AD600ARZ-R7 datasheet(17/28 Pages) AD

17 / 28 page

AD600/AD602

Rev. E | Page 17 of 28

This is a Class AB amplifier. As VIN increases in a positive

direction, Q1 conducts more heavily and its re becomes lower

while Q2 increases. Conversely, increasingly negative values of

VIN result in the re of Q2 decreasing, while the re of Q1 increases.

The design is chosen such that the net emitter resistance is

essentially independent of the instantaneous value of VIN,

resulting in moderately low distortion. Low values of resistance

and moderately high bias currents are important in achieving

the low noise, wide bandwidth, and low distortion of this

preamplifier. Heavy decoupling prevents noise on the power

supply lines from being conveyed to the input of the X-AMP.

Table 4. Measured Preamplifier Performance

Measurement

Value

Unit

Gain (f = 30 MHz)

Bandwidth (−3 dB)

250

MHz

Input Signal for 1 dB Compression

V p-p

Distortion

VIN = 200 mV p-p

HD2

0.27

HD3

0.14

VIN = 500 mV p-p

HD2

0.44

HD3

0.58

System Input Noise

1.03

nV/√Hz

Spectral Density (NSD)

(Preamp plus X-AMP)

Input Resistance

1.4

kΩ

Input Capacitance

Input Bias Current

±150

μA

Power Supply Voltage

±5

Quiescent Current

A LOW NOISE AGC AMPLIFIER WITH 80 dB GAIN

RANGE

Figure 37 provides an example of the ease with which the

AD600 can be connected as an AGC amplifier. A1 and A2 are

cascaded, with 6 dB of attenuation introduced by the 100 Ω

resistor R1, while a time constant of 5 ns is formed by C1 and

the 50 Ω of net resistance at the input of A2. This has the dual

effect of lowering the overall gain range from 0 dB to 80 dB to

−6 dB to 74 dB and introducing a single-pole, low-pass filter

with a −3 dB frequency of about 32 MHz. This ensures stability

at the maximum gain for a slight reduction in the overall

bandwidth. The Capacitor C4 blocks the small dc offset voltage

at the output of A1 (which may otherwise saturate A2 at its

maximum gain) and introduces a high-pass corner at about

8 kHz, useful in eliminating low frequency noise and spurious

signals that can be present at the input.

REF

–

AD600

C1HI

A1CM

A1OP

VPOS

VNEG

A2OP

A2CM

C2HI

C1LO

A1HI

A1LO

GAT1

GAT2

A2LO

A2HI

C2LO

100Ω

INPUT

0.1µF

100pF

+5V

46.4kΩ

3.74kΩ

15pF

AD590

+5V

2N3904

+5V

–5V

+5V DEC

–5V DEC

0.1µF

POWER SUPPLY

DECOUPLING NETWORK

OUTPUT

+5V DEC

–5V DEC

–

VPTAT

VG´

300µA

(AT 300K)

1µF

806Ω

Figure 37. This Accurate HF AGC Amplifier Uses Three Active Components

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