Microwave Amplifiers Analysis and Design, G. GonzalezBroadband & High Power Amplifier Broadband...

Microwave Amplifiers Analysis and Design, G. Gonzalez Broadband & High Power Amplifier

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



300 F (MHz)400 700

13

|S21|2

(dB)

106

-3

0 +4

Task-LN07-01: Delivery 91.02.11

Plot overall gain in ADS.

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.


Balanced Amplifiers

Compensated matching networks degrade VSWRs.

A balanced amplifier have good flatness and VSWR.

3dB-Coupler(Power Divider)

3dB-Coupler(Power Divider)

E0ej0

0.707E0e+j90

0.707E0e-j90


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.




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:



A balance amplifier by using Wilkinson divider.


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):




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:

=

=



Shunt Models:

For BJT we can show that:

=

=

=

=



Using CAD

For:

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



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



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:

Stable

VSWR is very bad

VSWR is bad

VSWR is bad



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.

Date post:	24-Dec-2015
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