Case Study Amp1: Block diagram of an RF amplifier including biasing networks. Design Specifications. Case Study: Amp1

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1 MIROWAVE AND RF DEIGN MIROWAVE AND RF DEIGN ase tudy: Amp1 Narrowband Linear Amplifier Design Presented by Michael teer ase tudy Amp1: Narrowband Linear Amplifier Design Design of a stable 8 GHz phemt Amplifier Reading: hapter 17, ection Index: _Amp1 Based on material in Microwave and RF Design: A ystems Approach, nd Edition, by Michael teer. citech Publishing, 014. Presentation copyright Michael teer lides copyright 013 M. teer. 1 Design pecifications Gain: maximum gain at 8 GHz Topology: three two-ports (input and output matching networks, and the active device) tability: broadband stability Bandwidth: maximum that can be achieved using two-element matching networks ource impedance: Z = 50 Ω Load impedance: Z L = 50 Ω Block diagram of an RF amplifier including biasing networks. AMPLIFIER RF BIA ONTROL HIP GATE OR BAE TRANITOR M 1 M OUTPUT LOW-PA FILTER OLLETOR OR DRAIN LOW-PA FILTER RF OUTPUT 3

2 Linear amplifier with input and output matching networks General amplifier configuration Z AMPLIFIER FEEDBAK OURE M 1 1 TRAN- [] ITOR 1 [ / ] L M Z L 4 5 Transistor hoice JFET n-type channel G D n-type substrate emi-insulating Reverse-biased junction A phemt is a JFET (jfet), junction field effect transistor. IEEE symbol ommon symbol Applying a voltage at the gate (a gate-source voltage) closes off the channel by extending the space-charge region (i.e no charge region) of a reverse-biased junction. The G voltage changes the resistance of the channel. Not quite accurate as for high D voltages it changes the drain current. This is called an depletion mode of operation. A GaAs jfet is called a MEFET (Metal-Epitaxy emiconductor Field Effect Transistor.) Largely replaced by phemt. 6 7

3 urrent-voltage characteristic of a depletion-mode JFET urrent-voltage characteristics of an enhancement-mode JFETs G n-type channel V th D n-type substrate emi-insulating Reverse-biased junction V G V G = 0.0 V = -0.1 V = -0. V = -0.3 V V D pecial doping in channel that results in a built-in reverse bias that shuts that must be overcome by positive gatesource voltage. G n-type channel D n-type substrate emi-insulating Reverse-biased junction V G =.0 V = 1.5 V = 1.0 V = 0.5 V = 0.5 V V D There are n type and p type JFETs but the performance of a pjfet is poor. o designs mostly use only njfets. 8 V th V G 9 urrent-voltage characteristics of depletionmode and enhancement-mode JFETs ENHANEMENT V =.0 V G = 1.5 V = 1.0 V = 0.5 V = 0.5 V JFET DEPLETION V = 0.0 V G = -0.1 V = -0. V = -0.3 V n-type channel urrent-voltage characteristics of depletion-mode JFETs G D n-type substrate emi-insulating Reverse-biased junction V D V D V th V G V th V G Note the need for a negative gate voltage. 10 V th V G There is not a useful p type phemt but there are p type JFETs but they do not work well. 11

4 phemt pseudomorphic High Electron Mobility Transistor n-type channel e.g. AlGaAs G D emi-insulating e.g. GaAs Reverse-biased junction Two modes of operation Enhancement mode Depletion mode JFET summary G n-type channel D n-type substrate emi-insulating Reverse-biased junction Generally depletion-mode Enhancementmode possible Only n-type V G = 0.0 V = -0.1 V = -0. V = -0.3 V ircuit model of fundamental operation V D 1 13 urrent-voltage characteristic of phemt V G = 0.0 V cattering parameters of an enhancement mode phemt transistor biased at V D = 5 V, = 55 ma, V G = 0.4 V. Extract from the data sheet of the FPD6836P70 discrete transistor. V DD L D R L V D 55 ma V DD V D, min 5 V V DD = -0. V = -0.4 V = -0.6 V V D 14 15

5 Amplifier classes Output characteristic High efficiency load line onventional load line Input characteristic A I G A Extract from Manufacturer s Datasheet A A AB AB AB B B B AB V G B V D AB A parameters of phemt transistor at V D = 5 V, = 55 ma,v G = 0.4 V. Definition of power measures AVAILABLE ATUAL AVAILABLE AVAILABLE IGNAL IGNAL OUTPUT IGNAL OUTPUT IGNAL P Ai P in D P ADo P Ao OURE M 1 OUTPUT M P in ATUAL IGNAL n 1 Z n 3 n 4 P L ATUAL OUTPUT IGNAL V in 1 AMPLIFIER Z L 18 n n 5 PORT 1 PORT o many definitions are needed as it is necessary to describe the amplifier before the matching networks have been designed and to know the ultimate performance at different stages. 19

6 Definition of gains AVAILABLE ATUAL AVAILABLE AVAILABLE IGNAL IGNAL OUTPUT IGNAL OUTPUT IGNAL P Ai P in D P ADo P Ao Definition of powers AVAILABLE ATUAL AVAILABLE AVAILABLE IGNAL IGNAL OUTPUT IGNAL OUTPUT IGNAL P Ai P in D P ADo P Ao OURE M 1 OUTPUT M OURE M 1 OUTPUT M P in P L P in P L The input and output matching networks are lossless so that the actual device input signal power, P ind, is the power delivered by the source. imilarly, the actual output signal power delivered to the load, P L, is the power delivered by the active device. 0 1 Most useful gains AVAILABLE ATUAL AVAILABLE AVAILABLE IGNAL IGNAL OUTPUT IGNAL OUTPUT IGNAL P Ai P in D P ADo P Ao Most useful gains AVAILABLE ATUAL AVAILABLE AVAILABLE IGNAL IGNAL OUTPUT IGNAL OUTPUT IGNAL P Ai P in D P ADo P Ao OURE M 1 OUTPUT M OURE M 1 OUTPUT M P in P L P in P L ystem Gain Power Gain Transducer Gain Transducer Gain Available Gain Power actually delivered to the load relative to the input power delivered by the source. G but with the effect of M 1 removed. G with optimum M 1. G with optimum M 1. G with optimum M 1 and M 3

7 OURE Transducer Gain G with optimum M 1. Most useful gains AVAILABLE ATUAL AVAILABLE AVAILABLE IGNAL IGNAL OUTPUT IGNAL OUTPUT IGNAL P Ai P in D P ADo P Ao P in M 1 OUTPUT M Unilateral Transducer Gain G TU G T with 1 = 0. P L Maximum Unilateral Transducer Gain G TUmax G TU with optimum Development of gain expressions Developed using generalized scattering parameters (which can be defined different and complex load and source impedances). Then refer back to using transistor s 50- parameters Different gains useful at different stages of design. E.G. G TUmax used in selecting transistor, estimating design challenge. M 1 and M 4 5 Amplifier Gain in Terms of Transistor Parameters Transducer Gain Maximum unilateral transducer gain, G TUmax, of the phemt transistor For a unilateral two-port 1 = 0 Unilateral transducer gain Maximum unilateral transducer gain G TUmax I THE MOT IMPORTANT FIGURE OF MERIT THAT GUIDE INITIAL DEIGN (design is very hard if required gain is greater). 6 7

8 OURE Even more gains AVAILABLE ATUAL AVAILABLE AVAILABLE IGNAL IGNAL OUTPUT IGNAL OUTPUT IGNAL P Ai P in D P ADo P Ao M 1 OUTPUT M Input tability consideration Active device Output P in P L IN OUT L Transducer Gain G with optimum M 1. Maximum Available Power Gain 1 GMA k k k 1 1 G T with optimum M 1 and Maximum table Gain 1 GM 1 G MA at edge of stability k = 1. NOIE IN Unstable if IN > 1. NOIE OUT L Unstable if L OUT > 1. M 8 9 Amplifier stability Input Amplifier tability Output NOIE IN NOIE OUT L Unstable if IN > 1. Unstable if L OUT > 1. IN OUT Amplifier is unstable if L For stable amplification IN must be less than one at all frequencies L OUT must be less than one at all frequencies For passive source and load < 1, L < 1 Thus for unconditional stability require IN < 1 OUT < 1 or Unconditional stability if (as long as source and load are passive) and Note: magnitudes of complex numbers describe circles in the complex plane. Formulas have been developed for a stability circle (center and radius)

9 Amplifier tability Amplifier is unstable if Output stability circles on the L plane: 11 < 1 11 > 1 Representations of stability circles UN r L c L r L cl UN using shading to indicate the unstable region stability circle of an unconditionally stable two port using a dashed line to indicate the unstable region 3 33 Output stability circles for 11 < 1 Input stability circles for < 1 UN UN UN UN out out out 1 GHz. L must be in stable region for amplifier to be stable 8 GHz. 16 GHz. 1 GHz. must be in stable region for amplifier to be stable 8 GHz. 16 GHz

10 Unconditional stability criterion k-factor of phemt The amplifier is stable for any source and load (provided that 11 < 1 and < 1. UN UN onditionally stable Unconditionally stable Edwards-insky tability riterion, factor is the distance from the origin to the nearest point of the unstable region. UN > 1 for unconditional stability, The greater the more stable. factor of phemt > 1 for unconditional stability 38 39

11 Input and output factor of phemt ummary of stability considerations If an amplifier is unconditionally stable design is considerably simplified. Even if an amplifier is not unconditionally stable it could still be stable. Design is then tricky and is only done in special circumstances. E.g. very high frequency operation. onsider a cell phone power amplifier connected to an antenna Must be stable When antenna is covered by hand. Phone is placed on a metal surface. evere price if amplifier goes unstable In a communication system the whole EM spectrum could be polluted. Amplifier could self-destruct (thermal runaway) Back to design of the amplifier AMPLIFIER RF BIA GATE ONTROL OR BAE HIP TRANITOR M 1 M OUTPUT LOW-PA FILTER OLLETOR OR DRAIN LOW-PA FILTER RF OUTPUT G T = G with optimum M 1. G TUmax = G with 1 = 0, optimum M 1, M. G MA = G with optimum M 1, M G M = G MA but with k set to 1 (at edge of stability. U = G MA with 1 = 0. 4 G T and G MA are the only gains that do not modify the device. 43

12 Gain circles of the phemt at 8 GHz Z Output matching network design Plotted on the plane G with optimum M 9.96 db db db 1.96 db V IN OUT L Z L Design nearly always commences with the output matching network. The first design step is to choose an output matching network that will provide the appropriate impedances to ensure stability below 5 GHz and above 11 GHz. > 1 for unconditional stability db To do this the stability circles must be considered, as the device is only conditionally stable below 5 GHz and above 11 GHz. G MA = G with optimum M 1 and M Output stability circles for 11 < 1 UN Transistor Z OUT = R +jx phemt at 8 GHz Output Matching Network Z L = R L 1 GHz. L must be in stable region for amplifier to be stable UN 8 GHz. 16 GHz. apacitive at low frequency or perhaps open or short circuit. Unconditionally stable at operating frequency of amplifier. Ignore 1 However the output of the transistor really looks like a resistor in parallel with a capacitor. o R = R L = Look like a resistor or short at high frequencies. 46 R > R L 47

13 onsider output stability circles. onsider output stability circles UN UN UN UN 1 GHz. 8 GHz. 16 GHz. 1 GHz. 8 GHz. 16 GHz. Alternative design. Almost certainly will be unstable Output matching network design Active device Load R L R L R L X x X x X x R L Input matching network design R Input Matching Network Now consider 1 and loading Z IN Transistor R X L X P R L Active device o Lo Load R L Appropriate choice: X p =

14 Input stability circles for < 1 Input matching network design UN UN ource Active device R L X x R L R L X x X x R L out out out 1 GHz. 8 GHz. 16 GHz. X ource must be in stable region for amplifier to be stable R L X P X p = 34.3 R L R i Li Active device 5 53 L 3 V G 3 RF IN Final amplifier schematic. L 1 1 V DD L RF OUT 10 nh has a reactance of approximately 500 Ω at 8 GHz. 100 pf provides an RF short circuit at 8 GHz. imulated transducer gain is 13. db Design for a specific gain at 8 GHz Recall that simulated gain of design here is 13. db. Gain (gain circles) plotted on the plane with optimum M 1.96 db db db 9.96 db ompare to maximum available gain of db db Error is because 1 was ignored in synthesizing the output matching network. Design could also target a specified gain. 54 G MA = G with optimum M 1 and M 55

15 Final amplifier V DD L 3 V L G 3 RF IN L 1 1 RF OUT This is a surprisingly simple circuit. Bias circuit integrated into matching networks. 56