RFIC_Lecture_Note_No10_p167-p191 (Linear Amplifiers) -...

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ECE695F RFIC Prof. S. Mohammadi Amplifiers - Gain - BW - Noise - Stability - Linearity High-frequency Linear Amplifier High-freq linear amps widebandwidth Amps Tuned Amps (Narrow band) gain f BW 3dB gain f 3dB BW * Characteristics of wideband Amps - useful for * opto-electronic receivers constant gain vs frequency * instrumentation amplifiers network analyzer oscilloscope input constant gain +constant delay vs frequency linear phase vs frequency no phase distortion - - 167
ECE695F RFIC Prof. S. Mohammadi - low efficiency (high power supply consumption) T f Bandwidth gain = × - Therefore low gain True only for first order systems (one pole) - high input noise power f kTR v n = 4 2 ( ) f v n 2 * Characteristics of narrowband Amps - high gain ( ) T f bandwidth gain = × - high efficiency (lower power consumption) - useful for low noise mobile receivers good for mobile application Stability is of concern in both wideband and turned Amps, but more importantly in wideband Amps typical RF transistor - - 168
ECE695F RFIC Prof. S. Mohammadi potentially unstable f gain MSG MA G unconditionally stable region you need positive feedback to make the transistor oscillate transistor can oscillate without any external positive feedback So for a good RF transistor we always have too much gain at very low-frequency + feedback through C gd is always there (how?) Internal feedback through C gd + high gain oscillator * So for widebandwidth Amps since we have gain at lower frequency, the chance of instability at lower frequency is higher f gain * for tuned Amps, we generally do not have high gain at lower frequencies as far as the amplifier is stable at around the Bandwidth we are fine gain f BW 0dB freq. range to look for unstable condition - - 169
ECE695F RFIC Prof. S. Mohammadi - How do we design / analyze linear amps. * let s say you have an RF transistor and you want to design a linear amp with this transistor. How to proceed? 1st measure transistor for its RF performance (S-parm) use network analyzer Design using S-parm of the transistor 22 21 12 11 S S S S f GHz 1 . 0 1 2 3 Try to model the transistor Small-Signal Model Large-Signal Model -Spice model -BSIM model g r gd C o r gs m V g gs V gs C * only need S-parm measurement + insight to the physics of device for initial values need DC d g I V ds d V I + RF V C parm S +insight to device physics older technique that measures Caps vs. supplied bias need S-parm for different biases So large-signal modeling is a difficult task often your transistor supplier provides that - - 170
ECE695F RFIC Prof. S. Mohammadi For integrated circuit technologies * you cannot possibly measure every given geometery you measure certain geometery (RF +DC) generate a scalable large-signal model that fits all possible geometeries for instant geometery transistors g r W ( ) min L geometery you only model 4 transistors then you curve-fit ( ) W f your model r g for different that you measured - Scalable models are not very accurate, gain may be off by a couple of dB but you cannot afford not knowing the stability situation that is why fab foundries often supply S-parm data along with the model But raw S-parm data is useless when you deal with an integrated transistor - - 171

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