LINEAR TECHNOLOGY LTC6412 Technical data

DESIGN FEATURES L

9

2

BUFFER/
OUTPUT

AMPLIFIER

ATTENUATOR

CONTROL

REFERENCE AND BIAS CONTROL

+V

G

+IN

3

–IN

4

5

V

CM

V

CM

10

V

REF

11 1

–V

G

GND8GND12GND15GND

21

EN

222419136

SHDN

18

GND

20

GNDV

CC

V

CC

V

CC

V

CC

23

GND

DECL2

25

14

DECL1

7

–OUT

16

+OUT

17

EXPOSED

PAD

REFERENCE AND

BIAS CONTROL

• • •

• • •

• • •

+VG OR –VG VOLTAGE (V)

0

GAIN (dB)

–5

0

5

0.6

1.0

–10

–15

–20

0.2 0.4 0.8

10

15

20

1.2

–40°C
25°C
85°C

–VG: NEGATIVE
SLOPE MODE

+VG: POSITIVE
SLOPE MODE

FREQ = 140MHz

FREQUENCY (MHz)

–20

GAIN (dB)

–10

0

10

20

1 100 1000 10000

–30

10

G

MAX

G

MIN

–VG VOLTAGE (V)

0

–5

GAIN CONFORMANCE ERROR (dB)

–3

–1

1

0.2

G

MAX

G

MIN

0.4

0.6 0.8 1.0

3

5

–4

–2

0

2

4

1.2

FREQ = 140MHz

–40°C

85°C

25°C

Analog VGA Simplifies Design
and Outperforms Competing
Gain Control Methods

Introduction

Variable gain amplifiers (VGAs) are
widely used in communications and
imaging applications such as cel­lular radio, satellite receivers, global
positioning, radar, and ultrasound ap­plications. Most of these applications
involve transmit and receive signals
of varying amplitude that need to be
managed within the constraints of the
overall system design. On the transmit
side, the signal amplitude is usually
adjusted near a maximum limit im­posed by the transmit power amplifier
or below a power limit imposed by the
receivers or reflectors of the signal. On
the receive side, the signal amplitude is
usually amplified and tailored to take
optimum advantage of the demodula­tor or ADC that decodes the signal. In
both the transmit and receive case, the
optimum signal gain targets change
over time and temperature, so most
systems share a common require­ment of controlling signal amplitude
through the use of adjustable gain
stages commonly known as variable
gain amplifiers.

This article introduces the LTC6412,
Linear Technology’s first high fre­quency, analog-controlled VGA—now
added to Linear Technology’s existing
portfolio of digitally controlled VGAs.
The design considerations for analog

vs digital control are also discussed.
This is followed by a brief introduction
to the important design and perfor­mance features of the LTC6412 along
with a discussion of a few application
examples.

Analog vs Digital Control
of VGAs

The vast majority of modern com­munication and imaging equipment
contains significant digital hardware
in the form of microprocessors, con­trollers, memory, data busses and the
like, so the choice of analog vs digital
system control would seem to be a
forgone conclusion in favor of the digi-

Figure 1. Block diagram of the LTC6412

by Walter Strifler

tally controlled VGA. While this trend
statement is largely true, it overlooks
important distinctions between the
two types of VGA control.

The digitally controlled VGA is
a natural choice when the system
parameters that determine optimum
gain are known to the digital control
system and are readily available across
a data bus. This information is piped
to the data inputs of the VGA, and the
desired gain is step-adjusted during
noncritical periods in the time-slotted
signal.

The digital control scenario is the
goal of most system designs, but
it leaves many application gaps for

Figure 2. LTC6412 gain vs frequency over gain
control range

Linear Technology Magazine • September 2009

Figure 3. Differential gain vs control voltage
over temperature for the LTC6412

Figure 4. LTC6412 gain conformance error vs
control voltage over temperature

19

L DESIGN FEATURES

RF

OUT

50Ω

PEAK

RF

OUT

= 4dBm

–V

G

0.5V/DIV

0.5µs/DIV

V

CC

GND

SHDNEN

–V

G

V

REF

+V

G

–OUT

+OUT

–OUT

+OUT

–IN

+IN

–IN

+IN

10nF

10nF

10nF

C

F

1000pF

10nF

IF IN IF OUT

–IN

+IN

LTC6412 LTC6400-8

LT5537

V

CC

EN

GND

220Ω

10nF

2.2nF

470pF

10nF

1k 1k1k 1k

180nH 180nH

3.3V

3.3V

3.3V

3.3V

3.3V

3.3V

–

+

½LTC6244

590k

33k

2k

100Ω

AGC SET

–20dB TAP

0.1µF

OUT

INPUT

200mV/DIV

OUTPUT

200mV/DIV

20µs/DIV

INPUT

200mV/DIV

C=330pF

C=1000pF

C=4400pF

20µs/DIV

OUTPUT (200mV/DIV)

clever analog solutions. For example,
what if the information needed to
control the amplifier gain is not known
to the digital control system or no
practical data bus is available? What
if the RF signal through the ampli­fier chain cannot tolerate any step
disturbance in amplitude or phase?
These kinds of situations arise often
enough to sustain a healthy market
for analog-controlled VGAs. A few
such applications are discussed later
in this article.

Design Features

The LTC6412 is an 800MHz analog­controlled VGA manufactured on an
advanced silicon-germanium (SiGe)
BiCMOS process that offers the speed
and performance of a complementary
SiGe bipolar process along with the
flexibility and compactness of a CMOS
process. The term SiGe refers to the
material composition of the bipolar
base layers whereby a SiGe semicon­ductor alloy is used to create critical
bandgap discontinuities and drift
fields within the bipolar devices to
improve high speed performance.

Figure 5. LTC6412 gain control 10dB step
response at IF = 70MHz

Figure 1 shows a block diagram of
the LTC6412. The design employs an
interpolated, tapped attenuator circuit
architecture to generate the variable
gain characteristic of the amplifier.
The tapped attenuator is fed to a
buffer and output amplifier to com­plete the differential signal path. The
circuit architecture provides good RF
input handling capability along with
a constant output noise and output
IP3 characteristic that are desirable for
most IF signal chain applications.

The internal circuitry takes the gain
control signal from the ±VG terminals
and converts this to an appropriate set
of control signals to the attenuator lad-

der. The attenuator control preserves
OIP3 through the interpolated transi­tions and ensures that the linear-in-dB
gain response is continuous and
monotonic over the 31dB gain range
for both slow and fast moving input
control signals, all while maintaining a
fixed input and output terminal imped­ance. The control terminal inputs can
be configured for positive or negative
gain slope mode by connecting the
unused control terminal to the V

REF

pin provided.

The output amplifier employs an
open-collector topology and linearizing
techniques similar to the LT5554. En­hanced clamping circuits provide fast
overdrive recovery up to 15dB signal
compression. The entire circuit runs
off a 3.3V supply at a nominal total
supply current of 110mA.

Electrical Performance

The LTC6412 is a fully differential VGA
designed for AC-coupled operation in
signal chains from 1MHz–500MHz and
provides a typical maximum gain of
17dB and minimum noise figure (NF)
of 10dB over this frequency range.

Figure 6. Analog control loop application circuit at IF = 240MHz. LTC6412 bypass capacitors to
ground omitted for clarity.

20

Figure 7. Measured analog control loop circuit
response to 6dB step changes in input signal
amplitude for CF = 1000pF

Figure 8. Measured analog control loop
response to 6dB step changes in input signal
amplitude over a range of CF values

Linear Technology Magazine • September 2009