Fundamentals of Microelectronics. Bipolar Amplifier

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Irene Ryan
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1 Bipolar Amplifier Voltage Amplifier Performance Metrics - There are many metrics that are used to evaluate how good an amplifier is (1) (Voltage) Gain= Vout/ Vin. Can be found from small-signal Vin Vout(a) Vout(b) 4 volt [v] 2 0 Ex) Vout(a): gain = -5, Vout(b): gain = 7 (2) Input/Output Impedance (Rin, Rout): Small-Signal Impedance seen at the input/output terminal. - Rin = Vin/ Iin, Rout = Vout/ Iout time [us] - Voltage amplifier with finite Rin and Rout R s I in Voltage Amplifier V in - V in R in Vamp - R out I out R L V amp =A V in - Ideally, we want Vout=A Vin. From voltage divider, Vin = Rin/(RinRs) Vs & Vout = RL/(RoutRL) Vamp Vout = A RL/(RoutRL) Rin/(RinRs) Vs - Ideal amplifier : Rin = & Rout = 0. Vout=A Vin - Real amplifier: Rin & Rout are finite values, therefore gain degrades from ideal gain 1

2 - I/O impedance calculation: Use small-signal model Rin: apply small-signal voltage at the input while Vout open and find Vx/Ix Rout: apply small-signal voltage at the output while Vin short and find Vx/Ix - We use lower case for the small-signal voltage and current. When finding gain and I/O impedance, we assume that DC bias is already set up (In actual implementation, the DC bias must be set up by actual bias circuits) R c R in V in 2

3 (Bias not shown) R out V in (Bias not shown) V in R out 3

(3) Signal swing: In an amplifier, output and input voltages have finite Swing V p2p,swing =400mV 1.2V 1V V p2p,swing =10mV 0.705V 0.7V V in - Ex) Vin=0.75mV sinwt : Input has 0.

4 (3) Signal swing: In an amplifier, output and input voltages have finite Swing V p2p,swing =400mV 1.2V 1V V p2p,swing =10mV 0.705V 0.7V V in - Ex) Vin=0.75mV sinwt : Input has 0.7 DC 10mV peak-to-peak swing Vout=1-0.2 sinwt : output voltage has 1V DC plus 400mV peak-to-peak swing - We want all transistors in an amplifier to stay in forward-active mode in BJT amplifier. Otherwise, the gain will drop and the output will be distorted. - For example, if the gain is high, output signal will have large swing Transistor may go into saturation region temporarily leading to signal distortion - In other words, large output swing requires small voltage headroom (voltage headroom: required minimum voltage at output to keep all transistors in forward active mode) Ex) 4

5 Amplifier Design/Analysis Method Step 1: DC analysis - Also called as Large-Signal Analysis, Bias Analysis, Operating-Point Analysis - Goals: 1) to check transistors in forward-active region and 2) to determine IC - Method: Use Large-Signal Model of BJT with DC sources, and solve for IC Step 2: Small-Signal analysis - Also called as AC analysis - Goal: To find gain and I/O impedance - Method: Use small-signal model of BJT. Set all DC sources to 0 and analyze the circuit. <DC vs. Small-Signal Analysis > DC Analysis Open (No DC current) No change Small-Signal Analysis Short (Assume Cap=infinite) No change I DC No change Open V DC No change Short Biasing Circuit for BJT Amplifier - To use BJT for an amplifier, we need a circuit that places the BJT in forward-active region. Need Biasing Circuit. The biasing is derived from DC voltage using DC Analysis The amplifier gain is computed using small-signal analysis 5

6 1) Using Base Resistor RB - Idea: Put Resistor RB between VCC and Base to set Base voltage VBE Load Resistor Bias Resistor Small-signal voltage source R B Supply Voltage (DC source) C 0 NPN BJT Coupling Capacitor(Short in small-signal, Open in DC ) Ground (0V) 2) Using Resistive Divider Bias Circuit R 1 C 0 R 2 6

7 3) Using Emitter Degeneration R 1 C 0 R 2 C E 3) Self Bias (Always in active mode) R B C 0 7

8 5) PNP-Biasing - Similar except polarity modification BJT Amplifier Topology: categorized by the common-reference terminal for small-signal input and output. - Common-Emitter: Vin=VBE, Vout=VCE Emitter is common - Common-Base: Vin=VEB, Vout=VCB Base is common - Emitter-Follower (= Common-Collector): Vin=VBC, Vout=VEC Base is common Each of this amplifier has different 1) gain 2) I/O impedance, and 3) output swing/headroom characteristic. 8

9 1) Common-Emitter Amplifier - The most-commonly used type of amplifier - We assume biasing is already done (bias circuit not shown. Assume Q1 is in active mode) i) gain v π r π g m v π v out (Small- Signal Input) Small-Signal Analysis ii) Headroom - V RC V BE Output Headroom for Q1 GND Small Gain - V RC V BE Output Headroom for Q1 GND Large Gain - V RC =V BE Output Headroom for Q1 GND Maximum Gain 9

10 iii) I/O impedance a) Rin g m v π v π r π b) Rout g m v π v π r π r o 10

11 2) Common-Emitter With Degeneration - Problem of CE amplifier: Small-signal gain is high, but the gain depends entirely on gm of BJT gm is sensitive to VBE and temperature. Use degeneration resistor to de-sensitize the amplifier gain to gm. (In other words, degeneration improves the linearity of amplifier) i) gain v π r π g m v π Small-Signal Analysis Ex1) Q2 11

12 Ex2) Q 2 ii) a) Rin I/O impedance g m v π v π r π b) Rout g m v π v π r π r o 12

13 Common-Emitter with Degeneration and Source Impedance - Real source has finite source impedance, and this also impacts amplifier gain. - Let s find the gain by re-using the analysis for CE with degeneration voltage divider theory R S v x EX) V in R B R 1 R 2 C 1 I 1 13

14 Common-Emitter Amplifier with Bias Circuits - Actual amplifier needs bias circuits to set up collector current properly. - Bias circuits also impact the small-signal gain and I/O impedance, so it must be included in the analysis for accurate gain estimation Ex1) Bias Circuit R in R 1 C 0 R 2 Ex2) Bias Circuit R in R 1 C 0 R 2 C E 14

15 - A general case of CE amplifier: Let s consider source impedance RS and load impedance RL as well as bias circuit. R in R 1 C 2 R L R S C 1 R 2 15

16 Common-Base Amplifier: - Motivation: Sometimes we need an amplifier with small Rin (optical communication receiver, RF receiver, etc, when source impedance is small). - In case of CE, Rin = rpi= beta/gm tends to be too large (since beta is large number). Can only be lowered by increasing gm can only be increased by using larger current - CB topology is good for achieving low Rin at the cost of headroom penalty. a) Gain Bias Voltage (DC source) V b Small-signal Input Small-Signal Analysis V IN,DC b) Headroom CB I C CE I C V b V BE V BE V IN,DC V IN,DC 16

17 iii) I/O impedance c) Rin g m v π v π r π V b V X I X Small-Signal Analysis d) Rout I X g m v π v π r π r o V b V X Small-Signal Analysis 17

18 Common-Base with Input Source Impedance - A real world input source has finite output impedance ( in the Figure) - Let s find small-signal gain, R in and R out I C V b V BE Common-Base with Input Source and Base Impedance - A real world base bias voltage (=Vb) generator also has finite output impedance (R B in the Figure). - Let s find small-signal gain, R in I C R B V BE V b 18

19 Example) R B Q 2 R in Common-Base with Biasing Circuit Bypass Capacitor R 1 R in C 2 R 2 C 1 R S 19

20 Emitter Follower: - Motivation: Sometimes we need an amplifier with small Rout This is necessary to drive a load with small load impedance. Ex) Audio amplifier has to drive a speaker whose input impedance is on the order of a few ohms. - Both CE and CB have high output impedance This is because the gain is proportional to output impedance (In other words, you need high Rout to realize high gain) - We want an amplifier whose gain is low, but exhibits near-ideal I/O impedance characteristic, i.e., high Rin and low Rout Emitter Follower I C V IN,DC - Emitter Follower with Source Impedance RS R S I C 20

21 Summary) Input Output Schematic Common Emitter Common Base Emitter Follower No Degeneration With Degeneration /V in (V A = ) R in R out (V A ) When Use? to 21

22 Examples) 22

23 23

Physics of Bipolar Transistor

Physics of Bipolar Transistor Motivations - In many electronic applications, amplifier is the most fundamental building block. Ex Audio amplifier: amplifies electric signal to drive a speaker RF Power