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 )||(||)||(