Lecture 8
Lecture 8
RF Amplifier Design
• Amplifier Design
– Summary of Design Methods
– Transistor Biasing
• Voltage and Current Drive of Bipolar Transistors
• Temperature Dependence
–
–
–
–
–
Johan Wernehag
Electrical and Information Technology
Passive Biasing Circuits
Active Biasing Circuits
Biasing of Field Effect Transistors
Isolating the Bias Design from Signal Designs
Diagnostic test
Johan Wernehag, EIT
Johan Wernehag, EIT
Summary of Amplifier Design Methods
Transistor Biasing
Specific GT and F
1. Decide if the transistor is unconditionally
stable or not
2. Calculate stability circles if the transistor
is conditionally stable
3. Choose the design method for
specific gain
4. Assume that conjugate match will be
applied at the output
(ID and VDS for FET)
Stable
area
6. Select a S in the stable area that
provides a suitable compromise between
gain and noise figure
OUT
8. Calculate
L
=
regarding the selected
OUT*
– collector current IC and
collector-emitter voltage VCE for BJT
L
5. Calculate and draw the gain circle
(ga < GMSG) and noise circle (F)
7. Calculate
• How is the bias setting specified?
F
S
ga
S
OUT
S11
Stable
area
S22
S
and check for stability
L
• Why controlling the biasing?
– S-parameters or similar are only valid at a specific
operating point (small signal parameters)
– temperature essentially controls the properties of
the transistor
9. Design the matching networks and verify stability for all frequencies of interest
10.Design the biasing circuits for proper DC settings of the transistor
Johan Wernehag, EIT
Johan Wernehag, EIT
•1
Voltage or Current Drive of the Transistor
The collector current
increase exponentially
wrt the base voltage
• Voltage drive:
Voltage Drive of Bipolar Transistors
• Simplified
relationship:
IC
– where the thermal voltage
VBE
• k = Boltzmann’s constant = 1.38·10-23 [J/K]
• T = absolute temperature [K]
• q = charge of the electron = 1.6·10-19 [C]
The collector current
depends linearly wrt
the base current
• Current drive:
IC
• The saturation current IS varies between different transistors
• VT and IS are temperature dependent
IB
Johan Wernehag, EIT
Johan Wernehag, EIT
Temperature Dependence at Voltage Driven Biasing
Temperature Dependence at Current Driven Biasing
80mA
55 A
•
Example:
Calculate the collector
current at constant VBE
-20 to 80 is a standard
temp. range for a
commercial design
Johan Wernehag, EIT
The temperature dependence at current driven biasing is
considerably ”friendlier” compared to voltage driven biasing!
A factor of 2 compared to 10million!
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•2
Controlling the Operating Point
• The operating point needs to be controlled to avoid
thermal avalanche effect that might destroy the device
• Three common methods to
implement the feedback:
Current Driven Biasing
• The simplest circuitry:
– Moderate temperature dependency
– Sensitive to variations in the current gain,
– Requires large resistance values
0
– Loop gain:
– Large loop gain if the ratio VCC / VCE is large
– passive current or voltage
driven biasing
– active biasing
• Alternative circuit solution:
–
–
–
–
Not strictly current driven
Moderate temperature dependency
Less sensitive to variations in the current gain
Does not require large resistance values
– Loop gain :
– thermal feedback
Johan Wernehag, EIT
– Large loop gain if the ratio VCC / VCE is large
Johan Wernehag, EIT
Voltage Driven Biasing
Example of Active Biasing Circuit
• Series feedback
without feedback
• Decent temperature dependence
• Not sensitive to variations in the current
active biasing
gain
passive
current drive
• RC may be replaced with an RFC*
• Loop gain: gm·RE
• Not suitable at high frequencies
– due to difficulties to signal ground the emitter
without introducing stability problems
T2: low frequency transistor working
as current generator
T1: high frequency transistor
*RFC = Radio-Frequency Choke, a large reactance coil intended for high frequencies
Johan Wernehag, EIT
Johan Wernehag, EIT
•3
Control by Thermal Feedback
Biasing of Field Effect Transistors
• The FET shows a slight temperature dependence
compared to the BJT
• Large spreading in the threshold voltage compared to
the BJT
Thermally
connected
• Only voltage driven biasing possible
• Passive biasing circuits are not usable if there is a large
variation in the threshold voltage
The diode VD is thermally connected to the transistor
Good in PAs etc. where high currents heat it up
Johan Wernehag, EIT
Johan Wernehag, EIT
Isolating the Bias Design from the Signal Design
Example using
RFC’s
Isolating the Bias Design from the Signal Design
Example using
stubs
biasing circuit
biasing circuit
signal
output
signal
output
signal
input
signal
input
signal circuit
RFC = Radio-Frequency Choke
may be used up to medium high frequencies
Johan Wernehag, EIT
signal circuit
At high frequencies the RFC’s are replaced with line elements
Johan Wernehag, EIT
•4
Lab 3 – RF amplifier
Johan Wernehag, EIT
•5