BJT Amplifiers: Overview

Transcription

1 Indian Institute of Technology Jodhpur, Year 07 Analog lectronics (ourse ode: 34) Lecture 9 0: BJT Biasing, Amplifiers ourse Instructor: Shree Prakash Tiwari mail: sptiwari@iitj.ac.in Webpage: / / ourse related documents will be uploaded on Note: e information provided in the slides are taken form text books for microelectronics (including Sedra & Smith, B. azavi), and various other resources from internet, for teaching/academic use only BJT Amplifiers: Overview

2 Voltage Amplifier In an ideal voltage amplifier, the input impedance is infinite and the output impedance is zero. In reality, the input and output impedances depart from their ideal values. Input/Output Impedances e figures below show how input and output impedances are determined. All independent d sources are set to zero. impedance v i x x

3 Input Impedance xample Note that input/output impedances are usually regarded as small signal quantities. e input impedance is obtained by applying a small change in the input voltage and finding the resultant change in the input current: v i x x r Impedance at a Node When calculating I/O impedances at a port, we usually ground one terminal. We often refer to the impedance seen at a node rather than the impedance between two nodes (i.e. at a port). 3

4 Impedance seen at the ollector e impedance seen at the collector is equal to the intrinsic output impedance of the transistor, if the emitter is grounded. out r o Impedance seen at the ollector e impedance seen at the collector is equal to the intrinsic output impedance of the transistor, if the emitter is grounded. out r o 4

5 Impedance seen at the mitter e impedance seen at the emitter is approximately equal to the inverse of its transconductance, if the base is grounded. v i x x g out m g ( V ) A r m Impedance seen at the mitter e impedance seen at the emitter is approximately equal to the inverse of its transconductance, if the base is grounded. v i x x g out m g ( V ) A r m 5

6 Summary of BJT Impedances. Looking into the base, the impedance is r if the emitter is (ac) grounded.. Looking into the collector, the impedance is r o if emitter is (ac) grounded. 3. Looking into the emitter, the impedance is /g m if base is (ac) grounded and arly effect is neglected. Biasing of BJT Transistors must be biased because. ey must operate in the active region, and. eir small signal l model dlparameters are set by the bias conditions. 6

7 D Analysis vs. Small Signal Analysis Firstly, D analysis is performed to determine the operating point and to obtain the small signal model parameters. Secondly, independent sources are set to zero and the small signal model is used. Simplified Notation Hereafter, the voltage source that supplies power to the circuit is replaced by a horizontal bar labeled V, and input signal is simplified as one node labeled v in. 7

8 xample of Bad Biasing e microphone is connected to the amplifier in an attempt to amplify the small output signal of the microphone. Unfortunately, there is no D bias current running through h the transistor to set the transconductance. xample of Bad Biasing e microphone is connected to the amplifier in an attempt to amplify the small output signal of the microphone. Unfortunately, there is no D bias current running through h the transistor to set the transconductance. 8

9 Another xample of Bad Biasing e base of the amplifier is connected to V, trying to establish a D bias. Unfortunately, t the output tsignal produced dby the microphone is shorted to the power supply. Another xample of Bad Biasing e base of the amplifier is connected to V, trying to establish a D bias. Unfortunately, t the output tsignal produced dby the microphone is shorted to the power supply. 9

10 Biasing with Base esistor Assuming a constant value for V B, one can solve for both I B and I and determine the terminal voltages of the transistor. However, the bias point is sensitive to variations. Biasing with Base esistor Using KVL in the base emitter loop, V I B B V B = 0 or, I B = (V V B )/ B I = βi B = β(v V B )/ B Using KVL in the collector emitter loop, V I V = 0 or, V = V I Q(V,I ) is set 0

11 Improved Biasing: esistive Divider Using a resistive divider to set V B, it is possible to produce an I that is relatively insensitive to variations in, if the base current is small. Accounting for Base urrent With a proper ratio of to, I can be relatively insensitive to. However, its exponential dependence on // makes it less useful.

12 mitter Degeneration Biasing helps to absorb the change in V X so that V B stays relatively constant. is bias technique is less sensitive to (if I >> I B ) and V B variations. mitter Degeneration Biasing evenin s quivalent ircuit for the base emitter loop Base mitter Loop V β I B I B V B 0 or, I B V V B β ollector mitter Loop V V ( ) ( ) V I βi B β(v VB) β V I I V I (I IB)

13 mitter Degeneration Biasing Bias Stabilization I β(v V B ) β If << (β+), then I V V B So, I is independent of β V V ( ( ) ) mitter Degeneration Biasing helps to absorb the change in V X so that V B stays relatively constant. is bias technique is less sensitive to (if I >> I B ) and V B variations. 3

14 Bias ircuit Design Procedure. hoose a value of I to provide the desired smallsignal model parameters: g m, r, etc.. onsidering the variations in,, and V B, choose a value for V. 3. With V chosen, and V B calculated, V x can be dt determined. d 4. Select and to provide V x. Self Biasing Technique is bias technique utilizes the collector voltage to provide the necessary V x and I B. One important characteristic of this approach is that the collector has a higher potential than the base, thus guaranteeing active mode operation of the BJT. 4

15 Self Biasing Design Guidelines () () V B B V V B () provides insensitivity to. () provides insensitivity to variation in V B. mitter and ollector Feedback Bias 5

16 mitter and ollector Feedback Bias Applying KVL or, V (I +I B ) I B B VV B (β+)i B = 0 or, V (βi B +I B ) I B B V B (β+)i B = 0 or, V { B +(β+) ( + )}I B V B = 0 I B B V V B β ( ) V = V (I +I B )( + ) mitter and ollector Feedback Bias I B (V V B )β β ( ) V = V (I +I B )( + ) Bias Stabilization If B << (β+)( + ), then or, I (V ( V B ) ) So, I is independent of β 6

17 Summary of Biasing Techniques Transistor as an Amplifier (ac in active region) 7

18 PNP BJT Biasing Techniques e same principles that apply to NPN BJT biasing also apply to PNP BJT biasing, with only voltage and current polarity modifications. 8