Power Amplifiers

22 Pages·2007·0.24 MB·English

ECE 145A/218A

Power Amplifier Design 2

Power Amplifiers; part 2
1. PA impedance matching - large signal
a. Load pull contours
b. Package parasitics
c. Large signal input match
2. PA bias circuits: design for stability

How does mismatch affect output power?

“Load Pull”
V

DC
I = I sinθ

D M
I

DC
g V
m gs
I
Z V
D
L DS

V V Z = R + jX

in os L

V = −V sinθ

DS M

V V
Let Z = R = DC = DC .
L opt I 2 I
MAX DC

This is the optimum power match (neglecting knee
voltage).

5/28/07 1 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

V I
P = DC DC
opt
2

2
V 1
= DC = I2 R
DC opt
2R 2
R
opt
opt

V

DC

0

I

max
I

D
Full voltage and
I

DC
current swing.
(neglecting V )
knee
0
5/28/07 2 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

Ref. S. Cripps, RF Power Amplifiers for Wireless Communications, Artech House, 1999.
5/28/07 3 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

To determine the influence of a mismatched load on the
power amplifier output power and also efficiency, we must
trace out a contour on the Smith Chart that will give a
power of P / p. Here, p is the power reduction factor.
opt

There are two solutions.

Resistive Terminations

⎧ V
1) pR ⎪⎨Im = pRDoCpt P = 1 VD2C = Popt
OPT o
2 pR p
⎪ opt
V =V
⎩ m DC

2) Ropt ⎧⎪Im = Imax / 2 =IDC 1 ID2CRopt Popt
p ⎨ P0 = =
2 p p
⎪V = I R / p
⎩ m DC opt

R = P / p
HI opt

R
opt
R

R = opt

LO P

5/28/07 4 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

R
Series reactance; When R = opt =R , add jX.
LO
p

jX

R / p = R V = I R + jX

opt LO M DC LO

in this limit when R = R , we have power limited by I .
LO MAX
Voltage V < V . So, series reactance will increase the
m DC
voltage swing but not affect current.
This is valid up to ±X , where R + jX = R
m LO m opt

Shunt susceptance; When R = pR =R , add jB
opt HI

jB 1
G =

pR
opt

at this limit, power is limited by the voltage swing,
V = V . I < I .
m DC D MAX

5/28/07 5 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

Shunt susceptance can be added. It will cause current to
increase until
1
G + jB =
m R
opt

These are called Load Pull Contours, and they follow
constant r or g circles on Smith Chart. It can be shown that
the intersection of these circles represents X or B .
m m

Po = P /p
Boundary for
opt

Boundary for P = P / p
0 opt
5/28/07 6 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

Ref. S. Cripps, RF Power Amplifiers for Wireless Communications, Artech House, 1999.

The load pull contour takes the place of gain circles for
power amplifier designs.
5/28/07 7 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

At higher frequencies, device capacitances and package
capacitance and inductance can be significant.

A

L

pkg

R
C
C L
out
1

B

Utilize C as part of matching network-
OUT
• a pi-section can accomplish this if
C and L is not extremely large.
OUT pkg

• Smith chart or design equations can be
used to design network. Q ≤ 5 is
reasonable for narrowband design.

• Note that we must provide the load line
match at plane A, not plane B.
5/28/07 8 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

What does the package do to Z ?
L

A
B

large small
x , b
L c
B
A

L
pkg

R
opt
C
out

We require that ZA = Ropt, so there is a problem – the
output capacitance and the package will transform Ropt
into something else. So, Ropt must be compensated to
account for the package.

5/28/07 9 of 22 Prof. S. Long
ECE 145A/218A

Power Amplifier Design 2

Input Matching
For PAs, we often have large voltage or current
variations on input. This can lead to large input
impedance variation with drive power, P .
IN

So, what procedure can be used to design input
network?

For Class A, we never cutoff or swing into knee
region of device I-V.

- 1st approximation: use small signal S parameter
calculation to find Γ at quiescent bias.
IN

- Conjugately match the input.

- If gain is adequate, stop. Otherwise, you may need to
optimize Γ experimentally in simulation. (source
S
pull)

5/28/07 10 of 22 Prof. S. Long