Lecture 8
RF Amplifier Design
Johan Wernehag
Electrical and Information Technology
Johan Wernehag, EIT
Lecture 8
• Amplifier Design
– Summary of Design Methods
– Transistor Biasing
• Voltage and Current Drive of Bipolar Transistors
• Temperature Dependence
–
–
–
–
–
Passive Biasing Circuits
Active Biasing Circuits
Biasing of Field Effect Transistors
Isolating the Bias Design from Signal Designs
Diagnostic test
Johan Wernehag, EIT
Summary of Amplifier Design Methods
Specific GT and F
1. Decide if the transistor is unconditionally
stable or not
2. Calculate stability circles if the transistor
is conditionally stable
F
S
3. Choose the design method for
specific gain
4. Assume that conjugate match will be
applied at the output
L
Stable
area S
5. Calculate and draw the gain circle
(ga < GMSG) and noise circle (F)
6. Select a S in the stable area that
provides a suitable compromise between
gain and noise figure
7. Calculate
OUT
8. Calculate
L
=
regarding the selected
OUT*
ga
OUT
S11
Stable
area L
S22
S
and check for stability
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
Transistor Biasing
• How is the bias setting specified?
– collector current IC and
collector-emitter voltage VCE for BJT
(ID and VDS for FET)
• 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
Johan Wernehag, EIT
Voltage or Current Drive of the Transistor
The collector current
increase exponentially
wrt the base voltage
• Voltage drive:
IC
IC
VBE
VBE
The collector current
depends linearly wrt
the base current
• Current drive:
IC
IC
IB
IB
Johan Wernehag, EIT
Voltage Drive of Bipolar Transistors
• Simplified
relationship:
IC
VBE
VT
IS e
IC
VBE
VT
IS e
1
VBE
– where the thermal voltage
VT
kT q 26 mV
T 300 K
• 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 saturation current IS varies between different transistors
• VT and IS are temperature dependent
Johan Wernehag, EIT
Temperature Dependence at Voltage Driven Biasing
IS A
IC A
80mA
55 A
T C
•
Example:
Calculate the collector
current at constant VBE
VBE
0, 7745 V
T C
IC
IC
VBE
VT
IS e
1 mA at T
300 K
VBE
-20 to 80 is a standard
temp. range for a
commercial design
Johan Wernehag, EIT
Temperature Dependence at Current Driven Biasing
IC
IC
0
IB
IB
0
T C
The temperature dependence at current driven biasing is
considerably ”friendlier” compared to voltage driven biasing!
A factor of 2 compared to 10million!
Johan Wernehag, EIT
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:
VCC
RB
VCC
– passive current or voltage
driven biasing
I
IB
RC1
R1
T2
I
C1
R2
I
C2
IB
– active biasing
T1
VCC
RC2
ID
– thermal feedback
IB
VD
Johan Wernehag, EIT
RC
I
C1
T1
C
Current Driven Biasing
VCC
• The simplest circuitry:
– Moderate temperature dependency
– Sensitive to variations in the current gain, 0
– Requires large resistance values
R
– Loop gain: 0 C
RB
– Large loop gain if the ratio VCC / VCE is large
RB
RC
I
IB
C
• Alternative circuit solution:
Not strictly current driven
Moderate temperature dependency
Less sensitive to variations in the current gain
Does not require large resistance values
R
RB2
– Loop gain : 0 C
RB3 RB1 RB2
– Large loop gain if the ratio VCC / VCE is large
VCC
–
–
–
–
Johan Wernehag, EIT
RB1
RC
I
RB3 I B
RB2
C
Voltage Driven Biasing
• Series feedback
VCC
• Decent temperature dependence
RC
• Not sensitive to variations in the current
gain
RB1
I
C
IB
• RC may be replaced with an RFC*
• Loop gain: gm·RE
RB2
RE
• Not suitable at high frequencies
– due to difficulties to signal ground the emitter
without introducing stability problems
*RFC = Radio-Frequency Choke, a large reactance coil intended for high frequencies
Johan Wernehag, EIT
Example of Active Biasing Circuit
VCC
I C mA
without feedback
RC1
R1
active biasing
T2
I
R2
I
C2
IB
passive
current drive
C1
T1
T C
RC2
T2: low frequency transistor working
as current generator
Johan Wernehag, EIT
T1: high frequency transistor
Control by Thermal Feedback
VCC
ID
Thermally
connected
IB
I
C1
T1
V
D
The diode VD is thermally connected to the transistor
Good in PAs etc. where high currents heat it up
Johan Wernehag, EIT
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
• Only voltage driven biasing possible
• Passive biasing circuits are not usable if there is a large
variation in the threshold voltage
Johan Wernehag, EIT
Isolating the Bias Design from the Signal Design
Example using
RFC’s
biasing circuit
signal
output
signal
input
signal circuit
RFC = Radio-Frequency Choke
may be used up to medium high frequencies
Johan Wernehag, EIT
Isolating the Bias Design from the Signal Design
Example using
stubs
biasing circuit
signal
output
signal
input
signal circuit
At high frequencies the RFC’s are replaced with line elements
Johan Wernehag, EIT
Lab 3 – RF amplifier
Johan Wernehag, EIT