Broadband Amplifiers

Broadband Amplifiers Broadening is obtaining flat gain in desired frequency range. Broadband Amplifiers can be obtained using: Compensated matching networks Negative feedback Balance amplifiers. Some of difficulties of this work: Variation [S] with frequency as shown: Degradation of noise figure with frequency as shown: A method develop by Mellor as shown in: Q1 and Q2 have a gain that decreases by increasing of frequency. Inter stage matching compensate it as: G f

Broadband Amplifiers Example: [S] a BJT is given as: Design a broadband amplifier with GT=10dB in frequency range of 300-700MHz. Refer to: Therefore in input matching isn't possible. Therefore in output is possible to get 4dB gain. 300 F (MHz) 400 700 13 |S21|2 (dB) 10 6 -3 +4 Task-LN07-01: Delivery 91.02.11 Plot overall gain in ADS.

Balanced Amplifiers Compensated matching networks degrade VSWRs. A balanced amplifier have good flatness and VSWR. 3dB-Coupler (Power Divider) E0ej0 0.707E0e+j90 0.707E0e-j90

Broadband Amplifiers Port 4 is isolated and then coupler is reduced a 3-port network: Overall scattering parameters a balanced amplifier: Advantages a Balanced amplifier: If VSWR a single amplifier is bad, VSWR in balanced configure is good. Stability will be high. Output power will be twice that obtained from a single amplifier. If one of the amplifiers fails, output operates with reduced gain 6dB. They are easy to cascade with other units, since each unit is isolated by coupler.

Broadband Amplifiers Example: A balanced amplifier including: Determine: S11, S22, (VSWR)in & (VSWR)out: Transducer gain GT. Transducer gain if B fails. Solution: if amplifier B fails:

Broadband Amplifiers A balance amplifier by using Wilkinson divider.

Broadband Amplifiers Feedback Amplifiers: Feedback amplifiers are broadband but: Degrades the noise figure. Reduces maximum power gain. Equivalent model of a amplifier without parasitic elements (low frequency):

= = Broadband Amplifiers [Y]= Admittance a FET: Using CH1, [Y] can be convert to [S]: To have S11=S22=0: In this condition: S21 show that gain dependents on R2 not on FET. By using a similar way, BIJ can be analyzed. = [Y]= Where: =

Broadband Amplifiers Shunt Models: For BJT we can show that: = = = =

Broadband Amplifiers Example: An amplifier having series-shunt feedback as: BJT have: Design for GT=10dB, VSWRin=VSWRout=1 Solution: With no feedback we have: Therefore certainly we have: Using: At microwave frequency the use series feedback R1 improve stability but degrade F For: Using CAD

Broadband Amplifiers |S21|2≈54dB Example: Design a BJT amplifier having GT=10dB from 10-1500MHz. Solution: Therefore, it is certainly capable providing GT=10dB Stability circles are shown: Analysis show that a 300ohm shunt resistor can be stable the output. |S21|2≈54dB Output stability circles Stability Circles

Broadband Amplifiers Stable Solution (cont.): Overall [S] is: |S21|2 is reduced but still sufficient to have GT=10dB. As presented in appendix “Example CAD.5”, to improve VSWR (broadening) a negative feedback is applied as: To improve performance, by using CAD, R2 & L2 can be tuned: VSWR is very bad VSWR is bad VSWR is bad

Broadband Amplifiers Go to appendix “Example CAD.5” Solution (cont.): To improve VSWRs, matching networks must be designed as: Task-LN07-02: Delivery 91.02.11 Go to appendix “Example CAD.5” Implement only BJT in ADS and show results. Modified the design as mentioned and then show results.