Electronic Circuits ELCT604 Lecture 10: Feedback Amplifiers

Transcription

1 Electronc Crcuts ELCT604 Lecture 10: Feedback Amplfers Assstant Professor Offce: C3.315 E-mal: 1

2 Introducton Negatve Feedback technque s appled to Electronc crcuts amplfers for the followng benefts: To Stablze the Gan aganst devce parameters varaton and agng To ncrease/decrease the amplfer s nput/output esstance Increase the amplfer s Bandwdth educe dstorton However, t has some drawbacks: Gan educton Tendency for Oscllaton (Unstable system?) 2

3 Introducton Feedback System Structure: The system conssts of a basc Amplfer wth gan A and a feedback network wth gan β The nput, feedback and output sgnals X can be voltage or current 3

4 Feedback System 4 Feedback System Equatons: X o A X fb X X X X X X X S fb S o A o X o A X 1 A Closed-loop Gan T A S A 1 Loop Gan For T>>>>1

5 Feedback System Gan Senstvty to ParameterVaratons What s the senstvty of the closed-loop gan A to any varatons n the basc amplfer s gan A? Senstvty da A A da 1 A A 1 da A A Senstvty da da da 1 A A A 1 A 1A da da 1 A A 1T 2 2 5

6 Feedback System Gan Senstvty to ParameterVaratons What s the senstvty of the closed-loop gan A to any varatons n the basc amplfer s gan A? Senstvty da da 1 A A 1T Any varaton n the basc amplfer s gan A wll lead to a change n the closed-loop gan dvded by (1+T) For ths reason the feedback amplfers are more mmune to dstorton as well 6

7 Feedback System Bandwdth of the Feedback Amplfer A What s the value of the hgher 3-dB frequency of the closed-loop gan A? Assume that the basc Amplfers gan s: AMF s 1 H AMF 1 s 1 H A 1 AMF 1T s 1T Feedback ncreases the Bandwdth of the Amplfer H A HF AMF s 1 H H, H 1T 7

8 Feedback Confguratons 8

9 Feedback Confguratons There are Four basc Feedback Confguratons (Connectons) They are categorzed based on whether the feedback/output sgnal s Current orvoltage current Feedback Sgnal Output Sgnal Feedback Confguraton Amplfer s Type Voltage Voltage Seres-Shunt Voltage Current Current Shunt-Seres Current Voltage Current Seres-Seres Trans-conductance Current Voltage Shunt-Shunt Trans-resstance Source Sgnal must be the same type as the feedback sgnal 9

10 Seres-Shunt Connecton The feedback network s connected n seres wth the I/P node and shunts the O/P sgnal The feedback voltage sgnal s proportonal to the output voltage Ths confguraton s used for Voltage Amplfers A v o A v 1 A S 10

11 Seres-Shunt Connecton What s the effect of Seres-Shunt feedback on the Input and Output esstances of the amplfer? Assume the basc amplfer has fnte n and out n, v v v s fb A n, n 1 Seres Feedback at the nput ncreases the Input esstance of the Amplfer n, v 1 A 11

12 Seres-Shunt Connecton 12 What s the effect of Seres-Shunt feedback on the Input and Output esstances of the amplfer? Short Crcut the Source voltage and add av T at the output out, out, V T T v v fb V T IT IT out Shunt Feedback at the output decreases the Output esstance of the Amplfer out 1 A V Av

13 Shunt-Seres Connecton The feedback network s connected n parallel wth the I/P node and n seres wth the O/P node The feedback current sgnal s proportonal to the output current Ths confguraton s used for Current Amplfers A o A 1 A S 13

14 Shunt-Seres Connecton What s the effect of Shunt-Seres feedback on the Input and Output esstances of the amplfer? Assume the basc amplfer has fnte n and out n n, v s n 1 A Shunt Feedback at the nput decreases the Input esstance of the Amplfer n, vs vs 1 A s 14

15 Shunt-Seres Connecton What s the effect of Shunt-Seres feedback on the Input and Output esstances of the amplfer? Open crcut the current Source out, V I A Seres Feedback at the output ncreases the Output esstance of the Amplfer T T out, out 1 fb IT V T IT A IT out 15

16 Seres-Seres Connecton The feedback network s connected n seres wth the I/P node and n seres wth the O/P node The feedback voltage sgnal s proportonal to the output current Ths confguraton s used for Trans-conductance Amplfers A o A v Note: A and β are not 1 dmensonless A S A A n, n 1 out, out 1 16

17 Shunt-Shunt Connecton 17 The feedback network s connected shunt wth the I/P node and shunt wth the O/P node The feedback current sgnal s proportonal to the output voltage Ths confguraton s used for Trans-resstance Amplfers A out, v o A Note: A and β are not 1 dmensonless A n, S n 1 A out 1 A

18 Feedback Crcuts analyss 18

19 Feedback Crcut Analyss Soluton Steps: 1. Identfy the Feedback Confguraton type whether t s seres or shunt at the Input and Output of the basc Amplfer 2. Calculate the feedback network gan β The nput sgnal contrbuton s neglected If the feedback network at the nput s shunt, then short the nput node to ground and calculate the feedback current If the feedback network at the nput s seres, then open crcut the nput node and calculate the feedback voltage Irrespectve of the feedback network connecton at the nput, f ts connected shunt/seres to the output node, then drve the feedback network wth a voltage/current Source 19

20 Feedback Crcut Analyss Soluton Steps (Cont.): 3. edraw the basc Amplfer to fnd the gan A Start wth the nput loop whle neglectng the feedback sgnal at the output of the amplfer If the feedback network s connected shunt to the output node, then short crcut the output node to ground. If the feedback network s connected seres to the output node, then open crcut the output node. For the output loop, neglect the feedback sgnal from the nput of the amplfer If the feedback network s connected shunt to the nput node, then short crcut the nput node to ground. If the feedback network s connected seres to the nput node, then open crcut the nput node 20

21 Feedback Crcut Analyss Soluton Steps (Cont.): 4. Calculate the basc amplfer gan A, nput and output resstances. 5. Use the feedback rules to calculate the closed loop gan, closed loop nput and output resstances. 21

22 Solved Examples 22

23 Example One Fnd the closed-loop Gan, Input and Output esstances of the amplfer? Soluton: 1. F (Feedback Network) s connected to the nput node and to the output node Shunt-Shunt Confguraton 23

24 Example One Soluton: 2. Calculatng β v fb o fb v v v o o F F 1 F 24

25 Example One Soluton: 3. edraw the basc Amplfer Snce we had shunt-shunt, we wll short crcut the nput and output node 25

26 Example One v Soluton: 4. Calculate the basc amplfer s gan, nput and output esstances A v o v F L out o F // L // V // F n a V F // L A a // // 26 F L out v F n, // n A F n, // out A F L out

27 Example One Soluton: 4. Calculate the Closed-loop amplfer s gan, nput and output esstances A F // L av F // n F // L out F // L 1 1 av F / / n // F L out F n, F // n 1 A out, // F L out 1 A 27

28 Example Two Fnd the closed-loop Gan, Input and Output esstances of the amplfer? Soluton: 1. Feedback Network s connected n seres to the nput and output loop Seres-Seres Confguraton 2. Calculatng β v fb E2 E1 3 o E 1 E 2 F E 1 v o fb o E 1 E1 E 2 E 1 E 2 F 28

29 Example Two Soluton (cont.): 3. edraw the basc Amplfer Snce we had seres-seres, we wll open crcut the nput and output node 29

30 Example Two Soluton (Cont.): 4. Calculate the basc amplfer s gan, nput and output esstances A v 4. Calculate the closed-loop gan,, nput and output esstances o Assgnment! 30