Cascode Stage
OUTLINE
Review of BJT AmplifiersCascode Stage
Reading: Chapter 9.1
Review: BJT Amplifier Design A BJT amplifier circuit should be designed to
1. ensure that the BJT operates in the active mode, 2. allow the desired level of DC current to flow, and3. couple to a small-signal input source and to an output “load”.
Proper “DC biasing” is required!(DC analysis using large-signal BJT model)
Key amplifier parameters: (AC analysis using small-signal BJT model) Voltage gain Av vout/vin
Input resistance Rin resistance seen between the input node and ground (with output terminal floating)
Output resistance Rout resistance seen between the output node and ground (with input terminal grounded)
Large-Signal vs. Small-Signal Models
The large-signal model is used to determine the DC operating point (VBE, VCE, IB, IC) of the BJT.
The small-signal model is used to determine how the output responds to an input signal.
The voltage across an independent voltage source does not vary with time. (Its small-signal voltage is always zero.)
It is regarded as a short circuit for small-signal analysis.
The current through an independent current source does not vary with time. (Its small-signal current is always zero.)
It is regarded as an open circuit for small-signal analysis.
Small-Signal Models for Independent Sources
Large-SignalModel
Small-SignalModel
Comparison of Amplifier Topologies
Common Emitter
Large Av < 0- Degraded by RE
- Degraded by RB/(+1)
Moderate Rin
- Increased by RB
- Increased by RE(+1)
Rout RC
ro degrades Av, Rout
but impedance seenlooking into the collector can be “boosted” by emitter degeneration
Common Base
• Large Av > 0- Degraded by RE and RS
- Degraded by RB/(+1)
• Small Rin
- Increased by RB/(+1)- Decreased by RE
• Rout RC
• ro degrades Av, Rout
but impedance seenlooking into the collector can be “boosted” by emitter degeneration
Emitter Follower
• 0 < Av ≤ 1- Degraded by RB/(+1)
• Large Rin
(due to RE(+1))
• Small Rout
- Effect of source impedance is reduced by +1- Decreased by RE
• ro decreases Av, Rin, and Rout
Common Emitter Stage
Cout
EBin
BE
m
C
in
out
RR
RrRR
RR
g
R
v
v
)1(
11
AV
)||(1|| rRgrRR EmOCout
AV
Common Base Stage
ES
E
BES
m
C
in
out
RR
RR
RRg
R
v
v
11
EB
min R
R
gR
1
1
AV
Cout RR )||(1|| rRgrRR EmOCout
AV
Emitter Follower
AV AV
11
S
mE
E
in
out
Rg
R
R
v
v
Ein RrR )1(
Es
mout R
R
gR ||
1
1
OEm
sout
OEin
S
mOE
OE
in
out
rRg
RR
rRrR
R
grR
rR
v
v
||||1
1
||1
11
||
||
Ideal Current Source
An ideal current source has infinite output impedance.
How can we increase the output impedance of a BJT that is used as a current source?
Equivalent CircuitCircuit Symbol I-V Characteristic
Recall that emitter degeneration boosts the impedance seen looking into the collector. This improves the gain of the CE or CB amplifier. However,
headroom is reduced.
Boosting the Output Impedance
rRrrRgR EOEmout ||||1
Cascode Stage In order to relax the trade-off between output
impedance and voltage headroom, we can use a transistor instead of a degeneration resistor:
VCE for Q2 can be as low as ~0.4V (“soft saturation”)
1211
12112
||
||)]||(1[
rrrgR
rrrrrgR
OOmout
OOOmout
1 if 1212 CEC III
Maximum Cascode Output Impedance The maximum output impedance of a cascode is
limited by r1.
1111max, 1 OOmout rrrgR
: If 12 rrO
PNP Cascode Stage
1211
121121
||
||)]||(1[
rrrgR
rrrrrgR
OOmout
OOOmout
False Cascodes
When the emitter of Q1 is connected to the emitter of Q2, it’s not a cascode since Q2 is a diode-connected device instead of a current source.
12
12
1
122
1122
1
21
1
||||1
||||1
1
Om
Om
mout
Om
OOm
mout
rg
rg
gR
rrg
rrrg
gR
Short-Circuit Transconductance
The short-circuit transconductance of a circuit is a measure of its strength in converting an input voltage signal into an output current signal.
0
outvin
outm v
iG
Voltage Gain of a Linear Circuit By representing a linear circuit with its Norton
equivalent, the relationship between Vout and Vin can be expressed by the product of Gm and Rout.
outminout
outinmoutoutout
RGvv
RvGRiv
Norton Equivalent Circuit
Computation of short-circuit
output current:
Example: Determination of Voltage Gain
11 Omv rgA
1
0
m
vin
outm g
v
iG
out
Determination of Gm Determination of Rout
1ox
xout r
i
vR
Comparison of CE and Cascode Stages Since the output impedance of the cascode is higher
than that of a CE stage, its voltage gain is also higher.
T
AOmv V
VrgA 11
11 Oinmout rvgv
21221 rrgrgA OmOmv
Voltage Gain of Cascode Amplifier Since rO is much larger than 1/gm, most of IC,Q1 flows
into diode-connected Q2. Using Rout as before, AV is easily calculated.
11 mminmout gGvgi
22121
2122121
||
||||1
OOmm
OOOmm
outmv
rrrgg
rrrrrgg
RGA
Practical Cascode Stage No current source is ideal; the output impedance is finite.
)||(|| 21223 rrrgrR OOmOout
Improved Cascode Stage In order to preserve the high output impedance, a
cascode PNP current source is used.
outmv
OOmOOmout
RgA
rrrgrrrgR
1
21223433 )||(||)||(