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Fundamentals of Microelectronics Chapter 5 Bipolar Amplifiers

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Fundamentals of Microelectronics Chapter 5 Bipolar Amplifiers 11/1/2010 Fundamentals of Microelectronics CH1 Why Microelectronics? CH2 Basic Physics of Semiconductors CH3 Diode Circuits CH4 Physics of Bipolar Transistors CH5 Bipolar Amplifiers CH6 Physics of MOS Transistors CH7 CMOS Amplifiers CH8 Operational Amplifier As A Black Box 1 Chapter 5 Bipolar Amplifiers 5.1 General Considerations 5.2 Operating Point Analysis and Design 5.3 Bipolar Amplifier Topologies 5.4 Summary and Additional Examples 2 1 11/1/2010 Bipolar Amplifiers CH5 Bipolar Amplifiers 3 Voltage Amplifier In an ideal voltage amplifier, the input impedance is infinite and the output impedance zero. But in reality, input or output impedances depart from their ideal values. CH5 Bipolar Amplifiers 4 2 11/1/2010 Input/Output Impedances Vx Rx = ix The figure above shows the techniques of measuring input and output impedances. CH5 Bipolar Amplifiers 5 Input Impedance Example I v x = rπ i x When calculating input/output impedance, small-signal analysis is assumed. CH5 Bipolar Amplifiers 6 3 11/1/2010 Impedance at a Node When calculating I/O impedances at a port, we usually ground one terminal while applying the test source to the other terminal of interest. CH5 Bipolar Amplifiers 7 Impedance at Collector Rout =ro With Early effect, the impedance seen at the collector is equal to the intrinsic output impedance of the transistor (if emitter is grounded). CH5 Bipolar Amplifiers 8 4 11/1/2010 Impedance at Emitter v 1 x = 1 ix gm + rπ 1 Rout ≈ gm (VA =∞) The impedance seen at the emitter of a transistor is approximately equal to one over its transconductance (if the base is grounded). CH5 Bipolar Amplifiers 9 Three Master Rules of Transistor Impedances Rule # 1: looking into the base, the impedance is r πππ if emitter is (ac) grounded. Rule # 2: looking into the collector, the impedance is ro if emitter is (ac) grounded. Rule # 3: looking into the emitter, the impedance is 1/gm if base is (ac) grounded and Early effect is neglected. CH5 Bipolar Amplifiers 10 5 11/1/2010 Biasing of BJT Transistors and circuits must be biased because (1) transistors must operate in the active region, (2) their small- signal parameters depend on the bias conditions. CH5 Bipolar Amplifiers 11 DC Analysis vs. Small-Signal Analysis First, DC analysis is performed to determine operating point and obtain small-signal parameters. Second, sources are set to zero and small-signal model is used. CH5 Bipolar Amplifiers 12 6 11/1/2010 Notation Simplification Hereafter, the battery that supplies power to the circuit is replaced by a horizontal bar labeled Vcc, and input signal is simplified as one node called Vin. CH5 Bipolar Amplifiers 13 Example of Bad Biasing The microphone is connected to the amplifier in an attempt to amplify the small output signal of the microphone. Unfortunately, there’s no DC bias current running thru the transistor to set the transconductance. CH5 Bipolar Amplifiers 14 7 11/1/2010 Another Example of Bad Biasing The base of the amplifier is connected to Vcc, trying to establish a DC bias. Unfortunately, the output signal produced by the microphone is shorted to the power supply. CH5 Bipolar Amplifiers 15 Biasing with Base Resistor VCC −VBE VCC −VBE IB = ,IC =β RB RB Assuming a constant value for VBE, one can solve for both IB and IC and determine the terminal voltages of the transistor. However, bias point is sensitive to βββ variations. CH5 Bipolar Amplifiers 16 8 11/1/2010 Improved Biasing: Resistive Divider R 2 V X = VCC R1 + R 2 R 2 VCC I C = I S exp( ) R1 + R 2 VT Using resistor divider to set VBE, it is possible to produce an IC that is relatively independent of βββ if base current is small. CH5 Bipolar Amplifiers 17 Accounting for Base Current V − I R Thev B Thev I C = I S exp VT With proper ratio of R 1 and R 2, I C can be insensitive to βββ; however, its exponential dependence on resistor deviations makes it less useful. CH5 Bipolar Amplifiers 18 9 11/1/2010 Emitter Degeneration Biasing The presence of R E helps to absorb the error in V X so V BE stays relatively constant. This bias technique is less sensitive to βββ (I 1 >> I B) and VBE variations. CH5 Bipolar Amplifiers 19 Design Procedure Choose an I C to provide the necessary small signal parameters, g m, r πππ, etc. Considering the variations of R 1, R 2, and V BE , choose a value for V RE. With V RE chosen, and V BE calculated, V x can be determined. Select R 1 and R 2 to provide V x. 20 10 11/1/2010 Self-Biasing Technique This bias technique utilizes the collector voltage to provide the necessary V x and I B. One important characteristic of this technique is that collector has a higher potential than the base, thus guaranteeing active operation of the transistor. CH5 Bipolar Amplifiers 21 Self-Biasing Design Guidelines R R >> B (1) C β 2( ) ∆VBE << VCC −VBE (1) provides insensitivity to βββ . (2) provides insensitivity to variation in V BE . CH5 Bipolar Amplifiers 22 11 11/1/2010 Summary of Biasing Techniques CH5 Bipolar Amplifiers 23 PNP Biasing Techniques Same principles that apply to NPN biasing also apply to PNP biasing with only polarity modifications. CH5 Bipolar Amplifiers 24 12 11/1/2010 Possible Bipolar Amplifier Topologies Three possible ways to apply an input to an amplifier and three possible ways to sense its output. However, in reality only three of six input/output combinations are useful. CH5 Bipolar Amplifiers 25 Study of Common-Emitter Topology Analysis of CE Core Inclusion of Early Effect Emitter Degeneration Inclusion of Early Effect CE Stage with Biasing 26 13 11/1/2010 Common-Emitter Topology CH5 Bipolar Amplifiers 27 Small Signal of CE Amplifier vout Av = vin Current vout − = gmvπ = gmvin RC Av = −gm RC CH5 Bipolar Amplifiers 28 14 11/1/2010 Limitation on CE Voltage Gain ICRC VRC VCC −VBE Av = Av = Av < VT VT VT Since g m can be written as I C/V T, the CE voltage gain can be written as the ratio of V RC and V T. VRC is the potential difference between V CC and V CE , and VCE cannot go below V BE in order for the transistor to be in active region. CH5 Bipolar Amplifiers 29 Tradeoff between Voltage Gain and Headroom CH5 Bipolar Amplifiers 30 15 11/1/2010 I/O Impedances of CE Stage v v X X Rout = = RC Rin = = rπ iX iX When measuring output impedance, the input port has to be grounded so that V in = 0. CH5 Bipolar Amplifiers 31 CE Stage Trade-offs CH5 Bipolar Amplifiers 32 16 11/1/2010 Inclusion of Early Effect Av =−gm(RC || rO) Rout =RC || rO Early effect will lower the gain of the CE amplifier, as it appears in parallel with R C. CH5 Bipolar Amplifiers 33 Intrinsic Gain Av = − g m rO V A Av = VT As R C goes to infinity, the voltage gain reaches the product of g m and r O, which represents the maximum voltage gain the amplifier can have. The intrinsic gain is independent of the bias current. CH5 Bipolar Amplifiers 34 17 11/1/2010 Current Gain iout AI = iin AI CE = β Another parameter of the amplifier is the current gain, which is defined as the ratio of current delivered to the load to the current flowing into the input. For a CE stage, it is equal to βββ. CH5 Bipolar Amplifiers 35 Emitter Degeneration By inserting a resistor in series with the emitter, we “degenerate” the CE stage. This topology will decrease the gain of the amplifier but improve other aspects, such as linearity, and input impedance. CH5 Bipolar Amplifiers 36 18 11/1/2010 Small-Signal Model g m RC Av = − 1+ g m RE R A = − C v 1 + RE g m Interestingly, this gain is equal to the total load resistance to ground divided by 1/g m plus the total resistance placed in series with the emitter. CH5 Bipolar Amplifiers 37 Emitter Degeneration Example I R A =− C v 1 +RE || rπ2 gm1 The input impedance of Q 2 can be combined in parallel with RE to yield an equivalent impedance that degenerates Q1. CH5 Bipolar Amplifiers 38 19 11/1/2010 Emitter Degeneration Example II R || r A = − C π 2 v 1 + RE g m1 In this example, the input impedance of Q 2 can be combined in parallel with R C to yield an equivalent collector impedance to ground. CH5 Bipolar Amplifiers 39 Input Impedance of Degenerated CE Stage VA = ∞ vX = π ir X + RE 1( +β)iX vX Rin = = rπ +(β + )1 RE iX With emitter degeneration, the input impedance is increased from r πππ to rπππ + ( βββ+1)R E; a desirable effect. CH5 Bipolar Amplifiers 40 20 11/1/2010 Output Impedance of Degenerated CE Stage VA = ∞ v π ⇒ vin = 0 = vπ + + gmvπ RE vπ = 0 rπ vX Rout = = RC iX Emitter degeneration does not alter the output impedance in this case. (More on this later.) CH5 Bipolar Amplifiers 41 Capacitor at Emitter At DC the capacitor is open and the current source biases the amplifier. For ac signals, the capacitor is short and the amplifier is degenerated by R E. CH5 Bipolar Amplifiers 42 21 11/1/2010 Example: Design CE Stage with Degeneration as a Black Box VA = ∞ vin iout = g m −1 1+ (rπ + g m )RE iout g m Gm = ≈ vin 1+ g m RE If g mRE is much greater than unity, G m is more linear.
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