Download File - David Callen

EE 414
Lab 3
Amplifier Design
Tyler Bohlke
David Callen
Introduction
In this lab we put into practice the methods of amplifier design learned in class. These
methods include, but are not limited to simultaneous conjugate match, tuning, stability, and sparameter calculation. Our task was to design an amplifier that was stable from 300 kHz to 6.0
GHz with an operating frequency of 1.1 GHz. The circuit had a specification to maintain a
bandwidth of +/- 5 MHz and have a gain of
20 log|𝑆21 | ≥ 16 − (6 ∗ 1.1) = 9.4 dB
In order to be sure we achieve this gain, we were encouraged to add 3 dB to our
calculated gain so we could ensure 20*log|S11| and 20*log|S22| (our return loss) stayed below -15
dB. The drawback to adding this gain was that it would ultimately make our circuit more
difficult to stabilize along our desired frequency range.
Step 1: Adding Solder Pads to the BJT
The first step in designing our amplifier was to add MLIN
structures (75x75 mil2) to each terminal of the BJT (emitter, base,
collector). These structures are used to simulate the solder pads of
the device once it is finally assembled. Figure 1 shows an MLIN
used in our design.
At this point it was necessary to add a 0.3 nH inductor to
the device to model our ground once the physical circuit is built.
This inductor is connected to a solder pad that is split in half, and
it is the most accurate model for a ground we can obtain for our
circuit.
Figure 1: MLIN structure
Step 2: Stabilizing the Initial Network
The second step in designing our amplifier was to stabilize the transistor (BFP620) before
we added any components to it. In order to stabilize the initial circuit, we needed find a suitable
value for the stabilization resistor RSTAB. This was done by finding our desired stability using
the following equation.
Once we found this value of K, we inserted a 1Ω resistor at the base of the transistor and
found a new K (all of this was done using ADS). We could now find the slope that these two
values of K and the 1Ω resistor created, and from there, find a value for RSTAB that would ensure
stabilization (K > 1, or µ > 1). All calculations to find RSTAB were done using either ADS or
MATLAB. The MATLAB code and final values are attached at the end of lab. Our final value
for RSTAB came out to be 2.4826Ω.
At this point the transistor portion of our circuit is complete. Figure 2 shows our circuit
at this point in the design stage.
Figure 2: The BJT with solder pads, ground, and RSTAB
Step 3: Simultaneous Conjugate Matching
Once the BJT was modeled with a proper stabilization resistor and our ground, it was
time to match each end to 50Ω terminations. To do this, first we had to convert our s-parameters
into z-parameters, with a normalization impedance (Z0) of 50Ω. We decided to use MATLAB
for the conversions, and the MATLAB code can be found at the end of this lab. For simplicity’s
sake, we used the Smith chart tool (in ADS) to obtain values for our inductors and capacitors for
the simultaneous conjugate match. For port S11 we chose to go with a high-pass match, and for
port S22 we again used a high pass match. The reason we chose high pass for each port was
because we would ultimately use the series capacitors to facilitate our DC blocking needs.
Figure 3 shows the ADS Smith
chart tool in use. The initial
value for our series capacitor at
the port 1 match was
14.4462pF, and our parallel
inductor was 4.3383nH. At
port 3, our initial value for the
series capacitor was 1.2246pF
and our parallel inductor had a
value of 14.580nH. It is
worthwhile to note that the
initial value for our matching
components are only to get our
terminals to match our Zin* and
Zout*. Because we will be
Figure 3: The Smith chart tool used to calculate our matching networks.
adding more components to our
amplifier, the matching networks will be frequently tuned to new values to ensure we maintain
our proper gain and stability. This conjugate match simply gives us a nice starting point.
Step 4: Stabilization Across all Frequencies
Once the simultaneous conjugate match was complete, it was time
to stabilize our amplifier across the necessary frequency range. For
this lab, our range was from 300 kHz to 6.0 GHz. Stabilization
requires us to add loss to the amplifier at frequencies other than the
design frequency. To do this, we must place either a series LC
component in parallel with a series resistor (figure 4), or a parallel
LC component in series with a shunt
resistor (figure 5). When we do this, we
chose our inductor and capacitor so they
Figure 4: Series LC in parallel with R resonate at the design frequency. With
the series LC in parallel with the
resistor, at resonant frequency (1.1GHz) they act as a short circuit,
and so the resistor will not affect the loss at that frequency. In
contrast, the parallel LC component in series with the resistor act as
an open circuit at resonant frequency, so the signal will not pass
Figure 5: Parallel LC in series with R
through those components, which are connected to ground. By
arranging where we place each of these components, we can
effectively stabilize our amplifier across all frequencies. We chose to place each of these blocks
at port 2 of our amplifier, because at our design frequency, that location gave us the most gain
and least amount of instability. We were able to simply tune the circuit to where (at this point)
20log|S11| and 20log|S22| were approximately -45 dB and our µ value (stability) was greater than
1 for all required frequencies.
Step 5: Adding Real Components and RETMA Values
At this point, this amplifier has it matching networks in place, is stable across the desired
frequency range, and has a return loss (20*log|S11|) of around -35dB to -40dB. We are not
finished with the design because all of our connections and components are ideal, and we are
using non-RETMA values for our resistors, capacitors, and inductors. It is at this point where
must design our circuit to the point where it can be realistically fabricated. We do this by
introducing solder pads (MLIN), inserting our physically modeled R, L, and C components, and
then finally changing those component values to RETMA values.
We started with the solder pads. We needed at least a 75x75mil2 solder pad to connect
one terminal to another. Several terminals had at least 3 necessary connections which meant we
needed to adjust the size of the
solder pad in order to acclimate it
to multiple connections. Figure 6
shows a connection of four
terminals, which required us to
adjust the physical dimension of
the pad by doubling it. As we
added more and more pads, the
circuit would slowly start lose
gain, and at some point would
become unstable. This was dealt
with by meticulously adjusting our
component values for our
Figure 6: A four terminal solder pad
matching networks. Depending on
the location of the pad, it would act slightly inductive, or slightly capacitive, and we would tune
our matching networks accordingly to absorb the parasitics.
Once all of the solder pads were in place, and our matching networks were sufficiently
tuned, it was time to add our models for our resistor, capacitor, and inductor. These models were
found and adjusted in the previous lab. One by one the models were inserted and the circuit was
tuned to maintain stability. After the models were all in place, it was time to switch them all to
RETMA values. These last two steps were the largest contributors to our loss of gain throughout
the design process. Before we started inserting component models and RETMA values, we
sitting at around -38 dB for return loss, after everything was complete (up to this point) our
return loss was a respectable -23.469 dB for S11 and -20.518 dB for S22.
The Final Circuit
Our final completed circuit can be seen in figures 7, 8, and 9. We have the DC
connections set up, but no DC bias regulator circuit. That will be designed in a future lab (the
oscillator). The full schematic can be found in the appendix at the end of this lab report.
Figure 7: The source terminal and Γin = Γs* match
Figure 8: The BJT and DC connection
Figure 9: The load terminal and Γout = Γl* match
Conclusion
In this lab we were able to successfully design and simulate an RF amplifier with a gain
of 9.4dB, a return loss of -23 dB and -20 dB for S11 and S22 respectively, and stability from 300
kHz to 6.0 GHz. This was an intense and detail-oriented lab. The most difficult part was tuning
our component values to ensure stability and the lowest possible return loss. While the effort put
forth was tremendous, the information gained was just as worthwhile. It is satisfying to see how
an amplifier of this sort is designed from start to finish, and it gave us an insight as to how more
effectively design amplifiers in the future.
Appendix
1.
2.
3.
4.
The Full image of our final circuit
The return loss (top) and stability verification (bottom) of the final circuit
The MATLAB code for finding input and output impedance (Zin and Zout)
The MATLAB outputs for input and output impedance (Zin and Zout)