TAMPERE UNIVERSITY OF TECHNOLOGY
Institute of Communications Engineering
8307030 Communication Circuits and Modules
Auditorium Exercise 4
Common-Base Low Noise Amplifier
In this exercise we use PSpice to simulate a Common-Base LNA. We simulate DC
operating points, AC-gain, Input Return Loss, Noise Figure and Transient
behavior.
1. Introduction
Before going to actual simulations, let’s add some useful macros to the file
C:/Cadence/PSD_14.2/tools/pspice/common/pspice.prb (it is possible that
these have already been added). These macros will facilitate our LNA simulations.
Search for the item [MACROS] in the file and add the following lines starting from
VSRCSQ = … as shown below.
[MACROS]
pi=3.14159265
VSRCSQ = 16.5757565E-21*V(RSRC:1,RSRC:2)/I(RSRC)
NF = 10*LOG10((V(INOISE)*V(INOISE))/VSRCSQ)
dBm(N,Z) = VdB(N)*sqrt(500/Z)
MAGGAMMA = M(2*V(IN_NODE)-V(SRC_NODE))
S11 = 20*LOG10(MAGGAMMA)
VSWR = (1+MAGGAMMA)/(1-MAGGAMMA)
Now start Cadence PSD 14.2 -> Capture CIS. The LNA is already drawn, and
the assistant will show you where to find the schematic file, as shown in Fig. 1a.
Fig. 1a. Common-Base LNA Schematic.
Fig. 1b. Example simulation results.
TAMPERE UNIVERSITY OF TECHNOLOGY
Institute of Communications Engineering
8307030 Communication Circuits and Modules
2. Basic simulations with a resistor-loaded CB-LNA
2.1 DC-operation points
Perform a bias point analysis and check the operating points.
2.2 AC-gain, S11 and NF
For simulating AC-gain, S11, and noise figure, modify the setup as shown in
Fig. 2.
Fig. 2. AC and noise analysis setup.
Simulate and plot the results. You can find automatic functions for plotting NF
and the input return loss (S11) in “Macros”. You are supposed to receive results
similar to those in Fig. 1b.
Check the biggest noise contributors and the amount of noise they are
producing at 1 MHz. You can find them in View -> Output File.
2.3 Effect of the bias current on AC-gain, S11 and NF
Parameterize the bias current and see what happens to S11, the gain, and the
noise figure.
TAMPERE UNIVERSITY OF TECHNOLOGY
Institute of Communications Engineering
8307030 Communication Circuits and Modules
3. Band-limited CB-LNA
Replace the collector resistor with a parallel tank that has a 1µH inductor. What
is the size of the capacitor you need in order to achieve a central frequency of 10
MHz? Simulate and compare the results to the CB-LNA that has a collector
resistor.
4. Variable-gain CB-LNA
When the receiver is operating near the base station, it may be necessary to
reduce the gain. This can be done with a variable-gain amplifier, shown in Fig. 3.
Fig. 3. Variable-gain LNA.
Repeat the simulations for this topology. What value of voltage Vagc do you
need in order to achieve a 6 dB loss in the total gain?
5. Compression point simulation
One way to approximate the 1-dB compression point in PSpice is to run
transient analysis multiple times while varying the input amplitude between
simulation runs, and then studying the relationship between the output and input
TAMPERE UNIVERSITY OF TECHNOLOGY
Institute of Communications Engineering
8307030 Communication Circuits and Modules
voltages, i.e. the gain. When increasing the input amplitude, the gain begins to
decrease at some point.
Run transient analysis starting with an amplitude of 1 mV. Plot V(out) and
V(in_node).
Add a new plot (Plot -> Add new plot to Window). Plot a new curve by typing
the following line in the box “Trace expression”:
dB(rms(V(out))) - dB(rms(V(in_node)))
When you are running several analyses, it is helpful to check the option ‘Last
plot’ in Simulation Settings -> Probe Window. This preserves the previous results.
The power in dBm is PdBm
VRMS 2
= 10* Log10 (
).
R *0.001W
What is the value of the input 1-dB compression point in dBm?
Run the compression point simulation with the variable-gain LNA. What do
you notice?
6. White’s Cascode output stage
If you still have some extra time, you can replace the load resistor with the
proper output stage and repeat the simulations. The complete schematic is shown
in Fig. 4.
Fig. 4. LNA with White’s Cascode output stage.