基于ADS的功率放大器详解

说明档
写本文的目的在于如何用ADS设计大功率放大器，只是一个总结档。

所用芯片为MW6S9060N, 参考其他资料文献，并用于ADS中一步一步进行仿真。

特别说明：因为本文并不是为了设计PA，而是学习之用，文中有些部分实际中并不需要，例如偏置电压，不需要仿真，设计之后具体调节偏置电压以达到最佳IP3要求等。

线性度也是如此，因为当时就是想感受一下整个设计流程，就没调整线性度，真正设计的时候可以用功率回退10dB或者预失真技术解决线性度问题。

实际调试过程中，应该注重整体电路调整，不作描述。

文中可能会出现一些问题和错误，希望大家指正。

衷心期待各位射频设计高手指点。

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Topic: The Power Loadpull Technique For The
Impedance
Written by: Gaolong Sun
Date: 11th Sep, 2009
1. Introduction
This paper focus on a design of high power amplier in communication system. With the characteristic of high output power, high efficiency, gain, OIP3, power amplifier can be widely used in the world. This paper will use MW6S004N to show the procedure to how to design a power amplifier.
2. Basic knowledge a. RF amplifier block diagram:
b. Maximum available power from source () is transferred to load when
AVS P Ω==500Z Z IN c. Reflection factor: 00
Z Z Z Z V V IN IN +−==Γ+−
d. Voltage standing wave ratio (VSWR):
|
|1||1||||Γ−Γ+=
=
MIN MAX V V VSWR 1
1
||+−=
ΓVSWR VSWR
e. Return loss:
||log 20)(Γ−=dB RL
For one port network: )(||log 20||log 201111dB S S RL −=−=Γ−= For two port network: ||log 20||log 201111S RL −=Γ−= ||log 20||log 202222S RL −=Γ−= f. Mismatch loss:
Mismatch factor:
2||1Γ−=M Mismatch loss: )||1log(10)log(102Γ−−=−=M ML g. Gain:
)()/log(10)(21dB S P P dB G in out == h. Noise figure:
)]//()/log[(10)(o o i i N S N S dB NF = i. Power out at 1dB compression point: dB P 11)()()(1−=−dB G dBm P dBm P out in dB j. Efficiency:
Collector efficiency: DC
DC out
DC out C I V P P P =
=
η Power added efficiency: DC
in
out PAE P P P −=
η Total efficiency: in
DC out
T P P P +=η
k. Distortion: Harmonic Distortion
AM to PM conversion Intermodulation distortion OIP3
3.DC bias
We design the circuit with high output power and high efficiency. So before we begin the design, it is necessary to let the transistor work at an appropriate position which is called quiescent operating point.
The schematic diagram:
Thus, DC bias simulation is shown:
4.Stability
a. Unconditionally stable:
For passive matching networks: S Γ<1 and L Γ<1 Unconditionally stability: ||=|+
IN Γ11S L
L S S S Γ−Γ2212
211|<1
||=|+
OUT Γ22S S
S S S S Γ−Γ1112
211|<1
b. Potentially unstable: For certain cause that L Z ||=|+
IN Γ11S L L S S S Γ−Γ2212
211|>1
||=|+
OUT Γ22S S
S S S S Γ−Γ1112
211|>1
c. Output Stability Circles on -Plane L ΓLet ||=|+
IN Γ11S L
L S S S Γ−Γ2212
211|=1, so | |IN Γ-|=, we call this as output stability circle.
L C L r The centre of a circle: 2
222*
*
1122||||)(Δ−Δ−=S S S C L , Radius: 222222112||||||Δ−=S S S r L Where 21122211S S S S −=Δ
Thus, we obtain output stability circle construction as follows:
Output stability
Now, we check the centre of a circle (L Γ=0), from the formula above, . ||||11S IN =ΓIf ||>1, =0, there will be unstable region. 11S L ΓIf ||<1, =0, there will be stable region. 11S L ΓAlso, it is the same to the input stability circle.
d. Necessary and sufficient conditions for unconditional stability:
|<1 and <1
|11S ||22S Stability factor K=
|
|2||||||121122
2222|11S S S S Δ+−−>1 and ||Δ<1
Where 21122211S S S S −=Δ
Thus, we simulate the stability factor K of the device to see whether or not the transistor is working at a stable area.
After simulation, we can see the stability circle is out of the Γ-Plant. That means whatever we match the circuit, the device is always unconditionally stable.
5. Loadpull setup
As we known, Loadpull setup in ADS can tell us the efficiency and the output power of the device. We can easily read the data in the graph about PAE η and , and change the range of resistance to choose a position which has a high efficiency and high output power.
out P
The schematic is shown as follows:
Here we analyse the result, we can change the positon of centre and the radius of the circle.
11
S
Result from simulation, it is shown PAE=22.07% and Power delivered=29.08dBm. It is a tradeoff between PAE and output power, choose one point which is the high efficiency and high output power, write down the data (S11 centre: -0.311+j*0.206 and S11 _rho_actual: 0.620)
Now we can generate our output matching circuit by a smith chart:
Validation:
Now check the result of simulation with the circuit:
It is clear that the matching is good at 1817.5GHz.
Sourcepull setup is the same as loadpull setup, here we only give a result of simulation.
The whole circuit is shown as
follows:
6. Power
1)()()(1−=−dB G dBm P dBm P out in dB
Result from simulation, we can see that the linear is not very good here. The picture gives us and the output power. dB P 1
7. Efficiency
8.OIP3
After all, IM3 is also an important specification, because it can not be removed by filter, it is a in-band signal. If IM3 is not good, PA will not have a good performance.
There is about 30dBc here, we can change the current to adjust OIP3 which we need.
9.Conclusion
ADS can help engineers to design high power amplier fastly. Also, we can easliy simulate the result and analyse the data. With a bisic theory of RF, how to put the theory into practice is a big issue. With the help of ADS, we will avoid from the long formula to finish our designs.。