Peoples Democratic Republic of Algeria
Ministry of Higher Education and Scientific Research
University MHamed BOUGARA Boumerdes
Institute of Electrical and Electronic Engineering
Department of Electronics
Lab Experoment #:4
Microwave Amplifiers Design
Presented By:
- BRAHIMI Mohamed Amine
- BOUAICHA Abdenour
Supervisor:
Dr.CHALLAL
Abstract :
In This Lab we consider the analysis and design of microwave transistor
amplifiers based on S parameters approach with the following
specifications.
1. The frequency is set at 0.1 GHz
2. The S-matrix used is based on the transistor BTP 520 with Vce=2V
and Ic= 20mA.
Introduction:
Throughout the previous experiments we dealt with matching networks and
stability considerations, the aim of such topics was to gain the necessary
tools that allows us to implement more practical circuits. In this lab
experiment , microwave transistor amplifiers are considered.
Objectives:
- Designing high power amplifier using ADS
Theoretical part :
We have already talked about stabiliry in previous labs so here we present
only the algorithm of amplifier design
1. First we check the stability
2. if the amplifier is unconditionally stable we calculate sm and lm
3. We implement the input and output matching networks.
4. If 2 is not valid than maximum gain design is not possible so we fix
the gain at a given value, usually we start from the maximum and we
decrement 1 dB each time till we reach a gain that leads to a stable
s l.
5. We do the same as in 3.
Practical Part:
- In this experiment we are going to design a microwave transistor
power amplifier using a BPF microwave transistor at a frequency of
0.1 GHz. The s parameters of this transistor are shown below
f(GHz) S11 S12 S21 S22
0.1 0.7251 0.004192.8 31.637171.4 0.9363
8.4 4.4
Figure 1: The S parameters of the given transistor
- A transistor at a given frequency is totally described by its S matrix
and hence if we choose any two-port network with the same
parameters at the same frequency the response will be identical. The
S-parameter base two-port network is available in ADS as S2P_Eqn.
Using this latter with the above S-parameters we have built the
following circuit.
Figure 2: The BFP 520 transistor as a two-port network
1. Checking stability
- Using ADS we calculated the stability factor K.
Figure 3: The Stability factor using ADS
K=0.0416886 <1
- Note that calculating is not needed to determine stability here as K
greater than 0 is a necessary condition for unconditional probablilty so we
conclude that the transistor is potentially untable.
2. Using the S_StableCircle and L_StableCirle components available in ADS
under the header Simulation S-parameter we draw the source as well as
the load stability circle.
Figure 4: The source as well as the load Stability circles
- For both circles the stability region is outside the circle because |S11| and
|S22| are both less than 1 ( stable reflection coefficients for input and
output ports).
4. Designing the amplifer
The amplifier is not unconditionally stable and hence maximum gain design
is not possible since the corresponding s and l will not be stable or at
least one port will oscillate. So one must look for the gain that is as high as
possible and leads to a stable amplifier at the same time.
Figure 5 shows the power gain circles for many gains (each one is less that
the other by 1 dB)
Figure 5: Power gain circles for different gains
- We draw a line from the center of the smith chart that passes by the
center of the gain circles as shown in the following pictures, we know
that the point where this line intersects the gain circles from the side
closer to the center of smith chart is the one that yields the more
stable source reflection coefficient.
We start from the second circle whose gain is 1 dB less than maximum
and then the third and so on till we find a circle that leads to a stable
amplifier.
1. For the second circle :
Gain : 38.874053
zs=2.55 +i*0.95 ( at the intersection of the line with the circle)
s=0.4743 + 0.1407i
out= 1.0021 - 0.1582i
We stop here since out is not stable.
2. For the third circle :
Gain : 37.874053
zs=2.0200 + 0.5500i
s=0.3590 + 0.1167i
out=0.9804 - 0.1296i
l=0.9804 + 0.1296i
Zl= 1.2816 +15.0849i
- We can match here at this level but in terms of stability the
component is very reactive and at the verge of instability so it is
better to move to the other circles.
3. For the Fourth circle :
Gain : 36.874053
Zs= 1.8000 + 0.3500i
s =0.2967 + 0.0879i
out=0.9674 - 0.1180i
out=0.9674 +0.1180i
Zl= 3.3443 -15.7461i
Those values are better in a sense that both the gain and stability is high
( so if some fluctuations happened in the system the stability is preserved)
1. For the given Zs we use ADS to build the source matching network :
Figure 6: Designing the input matching network
2. We use the above Zl to implement the output matching network
Figure7: Designing the output matching network
3. After we finish with the matching networks we press build the circuit
to get the amplifier of the figure.
Figure8: The microwave amplifier of gain G=36.874053 dB
Conclusion :
In this lab we have went through the design of microwave power
amplifiers, we have seen that in the design of potentially unstable amplifiers
one should look for the gain that is both high and leads to a stable input and
output ports. The stability should be checked for both ports so one must be
careful in choosing the input reflection coefficient as a stable Input doesnt
mean necessarily a stable output.
