USD EEE 194RF - EEE 194RF LECTURE NOTES

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Class C Amplifier• Class C amplifier operates for less than half of the input cycle. It’s efficiency is about 75% because the active device is biased beyond cutoff.• It is commonly used in RF circuits where a resonant circuit must be placed at the output in order to keep the sine wave going during the non-conducting portion of the input cycle.Types of Signal DistortionTypes of distortion in communications:• harmonic distortion• intermodulation distortion• nonlinear frequency response• nonlinear phase response• noise• interferenceNon-sinusoidal Waveform• Any well-behaved periodic waveform can be represented as a series of sine and/or cosine waves plus (sometimes) a dc offset:e(t)=Co+ΣAncos nω t + ΣBnsin nω t (Fourier series)External Noise• Equipment / Man-made Noise is generated by any equipment that operates with electricity• Atmospheric Noise is often caused by lightning•Space Noiseis strongest from the sun and, at a much lesser degree, from other starsInternal Noise•Thermal Noiseis produced by the random motion of electrons in a conductor due to heat. Noise power, PN = kTBwhere T = absolute temperature inoKk = Boltzmann’s constant, 1.38x10-23J/KB = noise power bandwidth in HzNoise voltage, kTBR4VN=Internal Noise (cont’d)• Shot Noise is due to random variations in current flow in active devices.• Partition Noise occurs only in devices where a single current separates into two or more paths, e.g. bipolar transistor.• Excess Noise is believed to be caused by variations in carrier density in components.• Transit-Time Noise occurs only at high f.Noise Spectrum of Electronic DevicesDeviceNoiseShot and Thermal NoisesExcess orFlicker NoiseTransit-Time orHigh-FrequencyEffect Noise1 kHz fhcfNoise Figure• Noise Figure is a figure of merit that indicates how much a component, or a stage degrades the SNR of a system:NF = (S/N)i/ (S/N)owhere (S/N)i = input SNR (not in dB)and (S/N)o= output SNR (not in dB)NF(dB)=10 log NF = (S/N)i (dB) - (S/N)o(dB)Equivalent Noise Temperature and Cascaded Stages• The equivalent noise temperature is very useful in microwave and satellite receivers.Teq= (NF - 1)Towhere Tois a ref. temperature (often 290 oK)• When two or more stages are cascaded:...AA1NFA1NFNFNF213121T+−+−+=Class C Amplifier• Class C amplifier operates for less than half of the input cycle. It’s efficiency is about 75% because the active device is biased beyond cutoff.• It is commonly used in RF circuits where a resonant circuit must be placed at the output in order to keep the sine wave going during the non-conducting portion of the input cycle.Simple Oscillator Using StabilityLEmitterBiasing,coupling,matching,etc.CollecterBiasing,coupling,matching,etc.LoadNetworkTerminatingNetworkΓinΓoutΓLΓTChoose transistor (BJT or FET) wisely so that common-base S11> 1 and S22>1 at oscillation frequency: This will cause instability.NE021 npn High Frequency BJTS22>1: Potential InstabilitySimple Oscillator Design: KISS!• Select transistor that is potentially unstable at oscillation frequency• Chose GT for terminating network that will make |GIN|>1• Calculate GL for the load network that will resonate ZIN at oscillation frequency• If ZIN= RIN+ jXIN, then ZL= RL+ jXL, where RL= |RIN| /3 and XL= –XINHartley Oscillators211;21LLLCLfTTo+==π121LLLB+=12LLB =Colpitts Oscillator21212121CCCCC;LCf;CCBTTo+===πClapp OscillatorThe Clapp oscillator is a variation of the Colpitts circuit. C4is added in series with L in the tank circuit. C2and C3 are chosen large enough to “swamp” out the transistor’s junction capacitances for greater stability. C4is often chosen to be << either C2 or C3, thus making C4the frequency determining element, since CT= C4.432322111121;CCCCLCfCCCBTTo++==+=πMixers•A mixer is a nonlinear circuit that combines two signals in such a way as to produce the sum and difference of the two input frequencies at the output.•A square-law mixer is the simplest type of mixer and is easily approximated by using a diode, or a transistor (bipolar, JFET, or MOSFET).Dual-Gate MOSFET MixerGood dynamic range and fewer unwanted o/p frequencies.Balanced Mixers•A balanced mixer is one in which the input frequencies do not appear at the output. Ideally, the only frequencies that are produced are the sum and difference of the input frequencies.Circuit symbol:f1f2f1+ f2Equations for Balanced MixerLet the inputs be v1 = sin ω1t and v2 = sin ω2t.A balanced mixer acts like a multiplier. Thusits output, vo= Av1v2= A sin ω1t sin ω2t.Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)]Therefore, vo= A/2[cos(ω1-ω2)t-cos(ω1+ω2)t].The last equation shows that the output of the balanced mixer consists of the sum and difference of the input frequencies.Balanced Ring Diode MixerBalanced mixers are also called balanced modulators.Voltage-Controlled Oscillator•VCOs are widely used in electronic circuits for AFC, PLL, frequency tuning, etc.• The basic principle is to vary the capacitance of a varactor diode in a resonant circuit by applying a reverse-biased voltage across the diode whose capacitance is


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