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2. CURRENT REUSE CASCADE MPL IFIER

J n order to achieve a system with low power consumption operati11g in the

6 GHz band the power consumption of each build ing block has to be

minimized by focusing on the el imination

of

any possible power wastages .

n addition  due to the high operating frequency the rniUimeter-wave CMOS

amplifier has to operate at a significant fraction

of

the cut-off frequency of the

transistor. Currently it

is

difficult to obtain sufficient gain using a conventional

single-stage amplifier. However increasing the number of stages in a cascade

connection increases the power consumption. Therefore  the approach to

optimum power usage is to improve the gain ofeach single-stage block. In this

section  a 60 GHz ampl ifier has been designed to demonstrate a building block

for

reduced power consumption. This amp

li

fie r implements a current-reuse

technique  to

be

explained in the follow ing sections. Although there are otber

amplifiers that use an alternative current-reuse teclmique

67

-

7

for

bo

osting the

gain

of

microwave c

ir

cuits without having an accompanied increase in power

consumption the technique is difficult to apply at millimeter-wave frequencies

since many passive components are required in the circuit The passive

components usually leads to high insertion losses at high frequencies. n th is

work  a current-reuse technique

fo

r gain-boosting without consuming

additional power bas beeo proposed through a simple

to

pology.

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2 1 Principles o Operation

Du

ri

ng operation, this amp

li

fier reuses

th

e drain current to bias two

transistors, as in the case of the cascode topology, and feeding

th

e signal

from one stage

to

the other through a resonance path. A review

of

the

conventional cascode topology, shown in Fig. I0-4 a),

re

veals that the

cascode transistor M2 does not effectively contribute to the gain.

The

common-gate cascode transistor may increase the output impedance by a

factor of g

 

r r

  2

where

2

and r

  2

is the transconductance and the dra in

sou

rc

e resis

ta

nce

of

M2, but it does not increase the ga

in

in the amplifier

because the load impedance is always

li

mited to a low value in a microwave

circuit. The gain is provided by the common-source transistor of the

topology.

As

a result, M2 is main ly used to improve the

re

verse isolation. To

improve the gain, the function of the common-gate transistor M2 should

be

re-visited. From here, a new cascade circuit shown in Fig. 10-4 b) is

proposed , where

an

inductor at the gate of a common-gate transistor is

inse11cd By using this

ga

te inductor, the operation of M2 changes from a

common-gate amplifier

to

that of a common-source. A cascade connection is

formed by the feed-through

of

the high-frequency signal through the gate

sourcc capaci t

or

of

M2, which results in gain-boosting. This circuit

is

referred

to

as a current-reuse cascade amp lifier CRCA), hereafter.

The

operation of the

CRCA

is further desc

ri

bed below.