1. ELEC 412 - Lecture 191 ELEC 412 RF & Microwave Engineering Fall 2004 Lecture 19

2. ELEC 412 - Lecture 192 Stub Tuner Matched RF Amplifiers Stub tuners can be used to match sources and load to S 11 * and S 22 * of the RF BJT or FET Either open or short circuit stubs may be used When using short circuit stubs, place a capacitor between the stub and ground to produce RF path to ground – Do not short directly to ground as this will affect transistor DC biasing High resistance /4 transformers or RFC’s may be used to provide DC path to transistor for biasing without affecting the RF signal path

3. ELEC 412 - Lecture 193 Stub Tuner Matched RF Amplifier Series Resonant Ckt at Operating Frequency: Short Ckt at Resonance, Open Circuit at DC /4 Transformer: Transforms Short Circuit at Resonance to Open circuit at BJT Collector Thus Isolating RC from RF Signal Path Stub tuners of two types: Base-Side: Open Circuit Stub w/ Isolation from DC Bias Circuit Using RFC. Collector-Side: RF Short Circuit Stub via By-Pass Capacitor The BJT “Self-Bias” Configuration Is Shown Which Produces Excellent Quiescent Point Stability Power Supplies Are Cap By-Passed and RF Input and Output are Cap Coupled

4. ELEC 412 - Lecture 194 Stub Tuner Matched RF Amplifier Simpler method of bias isolation at BJT collector: C BP is RF short-circuit which when transformed by the Quarter-Wave Transformer is open circuit at the Single Stub Tuner and provides DC path for the Bias Network

5. ELEC 412 - Lecture 195 Design Strategy: RF Amplifiers Objective: Design a complete class A, single-stage RF amplifier operated at 1 GHz which includes biasing, matching networks, and RF/DC isolation.

6. ELEC 412 - Lecture 196 Design Strategy: RF Amplifier Design DC biasing conditions Select S-parameters for operating frequency Build input and output matching networks for desired frequency response Include RF/DC isolation simulate amplifier performance on the computer

7. ELEC 412 - Lecture 197 Design Strategy: Approach For power considerations, matching networks are assumed lossless

8. ELEC 412 - Lecture 198 Power Relationships Transducer Power Gain

9. ELEC 412 - Lecture 199 Stability of Active Device

10. ELEC 412 - Lecture 1910 Stability of Amplifiers In a two-port network, oscillations are possible if the magnitude of either the input or output reflection coefficient is greater than unity, which is equivalent to presenting a negative resistance at the port. This instability is characterized by |  in| > 1 or |  out| > 1 which for a unilateral device implies |S11| > 1 or |S22| > 1.

11. ELEC 412 - Lecture 1911 Stability Requirements Thus the requirements for stability are and These are defined by circles, called stability circles, that delimit |  in | = 1 and |  L | = 1 on the Smith chart.

12. ELEC 412 - Lecture 1912 Stability Regions: Stability Circles Regions of amplifier stability can be depicted using stability circles using the following: Output stability circle: Input stability circle:

13. ELEC 412 - Lecture 1913 Stability Regions: Stability Circles Where:

14. ELEC 412 - Lecture 1914 Stability Regions: Output Stability Circles

15. ELEC 412 - Lecture 1915 Stability Regions: Input Stability Circles

16. ELEC 412 - Lecture 1916 Different Input Stability Regions Dependent on ratio between r s and |C in |

17. ELEC 412 - Lecture 1917 Unconditional Stability Stability circles reside completely outside |  S | =1 and |  L | =1. Rollet Factor:

18. ELEC 412 - Lecture 1918 Constant Gain Amplifier

19. ELEC 412 - Lecture 1919 Constant Gain Circles in the Smith Chart To obtain desired amplifier gain performance

20. ELEC 412 - Lecture 1920 Circle Equation and Graphical Display

21. ELEC 412 - Lecture 1921 Gain Circles Max gain  imax =1/(1-|S ii | 2 ) when  i = S ii * ; gain circle center is at d gi = S ii * and radius r gi =0 Constant gain circles have centers on a line connecting origin to S ii * For special case  i = 0, g i = 1-|S ii | 2 and d gi = r gi = |S ii |/(1+|S ii | 2 ) implying  i = 1 (0 dB) circle always passes through origin of  i plane

22. ELEC 412 - Lecture 1922 Trade-off Between Gain and Noise