1 Conventional amplifier Collector Emitter Base Rb1 Rb2 Rc ReCe RL Vcc Vin Vout Av = Vout/Vin = - (Rc//RL) / re re = ac resistance of the emitter

2 High-frequency transformer- coupled amplifier Collector Emitter Base Rb1 Rb2 RL Re Ce Vcc C1 Vin Vout f = 1 / (2 pi sqrt(L C1) Q = f / B L Example 2.1

3 Practical common emitter amplifier with better impedance matching Collector Emitter Base Rb1 Rb2 RL Re Ce Vcc C1 Vin Vout L Rd Cd Better impedance matching Higher Q

4 Common base RF amplifier RL ReCb Vin Vout LL Vcc

5 Wideband amplifier- Class A Collector Emitter Base Rb1 Rb2 RL Re Ce Vcc Vin Vout L Linear amplifier Generally used as single-ended audio amplifiers

6 Wideband amplifier- Class B Rb1 RL Vcc Vin Vout Compared to Class A: Greater efficiency Larger distortion

7 Amplifier- Class C Collector Emitter Base RL Vcc Vin Vout L High efficiency Larger distortion

8 Operating condition Class C amplifiers would improve the efficiency by operating in nonlinear regime, however the input has to be a sinusoidal wave Some means are needed to remove the distortion and restore the signal to its original sine shape

9 Operation principle The active device conducts for less than 180 degrees of the input cycle The output resembles a series of pulses more than it does the original signal The pulses can be converted back to sine waves by an output tuned circuit

10 Circuit configuration Collector Emitter Base RL Vcc Vin Vout L Input Output Nonlinear amplifier Sine input -> nonlinear current output -> sine output Fig. 2.12

11 Pros and Cons of the Class C amplifiers Pros: High efficiency, no current in absence of signal Cons: The output tuned circuit must be adjusted fairly close to the operating frequency The amplification is nonlinear

12 Comparison of three amplifiers ClassABC Conduction angle < 180 Maximum efficiency 50%78.5%100% Likely practical efficiency 25%60%75%

13 Neutralization Collector Emitter Base Rb1 Rb2 RL Re Ce Vcc Vin Vout L Rd Cd Cn Neutralization capacitor

14 Oscillator A B Barkhausen criteria: A x B = 1 Phase shift must total 0 or integer multiple of 360 degrees

15 Using non-inverting amplifier Hartley oscillator B = N1 / (N1 + N2) f = 1/2pi sqrt(LC) N2 N1

16 Using inverting amplifier (Hartley oscillator) B = -N1 / N2B = (N1 + N2) / N1 Example 2.2 N2 N1 N2 N1

17 Colpitts oscillator (non-inverting amplifier) B = Xc1 / Xct = C2 / (C1 + C2) C2 C1

18 Colpitts oscillator (inverting amplifier) Example 2.3 B = -Xc1/Xc2 = - C2/C1 C2 C1

19 Clapp oscillator

20 Varactor tuned oscillator Example 2.5 C=C0/sqrt(1+2V)

21 Oscillation frequency of LC circuit See MIT open course ware

22 Another application of high Q filter Before After Clock recovery by strong filtering effect PTL Oct

23 Crystal Crystal oscillators achieve greater stability by using a small slab of quartz as a mechanical resonator, in place of an LC tuned circuit CsCp Two resonance frequency related to Cs and Cp, respectively

24 f T = f 0 + k f 0 (T-T 0 ) Example 2.6 A portable radio transmitter has to operate at temperatures from –5 to 35 degrees. If the frequency is derived from a crystal oscillator with a temperature coefficient of +1ppm/degree C and it transmits at exactly 146 MHz at 20 degree, find the transmitting frequencies at the two extremes of the operating range Temperature dependence

25 Mixers 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 Any nonlinear device can operate as a mixer Vout = A Vi + B Vi 2 + C Vi 3 + … f1f1 f2f2 f 1 +f 2 f 1 - f 2 Second order effects

26 Square law mixers Vout = A Vi + B Vi 2 If inputs are two frequencies, the outputs will be: Original frequencies, double frequencies, sum frequencies, and differential frequencies Example 2.7

27 Diode mixers The V-I curve for a typical silicon signal diode is nonlinear Diode mixers can operate between reverse and forward biased states Or they can operate with a small forward bias

28 Transistor mixers Collector Emitter Base Rb1 Rb2 RL Re Vcc f1 Vout L f2

29 Balanced mixers A multiplier circuit, where the output amplitude is proportional to the product of two input signals, can be used as a balanced mixer V1 = sinω 1 t V2 = sinω 2 t Vo = V1 x V2 = 0.5 x [cos(ω 1 t - ω 2 t) – cos(ω 1 t + ω 2 t)]

30 Applications of balanced mixers AM Modulation Data (… …) Carrier Output Signal AM de-modulation Signal input Local oscillator Output Signal Filter

31 Detection schemes Signal input Output Signal Filter Self-mixing homodyne detection Homodyne and heterodyne detection One example of heterodyne detection

32 Phase detector using mixer Signal input The DC output depends on the phase of the two paths

33 Phase locked loop Phase detector LPFAmp VCO Output Input Capture range Lock range Example 2.8

34 Simple frequency synthesizer Phase detector LPFAmp VCO Output Input / N divider FM and AM channel spacing Example 2.9

35 A practical example – 29M to 10G synchronization circuit 29MHz / 6 circuit 29MHz amplification, digitization and frequency division circuit (All capacitors are 0.1uF).

36 5M to 10G synchronization circuit

37 Spectrum of 4.827MHz square signal wave. Span: 500Hz, RB: 30Hz. Experimental results

38 Pre-scaling Phase detector LPFAmp VCO Output Input Fixed /M Programmable /N Fixed /Q Example 2.10

39 Frequency translation The movement of a block of frequencies is called a frequency translation Two configurations: Synthesizer with frequency shifting Synthesizer with mixer in the loop Example 2.11

40 Transmission lines Coaxial cables (solid dielectric, air dielectric) Parallel line cables (television twin-lead, open- wire line, shielded twin-lead) Twisted pair cable

41 Two models of short transmission line section Balanced line Unbalanced line

42 Step and pulse response of lines Characteristic impedance: the ratio of voltage to current through the transmission line with a step signal Concept of matched line Characteristic impedance Z0 = sqrt[(R + jwL) / (G + jwC)] Many lines approach Z0 = sqrt(L/C) Example 14.1, 14.2

43 Reflection (step input) Open end scenario Short end scenario Pulse input…

44 Some definitions Γ = Vr/Vi: reflection efficient Γ = (ZL – Z0) / (ZL + Z0) Meaning of the above equation: 1.To have zero reflection, ZL has to be equal to Z0 2.By measuring Γ, ZL can be derived to probe the internal characteristic of the load Example 14.13

45 An example to know the internal parameters of a tunable laser Source Transmission line S11 S11 = (ZL – Z0) / (ZL + Z0) ParametersReflector biased at 10 mA Is (A)1.79E10-5 q 4.47 Rp (ohm)0.1 Rs (ohm)0.1 Rsub (ohm)1.0 Cp (pF)4.58 Cs (pF)355 Lp (nH)21.4

46 Voltage driver is better than current driver Current responseOptical response Y. Su et al, IEEE PTL Sept. 2004

47 Wave propagation In a matched line, a sine wave moves down the line and disappear into the load. Such a signal is called a traveling wave Example 14.5 RF Phase shifter

48 Standing waves The interaction between the incident and reflected waves causes what appears to be a stationary pattern of waves on the line, which are called standing waves SWR = Vmax/Vmin For a matched line, the SWR = 1

49 Relation between Γ and SWR SWR = (1+ |Γ|) / ( 1 - |Γ|) If ZL >Z0, SWR = Z0 / ZL Example 14.6

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