Philips Semiconductors

Product specification

NE/SA5209

Wideband variable gain amplifier

1990 Aug 20

5

NE5209 APPLICATIONS

The NE5209 is a wideband variable gain amplifier (VGA) circuit

which finds many applications in the RF, IF and video signal

processing areas. This application note describes the operation of

the circuit and several applications of the VGA. The simplified

equivalent schematic of the VGA is shown in Figure 2. Transistors

Q1-Q6 form the wideband Gilbert multiplier input stage which is

biased by current source I1. The top differential pairs are biased

and the RF input appears at the lower differential pair. The circuit

topology and layout offer low input noise and wide bandwidth. The

second stage is a differential transimpedance stage with current

feedback which maintains the wide bandwidth of the input stage.

The output stage is a pair of emitter followers with 50

Ω output

impedance. There is also an on-chip bandgap reference with

buffered output at 1.3V, which can be used to derive the gain control

voltage.

Both the inputs and outputs should be capacitor coupled or DC

isolated from the signal sources and loads. Furthermore, the two

inputs should be DC isolated from each other and the two outputs

should likewise be DC isolated from each other. The NE5209 was

designed to provide optimum performance from a 5V power source.

However, there is some range around this value (4.5 - 7V) that can

be used.

The input impedance is about 1k

Ω. The main advantage to a

differential input configuration is to provide the balun function.

Otherwise, there is an advantage to common mode rejection, a

specification that is not normally important to RF designs. The

source impedance can be chosen for two different performance

characteristics: Gain, or noise performance. Gain optimization will

be realized if the input impedance is matched to about 1k

Ω. A 4:1

balun will provide such a broadband match from a 50

Ω source.

Noise performance will be optimized if the input impedance is

matched to about 200

Ω. A 2:1 balun will provide such a broadband

match from a 50

Ω source. The minimum noise figure can then be

expected to be about 7dB. Maximum gain will be about 23dB for a

single-ended output. If the differential output is used and properly

matched, nearly 30dB can be realized. With gain optimization, the

noise figure will degrade to about 8dB. With no matching unit at the

input, a 9dB noise figure can be expected from a 50

Ω source. If the

source is terminated, the noise figure will increase to about 15dB.

All these noise figures will occur at maximum gain.

The NE5209 has an excellent noise figure vs gain relationship. With

any VGA circuit, the noise performance will degrade with decreasing

gain.

The 5209 has about a 1.2dB noise figure degradation for

each 2dB gain reduction. With the input matched for optimum gain,

the 8dB noise figure at 23dB gain will degrade to about a 20dB

noise figure at 0dB gain.

The NE5209 also displays excellent linearity between voltage gain

and control voltage. Indeed, the relationship is of sufficient linearity

that high fidelity AM modulation is possible using the NE5209. A

maximum control voltage frequency of about 20MHz permits video

baseband sources for AM.

A stabilized bandgap reference voltage is made available on the

NE5209 (Pin 7). For fixed gain applications this voltage can be

resistor divided, and then fed to the gain control terminal (Pin 8).

Using the bandgap voltage reference for gain control produces very

stable gain characteristics over wide temperature ranges. The gain

setting resistors are not part of the RF signal path, and thus stray

capacitance here is not important.

The wide bandwidth and excellent gain control linearity make the

NE5209 VGA ideally suited for the automatic gain control (AGC)

function in RF and IF processing in cellular radio base stations,

Direct Broadcast Satellite (DBS) decoders, cable TV systems, fiber

optic receivers for wideband data and video, and other radio

communication applications. A typical AGC configuration using the

NE5209 is shown in Figure 3. Three NE5209s are cascaded with

appropriate AC coupling capacitors. The output of the final stage

drives the full-wave rectifier composed of two UHF Schottky diodes

that a quiescent current of about 2mA in each leg is achieved. An

NE5230 low voltage op amp is used as an integrator which drives

frequency ripple from the full-wave rectified signal. A voltage

divider is used to generate the reference for the non-inverting input

of the op amp at about 1.7V. Keeping D3 the same type as D1 and

D2 will provide a first order compensation for the change in Schottky

voltage over the operating temperature range and improve the AGC

performance. R6 is a variable resistor for adjustments to the op

amp reference voltage. In low cost and large volume applications

this could be replaced with a fixed resistor, which would result in a

slight loss of the AGC dynamic range. Cascading three NE5209s

will give a dynamic range in excess of 60dB.

The NE5209 is a very user-friendly part and will not oscillate in most

applications. However, in an application such as with gains in

excess of 60dB and bandwidth beyond 100MHz, good PC board

layout with proper supply decoupling is strongly recommended.

Q1

Q2

VCC

Q3

Q4

Q5

Q6

VBG

BANDGAP

REFERENCE

OUTA

OUTB

Q8

Q7

A1

VAGC

0–1V

I1

I2

I3

50

Ω

50

Ω

R1

R2

R3

R4

INA

INB

+

–

SR00238

Figure 2. Equivalent Schematic of the VGA