Leonard C. Nelson College of Engineering and Sciences
Electrical and Computer Engineering Department
EE-456, RF Design
Fall
Laboratory Report 2:
Amplifier and Attenuator
Author: Jane Doe
Professor: Dr. Jane Doe
Report submitted on: ##/##/##
Purpose
Amplifiers and attenuators are two very common devices in microwave engineering. The
purpose of an amplifier is to provide gain in a system while the purpose of an attenuator is to
provide a loss in a system. The main objective of the experiment is to show that amplifiers
provide a power gain to the RF signal and use an external DC power supply to provide the gain,
meaning that it is not a lossless device nor is it a reciprocal device . As well as show that
attenuators provide loss meaning they are not a lossless device, but they are reciprocal.
Theory
Amplification is one of the most basic and prevalent microwave circuit functions. The main
purpose of an amplifier is to amplify the signal, this is done by introducing gain to a system.
Amplifiers are known as an active device meaning they require an external DC power supply to
be connected to the circuit on top of the AC supply that is connected to the device.
Fundamentally speaking an amplifier provides a power gain for the RF input to the RF output.
The signal does not have any loss in RF power from port 1 to port 2, but the DC power supplied
does have a loss when generating the gain needed for the RF signal. The only power dissipated in
the device is from the DC power supply. Since the amplifier is an active device it does not satisfy
the requirements to be reciprocal. Therefore it is not lossless or reciprocal.
Attenuator is a passive microwave device. Since this device is passive it does not require an
external DC power supply to be connected. By its basic functionality an attenuator introduces a
loss to a system. This means that the condition, |𝑆11 |2 + |𝑆21 |2 = 1 is not true for an attenuator.
The attenuators performance is characterized by the magnitude and the phase. This is reminiscent
of how a transfer function is represented. This means that an attenuator is able to be represented
by a transfer function. Defining an output of a two port network as 𝑆(𝑆) = 𝑆𝑆(𝑆 − 𝑆0 ) where
𝑆(𝑆 − 𝑆0 ) is the input with a time delay (the delay prevents the source from being distorted).
The transfer function can be derived 𝑆(𝑆) = 𝑆(𝑆)/𝑆(𝑆) = 𝑆𝑆−𝑆𝑆𝑆0 where 𝛼 is the
amplitude component and 𝑆−𝑆𝑆𝑆0 is the phase component. The amplitude will be a constant flat
line which means that across the frequency spectrum the attenuator is constant for many different
frequencies making the attenuator very wideband. Since the attenuator is passive and is
characterized by loss, it is reasonable to determine that it is a reciprocal device. This means for a
two port network attenuator, S11=S22 and S12=S21.
Measurement Procedure
The network analyzer provides important measurement tools to characterize the performance of a
high frequency network. To conduct the procedure a DUT (Device Under Test) is connected to
the scattering parameter ports of the network analyzer and each scattering parameter is displayed
on the screen. There are two different DUTs used in this procedure: Amplifier and Attenuator.
Figure 1: Amplifier DUT Connected to Scattering Parameter Test
Figure 2: Attenuator DUT Connected to Scattering Parameter Test
Measurement Results and Analysis
Figure 3: |S21| of Thru connection (0dB, Amplitude = 1)
Amplifier
Figure 4: Magnitude of S21
Figure 5: Power supply voltage and current
Figure 6: Gain at 3GHz
● When measuring the magnitude of S21 in Figure 4 the spectrum analyzer shows that the
signal continues from the negative dB range to zero dB and into the positive dB range.
Physically, the positive dB range means power gain (Power > 1 Watt). Mathematically,
|𝑆21 | = |𝑆2 |/|𝑆1 | when |𝑆2 | = 0. For |𝑆21 | to show a gain it is required that |𝑆2 | >
|𝑆1 |. In Figure 6 the magnitude of S21 is shown at 3GHz, the value is approximately 10
dB. In linear scale this is 10 Watts which is 10 times the power at 0 dB. Based on the
results shown for the amplifier the input RF power is approximately 1 mW and the output
RF power is 10 mW. This power gain is created by the DC power supply input. In Figure
5 the voltage is 10 Volts and the current is approximately 60 mA. This means the power
of the DC power supply is approximately 600 mW. This shows how low the efficiency of
the amplifier is, roughly 590 mW is lost to create the gain from 1 mW to 10 mW. Based
on the performance in Figure 4 the amplifier is not a wideband device.
Attenuator
Figure 7: Magnitude of S21 of attenuator
Figure 8: Phase of S21 of Attenuator
● Based on Figure 7 the magnitude shows that this is a -10 dB attenuator. Physically this
means that the output power of the device is attenuated by 10 dB compared to the input
power of the device. Based on the magnitude it can also be said that the attenuator is a
wideband device unlike the amplifier. When looking at the phase performance in Figure
8, the phase is very linear with respect to the frequency. A flat magnitude and a linear
phase are what compose a good transfer function. This means that the attenuator has a
wideband transfer function. The magnitude of S21 represents the power and the phase of
S21 represents time delay.
Conclusion
The amplifier and attenuator are two common microwave devices. When observing the
performance of the amplifier it is important to take note of the physical behavior that the
scattering parameters represent. By observing the magnitudes behavior it is easy to come to the
conclusion that the amplifier is not lossless and by its physical setup it is easy to conclude that it
is an active device. The amplifier can be classified as an active device that has power gain and is
not reciprocal. Observing the performance and setup of the attenuator in the same manner as the
amplifier it is easy to deduce that the attenuator is a passive device due to the lack of external DC
power supply as well as the device is not lossless. The attenuator can be classified as a passive
device that is reciprocal, but not lossless.