses an inverting cascode stage, and the second stage is a non-inverting differential amplifier stage. The circuit is fabricated in a D-mode technology with a 0.5pm gate length and a fTof25GHz. The transconductance is 170mS/ mm, the Idss is 140mA/mm, and the pinchoff voltage is -0.8V. Figure 4 shows a micrograph of the transimpedance amplifier. The chip is 2mm2. The feedback resistor is made with a thin-film nichrome process which has a sheet resistance of 50*5% ohdsquare. The total FET periphery is about 2mm. The current drain from V, is lOOmA and from Vas is 75mA. Figure 5 shows the measured optical frequency response of three preamplifier chips with Cdiode + Cstra of 0.6pF. The curve with the diamonds shows the response ofya preamp without inductor L , and a 3k-ohm Q. The 3dB bandwidth is about 2GHz. The response curve, marked with the squares is of a preamp with the inductor load of Figure 2. The feedback resistor value is increased to about 5k-ohms and has a 3dB bandwidth of about 2.2GHz. The curve marked with the circles is of the same 5k-ohm preamp, but without the inductor load. Without the inductor load, the 3dB bandwidth is reduced to 1.4GHz. Figure 6 shows noise characteristics of the same three preamps. The midband noise has been reduced to about 2.7pA/ &to about 2.1pNG due to the increase in h. (Equation 1) At a fre uency of lGHz, the noise has been reduced from about 5.7pNk to about 4pNG. This decrease in noise results from the reduction in noise from the load device. (Equation 5) Table 1 lists the performance results obtained with the preamplifier. These results compare favorably to recently reported monolithic transimpedance amplifiers. Reference 2 obtains 2GHz bandwidth with a 2k-ohm feedback resistor and has a noise of about 5pNGat 100MHz. A 800-ohm feedback resistor is used in Reference 3 to achieve 2GHz bandwidth.