Mathcad - ee217projtodonew2.mcd

More documents

Recommendations

Info

Process Description The process used to simulate the amplifier is a 0.5 µm Silicon Germanium BiCMOS process. The following process parameters are measured for a single bipolar transistor, whose emitter size measures 5µm by 2.5µm. Although this device size is suitable for prescalers and low frequency operations, larger devices are better suited for low noise amplifiers. Increasing the device size has the beneficial effect of reducing the base resistance, and thus the input referred thermal noise of the amplifier. Increasing the device size has the detrimental effect of reducing the gain, increasing the input referred current noise. A more detailed discussion of these effects is given later. Other process parameters I S C µ0 2.2 . 10 18 A 3.41 fF Reverse Saturation Current of Base Emitter Diode . V jc 0.386 V m jc 0.11 x jc 0.82872 Collector Emitter Capacitance Parameters C cs0 6.47. fF V js 0.34 V m js 0.178 Collector Substrate Capacitance Parameters V A 71.87 V Forward Early Voltage β 0 146.15 DC Beta τ F 2.15. pS Base Transit Time r b0 177.66 ohm R e0 11.71 ohm R c0 75.42 ohm Base, Collector, and Emitter Resistances C je0 10.82 . fF V je 1.48 V m je 0.679 Base Emitter Capacitance Parameters Scaling Model Parameters with Device Size The small signal model used for the transistor is a relatively simple model. The resistances and capacitances of the transistor are assumed to scale linearly with the area of the emitter. The resistances are reduced by a factor of N as the device size increases, and the capacitances are increased by N as the device size increase. In reality, for small devices the base resistance stays relatively constant with device size, because of the added resistance and capacitance associated the contacts. In most practical LNA’s large devices are used so linear scaling is a good assumption. Small-signal parameters
Architecture Selection Amplifier Evolution The simplest versions of common emitter and common base amplifiers are shown in the following figure. They are essentially the same amplifier with the ground connection placed on the emitter and the base respectively. The distortion output power of these two amplifiers is identical, but the concept of input referred distortion and noise power is invalid, because the source impedance is zero ohms. I out I out V S V S Fig. 1: Unmatched Undegenerated Common Emitter and Common Base LNA All real amplifier have some value for source impedance. For high frequency amplifiers, this impedance is typically provided by a low loss matching network to a 50 ohm input. The value of the source impedance seen by the amplifier is usually somewhere between the optimal value for minimum noise and maximum gain. For a typical common emitter amplifier with good performance, the desired source impedance is close to 50 ohms. For a common base amplifier the desired source impedance is much smaller than 50 ohms. The large impedance difference of a common base amplifier from 50ohms requires a high-Q matching network. High Q matching networks usually have narrower bandwidths, more in-band loss, and are more sensitivity to process variations. This will degrade the performance of the common base amplifier. The distortion performance of the matched undegenerated common emitter and common base are similar. I out I out Z S V S V S Z S Fig. 2: Matched Undegenerated Common Emitter and Common Base LNA Usually the distortion and stability performance of a undegenerated amplifier is very poor, so it is desirable to place local or global feedback around the amplifier to improve it’s performance. For a bipolar amplifer, this is easily done with some inductance in the emitter of the amplifier. This inductance also helps to make the optimal source impedance for minimum noise figure closer to the conjugate match source impedance. For a common-base it is much more difficult to linearize the amplifier, because impedances in the base have little effect on the amplifier distortion. Furthermore, inductances in the base tend to destabilize the amplifier. Incidentally, inductance in the base is an common effective way to build an oscillator, such as the Colplitt’s oscillator. Our goal here is to build an amplifier, not an oscillator, so inductance in the base is undesirable.
Page 1 and 2: 'HVLJQDQG$QDO\VLVRID *+]/RZ1RLVH$PS
Page 3 and 4: Introduction Low noise amplifiers a
Page 5: ∆f: The frequency spacing of the
Page 9 and 10: Common Base Amplifier To find the d
Page 11 and 12: KCL N, I C , s, Z S , Z L 1 1 1 Z S
Page 13 and 14: 100 10 1 0.1 0.01 0.01 0.1 1 10 100
Page 15 and 16: Ballast Resistor Sizing The term ba
Page 17 and 18: 2000 Ballast Resistor (ohms) 1500 1
Page 19 and 20: Z popt SZ , 0 ∆ S . 11 , S 22 , S
Page 21 and 22: Solve r . b i . bn Z . e β β . V
Page 23 and 24: Noise Correlation Except in special
Page 25 and 26: n is the noise resistance represent
Page 27 and 28: Optimal Source Impedance to Trade-O
Page 29 and 30: Gain Analysis Z2Γ( Z) Z Z G T Γ G
Page 31 and 32: A 3 N, I C , Z S , s 1 , s 2 , s 3
Page 33 and 34: Optimizing Area and Current I C 0.0
Page 35 and 36: Impedance Matching High-pass impeda
Page 37 and 38: Low Frequency Stabilization It is d
Page 39 and 40: To insure stability over the spectr
Page 41: It is possible to use on-chip induc
Page 49: ... . 1 s 1 s 2 s . 3 C je N, I . C

Mathcad - ee217projtodonew2.mcd

Create successful ePaper yourself

Delete template?

Save as template?