Low Noise Composite Amp With Input Matching

The composite circuit shown in Figure 9 eliminates the

need for a matching resistor to ground at the input. By

connecting two amplifiers in series, the first non-invert-

ing and the second inverting, an overall inverting gain is

realized.

output of the second amplifier to the non-inverting input

of the first amplifier closes the loop, and generates a set

resistor generates less noise than a matching resistor to

ground at the input.

Figure 9: Composite Amplifier

Input resistance and DC voltage gain of the amplifier are:

Match the source resistance by setting:

The noise of a simple input matching resistor connected

to ground can be calculated by setting G to 0 in this

equation. Thus, this circuit reduces the thermal noise

produced by the matching resistor by a factor of (1+G).

Rectifier Circuit

Wide bandwidth rectifier circuits have many applications.

Figure 10 shows a 200MHz wideband full-wave rectifier

circuit using a CLC449 and CLC522 amplifier. Schottky or

PIN diodes are used for D1 and D2. They produce an

active half-wave rectifier whose signals are taken at the

feedback diode connection. The CLC522 takes the

difference of the two half-wave rectified signal, producing

a full-wave rectifier. The CLC522 is used at a gain of 5 to

achieve high differential bandwidth. For best high

frequency performance, maintain low parasitic capaci-

tance from the diodes D1 and D2 to ground, and from the

input of the CLC522, to ground.

Figure 10: Full-Wave Rectifier

Flash A/D Application

The Typical Application circuit on the front page shows

the CLC449 driving a flash A/D. Flash A/D’s require fast

settling, low distortion, low noise and wide bandwidth to

achieve high Effective Number of Bits and Spurious Free

Dynamic Range (SFDR).

This circuit connects a CLC449 to a TDA8716, 8-bit,

120MHz Flash Converter. The input capacitance for

this converter is typically 13pF plus layout capacitance.

From the

Typical Performance Characteristics section, select

the output of the CLC449 to achieve settling to 0.1% in

approximately 11ns.

Keep the amplifier noise seen at the A/D input at least

3dB lower than the A/D’s noise, to avoid degrading A/D

noise performance.

CLC449 APPLICATIONS

+

-

CLC449

-

+

20

Ω

CLC449

+

-

R

R

1G

, where G

1

R

R

R

R

V

V

G

R

RR

in

ff1

g1

f2

g2

o

s

in

in

s

=

+

=+











 ⋅













=− ⋅

+









RR

in

s

=

e4 TR

R

R1 G

R

2

s

s

in

f

=⋅

⋅+

()









k

+

-

CLC449

+

-

500

Ω

CLC522

250

Ω

250

Ω

3

250

Ω

162

Ω

50

Ω

50

Ω

3

4

5

6

2

800

Ω

50

Ω

20

Ω

12

9

10

50

Ω

2

6

Ordering Information

Model

Temperature Range

Description

CLC449AJP

-40°C to +85°C

8-pin PDIP

CLC449AJE

-40°C to +85°C

8-pin SOIC

CLC449AMC

-55°C to +125°C

dice, MIL-STD-883

Contact factory for other packages and DESC SMD number.

http://www.national.com

8

Reliability Information

Transistor count

26